summaryrefslogtreecommitdiffstats
path: root/Documentation/s390/Debugging390.txt
blob: adbfe620c0615003c3628fac77c3d082d900aa09 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
              
                          Debugging on Linux for s/390 & z/Architecture
			               by
		Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com)
		Copyright (C) 2000-2001 IBM Deutschland Entwicklung GmbH, IBM Corporation
                              Best viewed with fixed width fonts 

Overview of Document:
=====================
This document is intended to give an good overview of how to debug 
Linux for s/390 & z/Architecture it isn't intended as a complete reference & not a
tutorial on the fundamentals of C & assembly, it dosen't go into
390 IO in any detail. It is intended to complement the documents in the
reference section below & any other worthwhile references you get.

It is intended like the Enterprise Systems Architecture/390 Reference Summary
to be printed out & used as a quick cheat sheet self help style reference when
problems occur.

Contents
========
Register Set
Address Spaces on Intel Linux
Address Spaces on Linux for s/390 & z/Architecture
The Linux for s/390 & z/Architecture Kernel Task Structure
Register Usage & Stackframes on Linux for s/390 & z/Architecture
A sample program with comments
Compiling programs for debugging on Linux for s/390 & z/Architecture
Figuring out gcc compile errors
Debugging Tools
objdump
strace
Performance Debugging 
Debugging under VM
s/390 & z/Architecture IO Overview
Debugging IO on s/390 & z/Architecture under VM
GDB on s/390 & z/Architecture
Stack chaining in gdb by hand
Examining core dumps
ldd
Debugging modules
The proc file system
Starting points for debugging scripting languages etc.
Dumptool & Lcrash
SysRq
References
Special Thanks

Register Set
============
The current architectures have the following registers.
 
16  General propose registers, 32 bit on s/390 64 bit on z/Architecture, r0-r15 or gpr0-gpr15 used for arithmetic & addressing. 

16 Control registers, 32 bit on s/390 64 bit on z/Architecture, ( cr0-cr15 kernel usage only ) used for memory management,
interrupt control,debugging control etc.

16 Access registers ( ar0-ar15 ) 32 bit on s/390 & z/Architecture
not used by normal programs but potentially could 
be used as temporary storage. Their main purpose is their 1 to 1
association with general purpose registers and are used in
the kernel for copying data between kernel & user address spaces.
Access register 0 ( & access register 1 on z/Architecture ( needs 64 bit 
pointer ) ) is currently used by the pthread library as a pointer to
the current running threads private area.

16 64 bit floating point registers (fp0-fp15 ) IEEE & HFP floating 
point format compliant on G5 upwards & a Floating point control reg (FPC) 
4  64 bit registers (fp0,fp2,fp4 & fp6) HFP only on older machines.
Note:
Linux (currently) always uses IEEE & emulates G5 IEEE format on older machines,
( provided the kernel is configured for this ).


The PSW is the most important register on the machine it
is 64 bit on s/390 & 128 bit on z/Architecture & serves the roles of 
a program counter (pc), condition code register,memory space designator.
In IBM standard notation I am counting bit 0 as the MSB.
It has several advantages over a normal program counter
in that you can change address translation & program counter 
in a single instruction. To change address translation,
e.g. switching address translation off requires that you
have a logical=physical mapping for the address you are
currently running at.

      Bit           Value
s/390 z/Architecture
0       0     Reserved ( must be 0 ) otherwise specification exception occurs.

1       1     Program Event Recording 1 PER enabled, 
	      PER is used to facilititate debugging e.g. single stepping.

2-4    2-4    Reserved ( must be 0 ). 

5       5     Dynamic address translation 1=DAT on.

6       6     Input/Output interrupt Mask

7       7     External interrupt Mask used primarily for interprocessor signalling & 
	      clock interrupts.

8-11  8-11    PSW Key used for complex memory protection mechanism not used under linux

12      12    1 on s/390 0 on z/Architecture

13      13    Machine Check Mask 1=enable machine check interrupts

14      14    Wait State set this to 1 to stop the processor except for interrupts & give 
	      time to other LPARS used in CPU idle in the kernel to increase overall 
	      usage of processor resources.

15      15    Problem state ( if set to 1 certain instructions are disabled )
	      all linux user programs run with this bit 1 
	      ( useful info for debugging under VM ).

16-17 16-17   Address Space Control

	      00 Primary Space Mode when DAT on
	      The linux kernel currently runs in this mode, CR1 is affiliated with 
              this mode & points to the primary segment table origin etc.

	      01 Access register mode this mode is used in functions to 
	      copy data between kernel & user space.

	      10 Secondary space mode not used in linux however CR7 the
	      register affiliated with this mode is & this & normally
	      CR13=CR7 to allow us to copy data between kernel & user space.
	      We do this as follows:
	      We set ar2 to 0 to designate its
	      affiliated gpr ( gpr2 )to point to primary=kernel space.
	      We set ar4 to 1 to designate its
	      affiliated gpr ( gpr4 ) to point to secondary=home=user space
	      & then essentially do a memcopy(gpr2,gpr4,size) to
	      copy data between the address spaces, the reason we use home space for the
	      kernel & don't keep secondary space free is that code will not run in 
	      secondary space.

	      11 Home Space Mode all user programs run in this mode.
	      it is affiliated with CR13.

18-19 18-19   Condition codes (CC)

20    20      Fixed point overflow mask if 1=FPU exceptions for this event 
              occur ( normally 0 ) 

21    21      Decimal overflow mask if 1=FPU exceptions for this event occur 
              ( normally 0 )

22    22      Exponent underflow mask if 1=FPU exceptions for this event occur 
              ( normally 0 )

23    23      Significance Mask if 1=FPU exceptions for this event occur 
              ( normally 0 )

24-31 24-30   Reserved Must be 0.

      31      Extended Addressing Mode
      32      Basic Addressing Mode
              Used to set addressing mode
	      PSW 31   PSW 32
                0         0        24 bit
                0         1        31 bit
                1         1        64 bit

32             1=31 bit addressing mode 0=24 bit addressing mode (for backward 
               compatibility ), linux always runs with this bit set to 1

33-64          Instruction address.
      33-63    Reserved must be 0
      64-127   Address
               In 24 bits mode bits 64-103=0 bits 104-127 Address 
               In 31 bits mode bits 64-96=0 bits 97-127 Address
               Note: unlike 31 bit mode on s/390 bit 96 must be zero
	       when loading the address with LPSWE otherwise a 
               specification exception occurs, LPSW is fully backward
               compatible.
 	  
	  
Prefix Page(s)
--------------	  
This per cpu memory area is too intimately tied to the processor not to mention.
It exists between the real addresses 0-4096 on s/390 & 0-8192 z/Architecture & is exchanged 
with a 1 page on s/390 or 2 pages on z/Architecture in absolute storage by the set 
prefix instruction in linux'es startup. 
This page is mapped to a different prefix for each processor in an SMP configuration
( assuming the os designer is sane of course :-) ).
Bytes 0-512 ( 200 hex ) on s/390 & 0-512,4096-4544,4604-5119 currently on z/Architecture 
are used by the processor itself for holding such information as exception indications & 
entry points for exceptions.
Bytes after 0xc00 hex are used by linux for per processor globals on s/390 & z/Architecture 
( there is a gap on z/Architecure too currently between 0xc00 & 1000 which linux uses ).
The closest thing to this on traditional architectures is the interrupt
vector table. This is a good thing & does simplify some of the kernel coding
however it means that we now cannot catch stray NULL pointers in the
kernel without hard coded checks.



Address Spaces on Intel Linux
=============================

The traditional Intel Linux is approximately mapped as follows forgive
the ascii art.
0xFFFFFFFF 4GB Himem                        *****************
                                            *               *
                                            * Kernel Space  *
                                            *               *
                                            *****************          ****************
User Space Himem (typically 0xC0000000 3GB )*  User Stack   *          *              *
				            *****************          *              *
					    *  Shared Libs  *          * Next Process *          
                                            *****************          *     to       *  
					    *               *    <==   *     Run      *  <==
					    *  User Program *          *              *
					    *   Data BSS    *          *              *
                                            *	 Text       *          *              *
         			            *   Sections    *          *              *
0x00000000         			    *****************          ****************

Now it is easy to see that on Intel it is quite easy to recognise a kernel address 
as being one greater than user space himem ( in this case 0xC0000000).
& addresses of less than this are the ones in the current running program on this
processor ( if an smp box ).
If using the virtual machine ( VM ) as a debugger it is quite difficult to
know which user process is running as the address space you are looking at
could be from any process in the run queue.

The limitation of Intels addressing technique is that the linux
kernel uses a very simple real address to virtual addressing technique
of Real Address=Virtual Address-User Space Himem.
This means that on Intel the kernel linux can typically only address
Himem=0xFFFFFFFF-0xC0000000=1GB & this is all the RAM these machines
can typically use.
They can lower User Himem to 2GB or lower & thus be
able to use 2GB of RAM however this shrinks the maximum size
of User Space from 3GB to 2GB they have a no win limit of 4GB unless
they go to 64 Bit.


On 390 our limitations & strengths make us slightly different.
For backward compatibility we are only allowed use 31 bits (2GB)
of our 32 bit addresses,however, we use entirely separate address 
spaces for the user & kernel.

This means we can support 2GB of non Extended RAM on s/390, & more
with the Extended memory management swap device & 
currently 4TB of physical memory currently on z/Architecture.


Address Spaces on Linux for s/390 & z/Architecture
==================================================

Our addressing scheme is as follows


Himem 0x7fffffff 2GB on s/390    *****************          ****************
currently 0x3ffffffffff (2^42)-1 *  User Stack   *          *              *
on z/Architecture.		 *****************          *              *
		                 *  Shared Libs  *          *              *      
                                 *****************          *              *  
			         *               *          *    Kernel    *  
		                 *  User Program *          *              *
		                 *   Data BSS    *          *              *
                                 *    Text       *          *              *
            			 *   Sections    *          *              *
0x00000000                       *****************          ****************

This also means that we need to look at the PSW problem state bit
or the addressing mode to decide whether we are looking at
user or kernel space.

Virtual Addresses on s/390 & z/Architecture
===========================================

A virtual address on s/390 is made up of 3 parts
The SX ( segment index, roughly corresponding to the PGD & PMD in linux terminology ) 
being bits 1-11.
The PX ( page index, corresponding to the page table entry (pte) in linux terminology )
being bits 12-19. 
The remaining bits BX (the byte index are the offset in the page )
i.e. bits 20 to 31.

On z/Architecture in linux we currently make up an address from 4 parts.
The region index bits (RX) 0-32 we currently use bits 22-32
The segment index (SX) being bits 33-43
The page index (PX) being bits  44-51
The byte index (BX) being bits  52-63

Notes:
1) s/390 has no PMD so the PMD is really the PGD also.
A lot of this stuff is defined in pgtable.h.

2) Also seeing as s/390's page indexes are only 1k  in size 
(bits 12-19 x 4 bytes per pte ) we use 1 ( page 4k )
to make the best use of memory by updating 4 segment indices 
entries each time we mess with a PMD & use offsets 
0,1024,2048 & 3072 in this page as for our segment indexes.
On z/Architecture our page indexes are now 2k in size
( bits 12-19 x 8 bytes per pte ) we do a similar trick
but only mess with 2 segment indices each time we mess with
a PMD.

3) As z/Architecture supports upto a massive 5-level page table lookup we 
can only use 3 currently on Linux ( as this is all the generic kernel
currently supports ) however this may change in future
this allows us to access ( according to my sums )
4TB of virtual storage per process i.e.
4096*512(PTES)*1024(PMDS)*2048(PGD) = 4398046511104 bytes,
enough for another 2 or 3 of years I think :-).
to do this we use a region-third-table designation type in
our address space control registers.
 

The Linux for s/390 & z/Architecture Kernel Task Structure
==========================================================
Each process/thread under Linux for S390 has its own kernel task_struct
defined in linux/include/linux/sched.h
The S390 on initialisation & resuming of a process on a cpu sets
the __LC_KERNEL_STACK variable in the spare prefix area for this cpu
( which we use for per processor globals).

The kernel stack pointer is intimately tied with the task stucture for
each processor as follows.

                      s/390
            ************************
            *  1 page kernel stack *
	    *        ( 4K )        *
            ************************
            *   1 page task_struct *        
            *        ( 4K )        *
8K aligned  ************************ 

                 z/Architecture
            ************************
            *  2 page kernel stack *
	    *        ( 8K )        *
            ************************
            *  2 page task_struct  *        
            *        ( 8K )        *
16K aligned ************************ 

What this means is that we don't need to dedicate any register or global variable
to point to the current running process & can retrieve it with the following
very simple construct for s/390 & one very similar for z/Architecture.

static inline struct task_struct * get_current(void)
{
        struct task_struct *current;
        __asm__("lhi   %0,-8192\n\t"
                "nr    %0,15"
                : "=r" (current) );
        return current;
}

i.e. just anding the current kernel stack pointer with the mask -8192.
Thankfully because Linux dosen't have support for nested IO interrupts
& our devices have large buffers can survive interrupts being shut for 
short amounts of time we don't need a separate stack for interrupts.




Register Usage & Stackframes on Linux for s/390 & z/Architecture
=================================================================
Overview:
---------
This is the code that gcc produces at the top & the bottom of
each function, it usually is fairly consistent & similar from 
function to function & if you know its layout you can probalby
make some headway in finding the ultimate cause of a problem
after a crash without a source level debugger.

Note: To follow stackframes requires a knowledge of C or Pascal &
limited knowledge of one assembly language.

It should be noted that there are some differences between the
s/390 & z/Architecture stack layouts as the z/Architecture stack layout didn't have
to maintain compatibility with older linkage formats.

Glossary:
---------
alloca:
This is a built in compiler function for runtime allocation
of extra space on the callers stack which is obviously freed
up on function exit ( e.g. the caller may choose to allocate nothing
of a buffer of 4k if required for temporary purposes ), it generates 
very efficient code ( a few cycles  ) when compared to alternatives 
like malloc.

automatics: These are local variables on the stack,
i.e they aren't in registers & they aren't static.

back-chain:
This is a pointer to the stack pointer before entering a
framed functions ( see frameless function ) prologue got by 
deferencing the address of the current stack pointer,
 i.e. got by accessing the 32 bit value at the stack pointers
current location.

base-pointer:
This is a pointer to the back of the literal pool which
is an area just behind each procedure used to store constants
in each function.

call-clobbered: The caller probably needs to save these registers if there 
is something of value in them, on the stack or elsewhere before making a 
call to another procedure so that it can restore it later.

epilogue:
The code generated by the compiler to return to the caller.

frameless-function
A frameless function in Linux for s390 & z/Architecture is one which doesn't 
need more than the register save area ( 96 bytes on s/390, 160 on z/Architecture )
given to it by the caller.
A frameless function never:
1) Sets up a back chain.
2) Calls alloca.
3) Calls other normal functions
4) Has automatics.

GOT-pointer:
This is a pointer to the global-offset-table in ELF
( Executable Linkable Format, Linux'es most common executable format ),
all globals & shared library objects are found using this pointer.

lazy-binding
ELF shared libraries are typically only loaded when routines in the shared
library are actually first called at runtime. This is lazy binding.

procedure-linkage-table
This is a table found from the GOT which contains pointers to routines
in other shared libraries which can't be called to by easier means.

prologue:
The code generated by the compiler to set up the stack frame.

outgoing-args:
This is extra area allocated on the stack of the calling function if the
parameters for the callee's cannot all be put in registers, the same
area can be reused by each function the caller calls.

routine-descriptor:
A COFF  executable format based concept of a procedure reference 
actually being 8 bytes or more as opposed to a simple pointer to the routine.
This is typically defined as follows
Routine Descriptor offset 0=Pointer to Function
Routine Descriptor offset 4=Pointer to Table of Contents
The table of contents/TOC is roughly equivalent to a GOT pointer.
& it means that shared libraries etc. can be shared between several
environments each with their own TOC.

 
static-chain: This is used in nested functions a concept adopted from pascal 
by gcc not used in ansi C or C++ ( although quite useful ), basically it
is a pointer used to reference local variables of enclosing functions.
You might come across this stuff once or twice in your lifetime.

e.g.
The function below should return 11 though gcc may get upset & toss warnings 
about unused variables.
int FunctionA(int a)
{
	int b;
	FunctionC(int c)
	{
		b=c+1;
	}
	FunctionC(10);
	return(b);
}


s/390 & z/Architecture Register usage
=====================================
r0       used by syscalls/assembly                  call-clobbered
r1	 used by syscalls/assembly                  call-clobbered
r2       argument 0 / return value 0                call-clobbered
r3       argument 1 / return value 1 (if long long) call-clobbered
r4       argument 2                                 call-clobbered
r5       argument 3                                 call-clobbered
r6	 argument 5                                 saved
r7       pointer-to arguments 5 to ...              saved      
r8       this & that                                saved
r9       this & that                                saved
r10      static-chain ( if nested function )        saved
r11      frame-pointer ( if function used alloca )  saved
r12      got-pointer                                saved
r13      base-pointer                               saved
r14      return-address                             saved
r15      stack-pointer                              saved

f0       argument 0 / return value ( float/double ) call-clobbered
f2       argument 1                                 call-clobbered
f4       z/Architecture argument 2                  saved
f6       z/Architecture argument 3                  saved
The remaining floating points
f1,f3,f5 f7-f15 are call-clobbered.

Notes:
------
1) The only requirement is that registers which are used
by the callee are saved, e.g. the compiler is perfectly
capible of using r11 for purposes other than a frame a
frame pointer if a frame pointer is not needed.
2) In functions with variable arguments e.g. printf the calling procedure 
is identical to one without variable arguments & the same number of 
parameters. However, the prologue of this function is somewhat more
hairy owing to it having to move these parameters to the stack to
get va_start, va_arg & va_end to work.
3) Access registers are currently unused by gcc but are used in
the kernel. Possibilities exist to use them at the moment for
temporary storage but it isn't recommended.
4) Only 4 of the floating point registers are used for
parameter passing as older machines such as G3 only have only 4
& it keeps the stack frame compatible with other compilers.
However with IEEE floating point emulation under linux on the
older machines you are free to use the other 12.
5) A long long or double parameter cannot be have the 
first 4 bytes in a register & the second four bytes in the 
outgoing args area. It must be purely in the outgoing args
area if crossing this boundary.
6) Floating point parameters are mixed with outgoing args
on the outgoing args area in the order the are passed in as parameters.
7) Floating point arguments 2 & 3 are saved in the outgoing args area for 
z/Architecture


Stack Frame Layout
------------------
s/390     z/Architecture
0         0             back chain ( a 0 here signifies end of back chain )
4         8             eos ( end of stack, not used on Linux for S390 used in other linkage formats )
8         16            glue used in other s/390 linkage formats for saved routine descriptors etc.
12        24            glue used in other s/390 linkage formats for saved routine descriptors etc.
16        32            scratch area
20        40            scratch area
24        48            saved r6 of caller function
28        56            saved r7 of caller function
32        64            saved r8 of caller function
36        72            saved r9 of caller function
40        80            saved r10 of caller function
44        88            saved r11 of caller function
48        96            saved r12 of caller function
52        104           saved r13 of caller function
56        112           saved r14 of caller function
60        120           saved r15 of caller function
64        128           saved f4 of caller function
72        132           saved f6 of caller function
80                      undefined
96        160           outgoing args passed from caller to callee
96+x      160+x         possible stack alignment ( 8 bytes desirable )
96+x+y    160+x+y       alloca space of caller ( if used )
96+x+y+z  160+x+y+z     automatics of caller ( if used )
0                       back-chain

A sample program with comments.
===============================

Comments on the function test
-----------------------------
1) It didn't need to set up a pointer to the constant pool gpr13 as it isn't used
( :-( ).
2) This is a frameless function & no stack is bought.
3) The compiler was clever enough to recognise that it could return the
value in r2 as well as use it for the passed in parameter ( :-) ).
4) The basr ( branch relative & save ) trick works as follows the instruction 
has a special case with r0,r0 with some instruction operands is understood as 
the literal value 0, some risc architectures also do this ). So now
we are branching to the next address & the address new program counter is
in r13,so now we subtract the size of the function prologue we have executed
+ the size of the literal pool to get to the top of the literal pool
0040037c int test(int b)
{                                                          # Function prologue below
  40037c:	90 de f0 34 	stm	%r13,%r14,52(%r15) # Save registers r13 & r14
  400380:	0d d0       	basr	%r13,%r0           # Set up pointer to constant pool using
  400382:	a7 da ff fa 	ahi	%r13,-6            # basr trick
	return(5+b);
	                                                   # Huge main program
  400386:	a7 2a 00 05 	ahi	%r2,5              # add 5 to r2

                                                           # Function epilogue below 
  40038a:	98 de f0 34 	lm	%r13,%r14,52(%r15) # restore registers r13 & 14
  40038e:	07 fe       	br	%r14               # return
}

Comments on the function main
-----------------------------
1) The compiler did this function optimally ( 8-) )

Literal pool for main.
400390:	ff ff ff ec 	.long 0xffffffec
main(int argc,char *argv[])
{                                                          # Function prologue below
  400394:	90 bf f0 2c 	stm	%r11,%r15,44(%r15) # Save necessary registers
  400398:	18 0f       	lr	%r0,%r15           # copy stack pointer to r0
  40039a:	a7 fa ff a0 	ahi	%r15,-96           # Make area for callee saving 
  40039e:	0d d0       	basr	%r13,%r0           # Set up r13 to point to
  4003a0:	a7 da ff f0 	ahi	%r13,-16           # literal pool
  4003a4:	50 00 f0 00 	st	%r0,0(%r15)        # Save backchain

	return(test(5));                                   # Main Program Below
  4003a8:	58 e0 d0 00 	l	%r14,0(%r13)       # load relative address of test from
						           # literal pool
  4003ac:	a7 28 00 05 	lhi	%r2,5              # Set first parameter to 5
  4003b0:	4d ee d0 00 	bas	%r14,0(%r14,%r13)  # jump to test setting r14 as return
							   # address using branch & save instruction.

							   # Function Epilogue below
  4003b4:	98 bf f0 8c 	lm	%r11,%r15,140(%r15)# Restore necessary registers.
  4003b8:	07 fe       	br	%r14               # return to do program exit 
}


Compiler updates
----------------

main(int argc,char *argv[])
{
  4004fc:	90 7f f0 1c       	stm	%r7,%r15,28(%r15)
  400500:	a7 d5 00 04       	bras	%r13,400508 <main+0xc>
  400504:	00 40 04 f4       	.long	0x004004f4 
  # compiler now puts constant pool in code to so it saves an instruction 
  400508:	18 0f             	lr	%r0,%r15
  40050a:	a7 fa ff a0       	ahi	%r15,-96
  40050e:	50 00 f0 00       	st	%r0,0(%r15)
	return(test(5));
  400512:	58 10 d0 00       	l	%r1,0(%r13)
  400516:	a7 28 00 05       	lhi	%r2,5
  40051a:	0d e1             	basr	%r14,%r1
  # compiler adds 1 extra instruction to epilogue this is done to
  # avoid processor pipeline stalls owing to data dependencies on g5 &
  # above as register 14 in the old code was needed directly after being loaded 
  # by the lm	%r11,%r15,140(%r15) for the br %14.
  40051c:	58 40 f0 98       	l	%r4,152(%r15)
  400520:	98 7f f0 7c       	lm	%r7,%r15,124(%r15)
  400524:	07 f4             	br	%r4
}


Hartmut ( our compiler developer ) also has been threatening to take out the
stack backchain in optimised code as this also causes pipeline stalls, you
have been warned.

64 bit z/Architecture code disassembly
--------------------------------------

If you understand the stuff above you'll understand the stuff
below too so I'll avoid repeating myself & just say that 
some of the instructions have g's on the end of them to indicate
they are 64 bit & the stack offsets are a bigger, 
the only other difference you'll find between 32 & 64 bit is that
we now use f4 & f6 for floating point arguments on 64 bit.
00000000800005b0 <test>:
int test(int b)
{
	return(5+b);
    800005b0:	a7 2a 00 05       	ahi	%r2,5
    800005b4:	b9 14 00 22       	lgfr	%r2,%r2 # downcast to integer
    800005b8:	07 fe             	br	%r14
    800005ba:	07 07             	bcr	0,%r7


}

00000000800005bc <main>:
main(int argc,char *argv[])
{ 
    800005bc:	eb bf f0 58 00 24 	stmg	%r11,%r15,88(%r15)
    800005c2:	b9 04 00 1f       	lgr	%r1,%r15
    800005c6:	a7 fb ff 60       	aghi	%r15,-160
    800005ca:	e3 10 f0 00 00 24 	stg	%r1,0(%r15)
	return(test(5));
    800005d0:	a7 29 00 05       	lghi	%r2,5
    # brasl allows jumps > 64k & is overkill here bras would do fune
    800005d4:	c0 e5 ff ff ff ee 	brasl	%r14,800005b0 <test> 
    800005da:	e3 40 f1 10 00 04 	lg	%r4,272(%r15)
    800005e0:	eb bf f0 f8 00 04 	lmg	%r11,%r15,248(%r15)
    800005e6:	07 f4             	br	%r4
}



Compiling programs for debugging on Linux for s/390 & z/Architecture
====================================================================
-gdwarf-2 now works it should be considered the default debugging
format for s/390 & z/Architecture as it is more reliable for debugging
shared libraries,  normal -g debugging works much better now
Thanks to the IBM java compiler developers bug reports. 

This is typically done adding/appending the flags -g or -gdwarf-2 to the 
CFLAGS & LDFLAGS variables Makefile of the program concerned.

If using gdb & you would like accurate displays of registers &
 stack traces compile without optimisation i.e make sure
that there is no -O2 or similar on the CFLAGS line of the Makefile &
the emitted gcc commands, obviously this will produce worse code 
( not advisable for shipment ) but it is an  aid to the debugging process.

This aids debugging because the compiler will copy parameters passed in
in registers onto the stack so backtracing & looking at passed in
parameters will work, however some larger programs which use inline functions
will not compile without optimisation.

Debugging with optimisation has since much improved after fixing
some bugs, please make sure you are using gdb-5.0 or later developed 
after Nov'2000.

Figuring out gcc compile errors
===============================
If you are getting a lot of syntax errors compiling a program & the problem
isn't blatantly obvious from the source.
It often helps to just preprocess the file, this is done with the -E
option in gcc.
What this does is that it runs through the very first phase of compilation
( compilation in gcc is done in several stages & gcc calls many programs to
achieve its end result ) with the -E option gcc just calls the gcc preprocessor (cpp).
The c preprocessor does the following, it joins all the files #included together
recursively ( #include files can #include other files ) & also the c file you wish to compile.
It puts a fully qualified path of the #included files in a comment & it
does macro expansion.
This is useful for debugging because
1) You can double check whether the files you expect to be included are the ones
that are being included ( e.g. double check that you aren't going to the i386 asm directory ).
2) Check that macro definitions aren't clashing with typedefs,
3) Check that definitons aren't being used before they are being included.
4) Helps put the line emitting the error under the microscope if it contains macros.

For convenience the Linux kernel's makefile will do preprocessing automatically for you
by suffixing the file you want built with .i ( instead of .o )

e.g.
from the linux directory type
make arch/s390/kernel/signal.i
this will build

s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
-fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce   -E arch/s390/kernel/signal.c
> arch/s390/kernel/signal.i  

Now look at signal.i you should see something like.


# 1 "/home1/barrow/linux/include/asm/types.h" 1
typedef unsigned short umode_t;
typedef __signed__ char __s8;
typedef unsigned char __u8;
typedef __signed__ short __s16;
typedef unsigned short __u16;

If instead you are getting errors further down e.g.
unknown instruction:2515 "move.l" or better still unknown instruction:2515 
"Fixme not implemented yet, call Martin" you are probably are attempting to compile some code 
meant for another architecture or code that is simply not implemented, with a fixme statement
stuck into the inline assembly code so that the author of the file now knows he has work to do.
To look at the assembly emitted by gcc just before it is about to call gas ( the gnu assembler )
use the -S option.
Again for your convenience the Linux kernel's Makefile will hold your hand &
do all this donkey work for you also by building the file with the .s suffix.
e.g.
from the Linux directory type 
make arch/s390/kernel/signal.s 

s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
-fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce  -S arch/s390/kernel/signal.c 
-o arch/s390/kernel/signal.s  


This will output something like, ( please note the constant pool & the useful comments
in the prologue to give you a hand at interpreting it ).

.LC54:
	.string	"misaligned (__u16 *) in __xchg\n"
.LC57:
	.string	"misaligned (__u32 *) in __xchg\n"
.L$PG1: # Pool sys_sigsuspend
.LC192:
	.long	-262401
.LC193:
	.long	-1
.LC194:
	.long	schedule-.L$PG1
.LC195:
	.long	do_signal-.L$PG1
	.align 4
.globl sys_sigsuspend
	.type	 sys_sigsuspend,@function
sys_sigsuspend:
#	leaf function           0
#	automatics              16
#	outgoing args           0
#	need frame pointer      0
#	call alloca             0
#	has varargs             0
#	incoming args (stack)   0
#	function length         168
	STM	8,15,32(15)
	LR	0,15
	AHI	15,-112
	BASR	13,0
.L$CO1:	AHI	13,.L$PG1-.L$CO1
	ST	0,0(15)
	LR    8,2
	N     5,.LC192-.L$PG1(13) 

Adding -g to the above output makes the output even more useful
e.g. typing
make CC:="s390-gcc -g" kernel/sched.s

which compiles.
s390-gcc -g -D__KERNEL__ -I/home/barrow/linux-2.3/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -fno-strict-aliasing -pipe -fno-strength-reduce   -S kernel/sched.c -o kernel/sched.s 

also outputs stabs ( debugger ) info, from this info you can find out the
offsets & sizes of various elements in structures.
e.g. the stab for the structure
struct rlimit {
	unsigned long	rlim_cur;
	unsigned long	rlim_max;
};
is
.stabs "rlimit:T(151,2)=s8rlim_cur:(0,5),0,32;rlim_max:(0,5),32,32;;",128,0,0,0
from this stab you can see that 
rlimit_cur starts at bit offset 0 & is 32 bits in size
rlimit_max starts at bit offset 32 & is 32 bits in size.


Debugging Tools:
================

objdump
=======
This is a tool with many options the most useful being ( if compiled with -g).
objdump --source <victim program or object file> > <victims debug listing >


The whole kernel can be compiled like this ( Doing this will make a 17MB kernel
& a 200 MB listing ) however you have to strip it before building the image
using the strip command to make it a more reasonable size to boot it.

A source/assembly mixed dump of the kernel can be done with the line
objdump --source vmlinux > vmlinux.lst
Also if the file isn't compiled -g this will output as much debugging information
as it can ( e.g. function names ), however, this is very slow as it spends lots
of time searching for debugging info, the following self explanitory line should be used 
instead if the code isn't compiled -g.
objdump --disassemble-all --syms vmlinux > vmlinux.lst  
as it is much faster

As hard drive space is valuble most of us use the following approach.
1) Look at the emitted psw on the console to find the crash address in the kernel.
2) Look at the file System.map ( in the linux directory ) produced when building 
the kernel to find the closest address less than the current PSW to find the
offending function.
3) use grep or similar to search the source tree looking for the source file
 with this function if you don't know where it is.
4) rebuild this object file with -g on, as an example suppose the file was
( /arch/s390/kernel/signal.o ) 
5) Assuming the file with the erroneous function is signal.c Move to the base of the 
Linux source tree.
6) rm /arch/s390/kernel/signal.o
7) make /arch/s390/kernel/signal.o
8) watch the gcc command line emitted
9) type it in again or alernatively cut & paste it on the console adding the -g option.
10) objdump --source arch/s390/kernel/signal.o > signal.lst
This will output the source & the assembly intermixed, as the snippet below shows
This will unfortunately output addresses which aren't the same
as the kernel ones you should be able to get around the mental arithmetic
by playing with the --adjust-vma parameter to objdump.




extern inline void spin_lock(spinlock_t *lp)
{
      a0:       18 34           lr      %r3,%r4
      a2:       a7 3a 03 bc     ahi     %r3,956
        __asm__ __volatile("    lhi   1,-1\n"
      a6:       a7 18 ff ff     lhi     %r1,-1
      aa:       1f 00           slr     %r0,%r0
      ac:       ba 01 30 00     cs      %r0,%r1,0(%r3)
      b0:       a7 44 ff fd     jm      aa <sys_sigsuspend+0x2e>
        saveset = current->blocked;
      b4:       d2 07 f0 68     mvc     104(8,%r15),972(%r4)
      b8:       43 cc
        return (set->sig[0] & mask) != 0;
} 

6) If debugging under VM go down to that section in the document for more info.


I now have a tool which takes the pain out of --adjust-vma
& you are able to do something like
make /arch/s390/kernel/traps.lst
& it automatically generates the correctly relocated entries for
the text segment in traps.lst.
This tool is now standard in linux distro's in scripts/makelst

strace:
-------
Q. What is it ?
A. It is a tool for intercepting calls to the kernel & logging them
to a file & on the screen.

Q. What use is it ?
A. You can used it to find out what files a particular program opens.



Example 1
---------
If you wanted to know does ping work but didn't have the source 
strace ping -c 1 127.0.0.1  
& then look at the man pages for each of the syscalls below,
( In fact this is sometimes easier than looking at some spagetti
source which conditionally compiles for several architectures )
Not everything that it throws out needs to make sense immeadiately

Just looking quickly you can see that it is making up a RAW socket
for the ICMP protocol.
Doing an alarm(10) for a 10 second timeout
& doing a gettimeofday call before & after each read to see 
how long the replies took, & writing some text to stdout so the user
has an idea what is going on.

socket(PF_INET, SOCK_RAW, IPPROTO_ICMP) = 3
getuid()                                = 0
setuid(0)                               = 0
stat("/usr/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory)
stat("/usr/share/locale/libc/C", 0xbffff134) = -1 ENOENT (No such file or directory)
stat("/usr/local/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory)
getpid()                                = 353
setsockopt(3, SOL_SOCKET, SO_BROADCAST, [1], 4) = 0
setsockopt(3, SOL_SOCKET, SO_RCVBUF, [49152], 4) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(3, 1), ...}) = 0
mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40008000
ioctl(1, TCGETS, {B9600 opost isig icanon echo ...}) = 0
write(1, "PING 127.0.0.1 (127.0.0.1): 56 d"..., 42PING 127.0.0.1 (127.0.0.1): 56 data bytes
) = 42
sigaction(SIGINT, {0x8049ba0, [], SA_RESTART}, {SIG_DFL}) = 0 
sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {SIG_DFL}) = 0
gettimeofday({948904719, 138951}, NULL) = 0
sendto(3, "\10\0D\201a\1\0\0\17#\2178\307\36"..., 64, 0, {sin_family=AF_INET,
sin_port=htons(0), sin_addr=inet_addr("127.0.0.1")}, 16) = 64
sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0
sigaction(SIGALRM, {0x8049ba0, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0
alarm(10)                               = 0
recvfrom(3, "E\0\0T\0005\0\0@\1|r\177\0\0\1\177"..., 192, 0, 
{sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84
gettimeofday({948904719, 160224}, NULL) = 0
recvfrom(3, "E\0\0T\0006\0\0\377\1\275p\177\0"..., 192, 0, 
{sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84
gettimeofday({948904719, 166952}, NULL) = 0
write(1, "64 bytes from 127.0.0.1: icmp_se"..., 
5764 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=28.0 ms

Example 2
---------
strace passwd 2>&1 | grep open
produces the following output
open("/etc/ld.so.cache", O_RDONLY)      = 3
open("/opt/kde/lib/libc.so.5", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/lib/libc.so.5", O_RDONLY)        = 3
open("/dev", O_RDONLY)                  = 3
open("/var/run/utmp", O_RDONLY)         = 3
open("/etc/passwd", O_RDONLY)           = 3
open("/etc/shadow", O_RDONLY)           = 3
open("/etc/login.defs", O_RDONLY)       = 4
open("/dev/tty", O_RDONLY)              = 4 

The 2>&1 is done to redirect stderr to stdout & grep is then filtering this input 
through the pipe for each line containing the string open.


Example 3
---------
Getting sophistocated
telnetd crashes on & I don't know why
Steps
-----
1) Replace the following line in /etc/inetd.conf
telnet  stream  tcp     nowait  root    /usr/sbin/in.telnetd -h 
with
telnet  stream  tcp     nowait  root    /blah

2) Create the file /blah with the following contents to start tracing telnetd 
#!/bin/bash
/usr/bin/strace -o/t1 -f /usr/sbin/in.telnetd -h 
3) chmod 700 /blah to make it executable only to root
4)
killall -HUP inetd
or ps aux | grep inetd
get inetd's process id
& kill -HUP inetd to restart it.

Important options
-----------------
-o is used to tell strace to output to a file in our case t1 in the root directory
-f is to follow children i.e.
e.g in our case above telnetd will start the login process & subsequently a shell like bash.
You will be able to tell which is which from the process ID's listed on the left hand side
of the strace output.
-p<pid> will tell strace to attach to a running process, yup this can be done provided
 it isn't being traced or debugged already & you have enough privileges,
the reason 2 processes cannot trace or debug the same program is that strace
becomes the parent process of the one being debugged & processes ( unlike people )
can have only one parent.


However the file /t1 will get big quite quickly
to test it telnet 127.0.0.1

now look at what files in.telnetd execve'd
413   execve("/usr/sbin/in.telnetd", ["/usr/sbin/in.telnetd", "-h"], [/* 17 vars */]) = 0
414   execve("/bin/login", ["/bin/login", "-h", "localhost", "-p"], [/* 2 vars */]) = 0 

Whey it worked!.


Other hints:
------------
If the program is not very interactive ( i.e. not much keyboard input )
& is crashing in one architecture but not in another you can do 
an strace of both programs under as identical a scenario as you can
on both architectures outputting to a file then.
do a diff of the two traces using the diff program
i.e.
diff output1 output2
& maybe you'll be able to see where the call paths differed, this
is possibly near the cause of the crash. 

More info
---------
Look at man pages for strace & the various syscalls
e.g. man strace, man alarm, man socket.


Performance Debugging
=====================
gcc is capible of compiling in profiling code just add the -p option
to the CFLAGS, this obviously affects program size & performance.
This can be used by the gprof gnu profiling tool or the
gcov the gnu code coverage tool ( code coverage is a means of testing
code quality by checking if all the code in an executable in exercised by
a tester ).


Using top to find out where processes are sleeping in the kernel
----------------------------------------------------------------
To do this copy the System.map from the root directory where
the linux kernel was built to the /boot directory on your 
linux machine.
Start top
Now type fU<return>
You should see a new field called WCHAN which
tells you where each process is sleeping here is a typical output.
 
 6:59pm  up 41 min,  1 user,  load average: 0.00, 0.00, 0.00
28 processes: 27 sleeping, 1 running, 0 zombie, 0 stopped
CPU states:  0.0% user,  0.1% system,  0.0% nice, 99.8% idle
Mem:   254900K av,   45976K used,  208924K free,       0K shrd,   28636K buff
Swap:       0K av,       0K used,       0K free                    8620K cached

  PID USER     PRI  NI  SIZE  RSS SHARE WCHAN     STAT  LIB %CPU %MEM   TIME COMMAND
  750 root      12   0   848  848   700 do_select S       0  0.1  0.3   0:00 in.telnetd
  767 root      16   0  1140 1140   964           R       0  0.1  0.4   0:00 top
    1 root       8   0   212  212   180 do_select S       0  0.0  0.0   0:00 init
    2 root       9   0     0    0     0 down_inte SW      0  0.0  0.0   0:00 kmcheck

The time command
----------------
Another related command is the time command which gives you an indication
of where a process is spending the majority of its time.
e.g.
time ping -c 5 nc
outputs
real	0m4.054s
user	0m0.010s
sys	0m0.010s

Debugging under VM
==================

Notes
-----
Addresses & values in the VM debugger are always hex never decimal
Address ranges are of the format <HexValue1>-<HexValue2> or <HexValue1>.<HexValue2> 
e.g. The address range  0x2000 to 0x3000 can be described described as
2000-3000 or 2000.1000

The VM Debugger is case insensitive.

VM's strengths are usually other debuggers weaknesses you can get at any resource
no matter how sensitive e.g. memory management resources,change address translation
in the PSW. For kernel hacking you will reap dividends if you get good at it.

The VM Debugger displays operators but not operands, probably because some
of it was written when memory was expensive & the programmer was probably proud that
it fitted into 2k of memory & the programmers & didn't want to shock hardcore VM'ers by
changing the interface :-), also the debugger displays useful information on the same line & 
the author of the code probably felt that it was a good idea not to go over 
the 80 columns on the screen. 

As some of you are probably in a panic now this isn't as unintuitive as it may seem
as the 390 instructions are easy to decode mentally & you can make a good guess at a lot 
of them as all the operands are nibble ( half byte aligned ) & if you have an objdump listing
also it is quite easy to follow, if you don't have an objdump listing keep a copy of
the s/390 Reference Summary & look at between pages 2 & 7 or alternatively the
s/390 principles of operation.
e.g. even I can guess that 
0001AFF8' LR    180F        CC 0
is a ( load register ) lr r0,r15 

Also it is very easy to tell the length of a 390 instruction from the 2 most significant
bits in the instruction ( not that this info is really useful except if you are trying to
make sense of a hexdump of code ).
Here is a table
Bits                    Instruction Length
------------------------------------------
00                          2 Bytes
01                          4 Bytes
10                          4 Bytes
11                          6 Bytes




The debugger also displays other useful info on the same line such as the
addresses being operated on destination addresses of branches & condition codes.
e.g.  
00019736' AHI   A7DAFF0E    CC 1
000198BA' BRC   A7840004 -> 000198C2'   CC 0
000198CE' STM   900EF068 >> 0FA95E78    CC 2



Useful VM debugger commands
---------------------------

I suppose I'd better mention this before I start
to list the current active traces do 
Q TR
there can be a maximum of 255 of these per set
( more about trace sets later ).
To stop traces issue a
TR END.
To delete a particular breakpoint issue
TR DEL <breakpoint number>

The PA1 key drops to CP mode so you can issue debugger commands,
Doing alt c (on my 3270 console at least ) clears the screen. 
hitting b <enter> comes back to the running operating system
from cp mode ( in our case linux ).
It is typically useful to add shortcuts to your profile.exec file
if you have one ( this is roughly equivalent to autoexec.bat in DOS ).
file here are a few from mine.
/* this gives me command history on issuing f12 */
set pf12 retrieve 
/* this continues */
set pf8 imm b
/* goes to trace set a */
set pf1 imm tr goto a
/* goes to trace set b */
set pf2 imm tr goto b
/* goes to trace set c */
set pf3 imm tr goto c



Instruction Tracing
-------------------
Setting a simple breakpoint
TR I PSWA <address>
To debug a particular function try
TR I R <function address range>
TR I on its own will single step.
TR I DATA <MNEMONIC> <OPTIONAL RANGE> will trace for particular mnemonics
e.g.
TR I DATA 4D R 0197BC.4000
will trace for BAS'es ( opcode 4D ) in the range 0197BC.4000
if you were inclined you could add traces for all branch instructions &
suffix them with the run prefix so you would have a backtrace on screen 
when a program crashes.
TR BR <INTO OR FROM> will trace branches into or out of an address.
e.g.
TR BR INTO 0 is often quite useful if a program is getting awkward & deciding
to branch to 0 & crashing as this will stop at the address before in jumps to 0.
TR I R <address range> RUN cmd d g
single steps a range of addresses but stays running &
displays the gprs on each step.



Displaying & modifying Registers
--------------------------------
D G will display all the gprs
Adding a extra G to all the commands is necessary to access the full 64 bit 
content in VM on z/Architecture obviously this isn't required for access registers
as these are still 32 bit.
e.g. DGG instead of DG 
D X will display all the control registers
D AR will display all the access registers
D AR4-7 will display access registers 4 to 7
CPU ALL D G will display the GRPS of all CPUS in the configuration
D PSW will display the current PSW
st PSW 2000 will put the value 2000 into the PSW &
cause crash your machine.
D PREFIX displays the prefix offset


Displaying Memory
-----------------
To display memory mapped using the current PSW's mapping try
D <range>
To make VM display a message each time it hits a particular address & continue try
D I<range> will disassemble/display a range of instructions.
ST addr 32 bit word will store a 32 bit aligned address
D T<range> will display the EBCDIC in an address ( if you are that way inclined )
D R<range> will display real addresses ( without DAT ) but with prefixing.
There are other complex options to display if you need to get at say home space
but are in primary space the easiest thing to do is to temporarily
modify the PSW to the other addressing mode, display the stuff & then
restore it.


 
Hints
-----
If you want to issue a debugger command without halting your virtual machine with the
PA1 key try prefixing the command with #CP e.g.
#cp tr i pswa 2000
also suffixing most debugger commands with RUN will cause them not
to stop just display the mnemonic at the current instruction on the console.
If you have several breakpoints you want to put into your program &
you get fed up of cross referencing with System.map
you can do the following trick for several symbols.
grep do_signal System.map 
which emits the following among other things
0001f4e0 T do_signal 
now you can do

TR I PSWA 0001f4e0 cmd msg * do_signal
This sends a message to your own console each time do_signal is entered.
( As an aside I wrote a perl script once which automatically generated a REXX
script with breakpoints on every kernel procedure, this isn't a good idea
because there are thousands of these routines & VM can only set 255 breakpoints
at a time so you nearly had to spend as long pruning the file down as you would 
entering the msg's by hand ),however, the trick might be useful for a single object file.
On linux'es 3270 emulator x3270 there is a very useful option under the file ment
Save Screens In File this is very good of keeping a copy of traces. 

From CMS help <command name> will give you online help on a particular command. 
e.g. 
HELP DISPLAY

Also CP has a file called profile.exec which automatically gets called
on startup of CMS ( like autoexec.bat ), keeping on a DOS analogy session
CP has a feature similar to doskey, it may be useful for you to
use profile.exec to define some keystrokes. 
e.g.
SET PF9 IMM B
This does a single step in VM on pressing F8. 
SET PF10  ^
This sets up the ^ key.
which can be used for ^c (ctrl-c),^z (ctrl-z) which can't be typed directly into some 3270 consoles.
SET PF11 ^-
This types the starting keystrokes for a sysrq see SysRq below.
SET PF12 RETRIEVE
This retrieves command history on pressing F12.


Sometimes in VM the display is set up to scroll automatically this
can be very annoying if there are messages you wish to look at
to stop this do
TERM MORE 255 255
This will nearly stop automatic screen updates, however it will
cause a denial of service if lots of messages go to the 3270 console,
so it would be foolish to use this as the default on a production machine.
 

Tracing particular processes
----------------------------
The kernel's text segment is intentionally at an address in memory that it will
very seldom collide with text segments of user programs ( thanks Martin ),
this simplifies debugging the kernel.
However it is quite common for user processes to have addresses which collide
this can make debugging a particular process under VM painful under normal
circumstances as the process may change when doing a 
TR I R <address range>.
Thankfully after reading VM's online help I figured out how to debug
I particular process.

Your first problem is to find the STD ( segment table designation )
of the program you wish to debug.
There are several ways you can do this here are a few
1) objdump --syms <program to be debugged> | grep main
To get the address of main in the program.
tr i pswa <address of main>
Start the program, if VM drops to CP on what looks like the entry
point of the main function this is most likely the process you wish to debug.
Now do a D X13 or D XG13 on z/Architecture.
On 31 bit the STD is bits 1-19 ( the STO segment table origin ) 
& 25-31 ( the STL segment table length ) of CR13.
now type
TR I R STD <CR13's value> 0.7fffffff
e.g.
TR I R STD 8F32E1FF 0.7fffffff
Another very useful variation is
TR STORE INTO STD <CR13's value> <address range>
for finding out when a particular variable changes.

An alternative way of finding the STD of a currently running process 
is to do the following, ( this method is more complex but
could be quite convient if you aren't updating the kernel much &
so your kernel structures will stay constant for a reasonable period of
time ).

grep task /proc/<pid>/status
from this you should see something like
task: 0f160000 ksp: 0f161de8 pt_regs: 0f161f68
This now gives you a pointer to the task structure.
Now make CC:="s390-gcc -g" kernel/sched.s
To get the task_struct stabinfo.
( task_struct is defined in include/linux/sched.h ).
Now we want to look at
task->active_mm->pgd
on my machine the active_mm in the task structure stab is
active_mm:(4,12),672,32
its offset is 672/8=84=0x54
the pgd member in the mm_struct stab is
pgd:(4,6)=*(29,5),96,32
so its offset is 96/8=12=0xc

so we'll
hexdump -s 0xf160054 /dev/mem | more
i.e. task_struct+active_mm offset
to look at the active_mm member
f160054 0fee cc60 0019 e334 0000 0000 0000 0011
hexdump -s 0x0feecc6c /dev/mem | more
i.e. active_mm+pgd offset
feecc6c 0f2c 0000 0000 0001 0000 0001 0000 0010
we get something like
now do 
TR I R STD <pgd|0x7f> 0.7fffffff
i.e. the 0x7f is added because the pgd only
gives the page table origin & we need to set the low bits
to the maximum possible segment table length.
TR I R STD 0f2c007f 0.7fffffff
on z/Architecture you'll probably need to do
TR I R STD <pgd|0x7> 0.ffffffffffffffff
to set the TableType to 0x1 & the Table length to 3.



Tracing Program Exceptions
--------------------------
If you get a crash which says something like
illegal operation or specification exception followed by a register dump
You can restart linux & trace these using the tr prog <range or value> trace option.



The most common ones you will normally be tracing for is
1=operation exception
2=privileged operation exception
4=protection exception
5=addressing exception
6=specification exception
10=segment translation exception
11=page translation exception

The full list of these is on page 22 of the current s/390 Reference Summary.
e.g.
tr prog 10 will trace segment translation exceptions.
tr prog on its own will trace all program interruption codes.

Trace Sets
----------
On starting VM you are initially in the INITIAL trace set.
You can do a Q TR to verify this.
If you have a complex tracing situation where you wish to wait for instance 
till a driver is open before you start tracing IO, but know in your
heart that you are going to have to make several runs through the code till you
have a clue whats going on. 

What you can do is
TR I PSWA <Driver open address>
hit b to continue till breakpoint
reach the breakpoint
now do your
TR GOTO B 
TR IO 7c08-7c09 inst int run 
or whatever the IO channels you wish to trace are & hit b

To got back to the initial trace set do
TR GOTO INITIAL
& the TR I PSWA <Driver open address> will be the only active breakpoint again.


Tracing linux syscalls under VM
-------------------------------
Syscalls are implemented on Linux for S390 by the Supervisor call instruction (SVC) there 256 
possibilities of these as the instruction is made up of a  0xA opcode & the second byte being
the syscall number. They are traced using the simple command.
TR SVC  <Optional value or range>
the syscalls are defined in linux/include/asm-s390/unistd.h
e.g. to trace all file opens just do
TR SVC 5 ( as this is the syscall number of open )


SMP Specific commands
---------------------
To find out how many cpus you have
Q CPUS displays all the CPU's available to your virtual machine
To find the cpu that the current cpu VM debugger commands are being directed at do
Q CPU to change the current cpu cpu VM debugger commands are being directed at do
CPU <desired cpu no>

On a SMP guest issue a command to all CPUs try prefixing the command with cpu all.
To issue a command to a particular cpu try cpu <cpu number> e.g.
CPU 01 TR I R 2000.3000
If you are running on a guest with several cpus & you have a IO related problem
& cannot follow the flow of code but you know it isnt smp related.
from the bash prompt issue
shutdown -h now or halt.
do a Q CPUS to find out how many cpus you have
detach each one of them from cp except cpu 0 
by issuing a 
DETACH CPU 01-(number of cpus in configuration)
& boot linux again.
TR SIGP will trace inter processor signal processor instructions.
DEFINE CPU 01-(number in configuration) 
will get your guests cpus back.


Help for displaying ascii textstrings
-------------------------------------
On the very latest VM Nucleus'es VM can now display ascii
( thanks Neale for the hint ) by doing
D TX<lowaddr>.<len>
e.g.
D TX0.100

Alternatively
=============
Under older VM debuggers ( I love EBDIC too ) you can use this little program I wrote which
will convert a command line of hex digits to ascii text which can be compiled under linux & 
you can copy the hex digits from your x3270 terminal to your xterm if you are debugging
from a linuxbox.

This is quite useful when looking at a parameter passed in as a text string
under VM ( unless you are good at decoding ASCII in your head ).

e.g. consider tracing an open syscall
TR SVC 5
We have stopped at a breakpoint
000151B0' SVC   0A05     -> 0001909A'   CC 0

D 20.8 to check the SVC old psw in the prefix area & see was it from userspace
( for the layout of the prefix area consult P18 of the s/390 390 Reference Summary 
if you have it available ).
V00000020  070C2000 800151B2
The problem state bit wasn't set &  it's also too early in the boot sequence
for it to be a userspace SVC if it was we would have to temporarily switch the 
psw to user space addressing so we could get at the first parameter of the open in
gpr2.
Next do a 
D G2
GPR  2 =  00014CB4
Now display what gpr2 is pointing to
D 00014CB4.20
V00014CB4  2F646576 2F636F6E 736F6C65 00001BF5
V00014CC4  FC00014C B4001001 E0001000 B8070707
Now copy the text till the first 00 hex ( which is the end of the string
to an xterm & do hex2ascii on it.
hex2ascii 2F646576 2F636F6E 736F6C65 00 
outputs
Decoded Hex:=/ d e v / c o n s o l e 0x00 
We were opening the console device,

You can compile the code below yourself for practice :-),
/*
 *    hex2ascii.c
 *    a useful little tool for converting a hexadecimal command line to ascii
 *
 *    Author(s): Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com)
 *    (C) 2000 IBM Deutschland Entwicklung GmbH, IBM Corporation.
 */   
#include <stdio.h>

int main(int argc,char *argv[])
{
  int cnt1,cnt2,len,toggle=0;
  int startcnt=1;
  unsigned char c,hex;
  
  if(argc>1&&(strcmp(argv[1],"-a")==0))
     startcnt=2;
  printf("Decoded Hex:=");
  for(cnt1=startcnt;cnt1<argc;cnt1++)
  {
    len=strlen(argv[cnt1]);
    for(cnt2=0;cnt2<len;cnt2++)
    {
       c=argv[cnt1][cnt2];
       if(c>='0'&&c<='9')
	  c=c-'0';
       if(c>='A'&&c<='F')
	  c=c-'A'+10;
       if(c>='a'&&c<='f')
	  c=c-'a'+10;
       switch(toggle)
       {
	  case 0:
	     hex=c<<4;
	     toggle=1;
	  break;
	  case 1:
	     hex+=c;
	     if(hex<32||hex>127)
	     {
		if(startcnt==1)
		   printf("0x%02X ",(int)hex);
		else
		   printf(".");
	     }
	     else
	     {
	       printf("%c",hex);
	       if(startcnt==1)
		  printf(" ");
	     }
	     toggle=0;
	  break;
       }
    }
  }
  printf("\n");
}




Stack tracing under VM
----------------------
A basic backtrace
-----------------

Here are the tricks I use 9 out of 10 times it works pretty well,

When your backchain reaches a dead end
--------------------------------------
This can happen when an exception happens in the kernel & the kernel is entered twice
if you reach the NULL pointer at the end of the back chain you should be
able to sniff further back if you follow the following tricks.
1) A kernel address should be easy to recognise since it is in
primary space & the problem state bit isn't set & also
The Hi bit of the address is set.
2) Another backchain should also be easy to recognise since it is an 
address pointing to another address approximately 100 bytes or 0x70 hex
behind the current stackpointer.


Here is some practice.
boot the kernel & hit PA1 at some random time
d g to display the gprs, this should display something like
GPR  0 =  00000001  00156018  0014359C  00000000
GPR  4 =  00000001  001B8888  000003E0  00000000
GPR  8 =  00100080  00100084  00000000  000FE000
GPR 12 =  00010400  8001B2DC  8001B36A  000FFED8
Note that GPR14 is a return address but as we are real men we are going to
trace the stack.
display 0x40 bytes after the stack pointer.

V000FFED8  000FFF38 8001B838 80014C8E 000FFF38
V000FFEE8  00000000 00000000 000003E0 00000000
V000FFEF8  00100080 00100084 00000000 000FE000
V000FFF08  00010400 8001B2DC 8001B36A 000FFED8


Ah now look at whats in sp+56 (sp+0x38) this is 8001B36A our saved r14 if
you look above at our stackframe & also agrees with GPR14.

now backchain 
d 000FFF38.40
we now are taking the contents of SP to get our first backchain.

V000FFF38  000FFFA0 00000000 00014995 00147094
V000FFF48  00147090 001470A0 000003E0 00000000
V000FFF58  00100080 00100084 00000000 001BF1D0
V000FFF68  00010400 800149BA 80014CA6 000FFF38

This displays a 2nd return address of 80014CA6

now do d 000FFFA0.40 for our 3rd backchain

V000FFFA0  04B52002 0001107F 00000000 00000000
V000FFFB0  00000000 00000000 FF000000 0001107F
V000FFFC0  00000000 00000000 00000000 00000000
V000FFFD0  00010400 80010802 8001085A 000FFFA0


our 3rd return address is 8001085A

as the 04B52002 looks suspiciously like rubbish it is fair to assume that the kernel entry routines
for the sake of optimisation dont set up a backchain.

now look at System.map to see if the addresses make any sense.

grep -i 0001b3 System.map
outputs among other things
0001b304 T cpu_idle 
so 8001B36A
is cpu_idle+0x66 ( quiet the cpu is asleep, don't wake it )


grep -i 00014 System.map 
produces among other things
00014a78 T start_kernel  
so 0014CA6 is start_kernel+some hex number I can't add in my head.

grep -i 00108 System.map 
this produces
00010800 T _stext
so   8001085A is _stext+0x5a

Congrats you've done your first backchain.



s/390 & z/Architecture IO Overview
==================================

I am not going to give a course in 390 IO architecture as this would take me quite a
while & I'm no expert. Instead I'll give a 390 IO architecture summary for Dummies if you have 
the s/390 principles of operation available read this instead. If nothing else you may find a few 
useful keywords in here & be able to use them on a web search engine like altavista to find 
more useful information.

Unlike other bus architectures modern 390 systems do their IO using mostly
fibre optics & devices such as tapes & disks can be shared between several mainframes,
also S390 can support upto 65536 devices while a high end PC based system might be choking 
with around 64. Here is some of the common IO terminology

Subchannel:
This is the logical number most IO commands use to talk to an IO device there can be upto
0x10000 (65536) of these in a configuration typically there is a few hundred. Under VM
for simplicity they are allocated contiguously, however on the native hardware they are not
they typically stay consistent between boots provided no new hardware is inserted or removed.
Under Linux for 390 we use these as IRQ's & also when issuing an IO command (CLEAR SUBCHANNEL,
HALT SUBCHANNEL,MODIFY SUBCHANNEL,RESUME SUBCHANNEL,START SUBCHANNEL,STORE SUBCHANNEL & 
TEST SUBCHANNEL ) we use this as the ID of the device we wish to talk to, the most
important of these instructions are START SUBCHANNEL ( to start IO ), TEST SUBCHANNEL ( to check
whether the IO completed successfully ), & HALT SUBCHANNEL ( to kill IO ), a subchannel
can have up to 8 channel paths to a device this offers redunancy if one is not available.


Device Number:
This number remains static & Is closely tied to the hardware, there are 65536 of these
also they are made up of a CHPID ( Channel Path ID, the most significant 8 bits ) 
& another lsb 8 bits. These remain static even if more devices are inserted or removed
from the hardware, there is a 1 to 1 mapping between Subchannels & Device Numbers provided
devices arent inserted or removed.

Channel Control Words:
CCWS are linked lists of instructions initially pointed to by an operation request block (ORB),
which is initially given to Start Subchannel (SSCH) command along with the subchannel number
for the IO subsystem to process while the CPU continues executing normal code.
These come in two flavours, Format 0 ( 24 bit for backward )
compatibility & Format 1 ( 31 bit ). These are typically used to issue read & write 
( & many other instructions ) they consist of a length field & an absolute address field.
For each IO typically get 1 or 2 interrupts one for channel end ( primary status ) when the
channel is idle & the second for device end ( secondary status ) sometimes you get both
concurrently, you check how the IO went on by issuing a TEST SUBCHANNEL at each interrupt,
from which you receive an Interruption response block (IRB). If you get channel & device end 
status in the IRB without channel checks etc. your IO probably went okay. If you didn't you
probably need a doctorto examine the IRB & extended status word etc.
If an error occurs more sophistocated control units have a facitity known as
concurrent sense this means that if an error occurs Extended sense information will
be presented in the Extended status word in the IRB if not you have to issue a
subsequent SENSE CCW command after the test subchannel. 


TPI( Test pending interrupt) can also be used for polled IO but in multitasking multiprocessor
systems it isn't recommended except for checking special cases ( i.e. non looping checks for
pending IO etc. ).

Store Subchannel & Modify Subchannel can be used to examine & modify operating characteristics
of a subchannel ( e.g. channel paths ).

Other IO related Terms:
Sysplex: S390's Clustering Technology
QDIO: S390's new high speed IO architecture to support devices such as gigabit ethernet,
this architecture is also designed to be forward compatible with up & coming 64 bit machines.


General Concepts 

Input Output Processors (IOP's) are responsible for communicating between
the mainframe CPU's & the channel & relieve the mainframe CPU's from the
burden of communicating with IO devices directly, this allows the CPU's to 
concentrate on data processing. 

IOP's can use one or more links ( known as channel paths ) to talk to each 
IO device. It first checks for path availability & chooses an available one,
then starts ( & sometimes terminates IO ).
There are two types of channel path ESCON & the Paralell IO interface.

IO devices are attached to control units, control units provide the
logic to interface the channel paths & channel path IO protocols to 
the IO devices, they can be integrated with the devices or housed separately
& often talk to several similar devices ( typical examples would be raid 
controllers or a control unit which connects to 1000 3270 terminals ).


    +---------------------------------------------------------------+
    | +-----+ +-----+ +-----+ +-----+  +----------+  +----------+   |
    | | CPU | | CPU | | CPU | | CPU |  |  Main    |  | Expanded |   |
    | |     | |     | |     | |     |  |  Memory  |  |  Storage |   |
    | +-----+ +-----+ +-----+ +-----+  +----------+  +----------+   | 
    |---------------------------------------------------------------+
    |   IOP        |      IOP      |       IOP                      |
    |---------------------------------------------------------------
    | C | C | C | C | C | C | C | C | C | C | C | C | C | C | C | C | 
    ----------------------------------------------------------------
         ||                                              ||
         ||  Bus & Tag Channel Path                      || ESCON
         ||  ======================                      || Channel
         ||  ||                  ||                      || Path
    +----------+               +----------+         +----------+
    |          |               |          |         |          |
    |    CU    |               |    CU    |         |    CU    |
    |          |               |          |         |          |
    +----------+               +----------+         +----------+
       |      |                     |                |       |
+----------+ +----------+      +----------+   +----------+ +----------+
|I/O Device| |I/O Device|      |I/O Device|   |I/O Device| |I/O Device|
+----------+ +----------+      +----------+   +----------+ +----------+
  CPU = Central Processing Unit    
  C = Channel                      
  IOP = IP Processor               
  CU = Control Unit

The 390 IO systems come in 2 flavours the current 390 machines support both

The Older 360 & 370 Interface,sometimes called the paralell I/O interface,
sometimes called Bus-and Tag & sometimes Original Equipment Manufacturers
Interface (OEMI).

This byte wide paralell channel path/bus has parity & data on the "Bus" cable 
& control lines on the "Tag" cable. These can operate in byte multiplex mode for
sharing between several slow devices or burst mode & monopolize the channel for the
whole burst. Upto 256 devices can be addressed  on one of these cables. These cables are
about one inch in diameter. The maximum unextended length supported by these cables is
125 Meters but this can be extended up to 2km with a fibre optic channel extended 
such as a 3044. The maximum burst speed supported is 4.5 megabytes per second however
some really old processors support only transfer rates of 3.0, 2.0 & 1.0 MB/sec.
One of these paths can be daisy chained to up to 8 control units.


ESCON if fibre optic it is also called FICON 
Was introduced by IBM in 1990. Has 2 fibre optic cables & uses either leds or lasers
for communication at a signaling rate of upto 200 megabits/sec. As 10bits are transferred
for every 8 bits info this drops to 160 megabits/sec & to 18.6 Megabytes/sec once
control info & CRC are added. ESCON only operates in burst mode.
 
ESCONs typical max cable length is 3km for the led version & 20km for the laser version
known as XDF ( extended distance facility ). This can be further extended by using an
ESCON director which triples the above mentioned ranges. Unlike Bus & Tag as ESCON is
serial it uses a packet switching architecture the standard Bus & Tag control protocol
is however present within the packets. Upto 256 devices can be attached to each control 
unit that uses one of these interfaces.

Common 390 Devices include:
Network adapters typically OSA2,3172's,2116's & OSA-E gigabit ethernet adapters,
Consoles 3270 & 3215 ( a teletype emulated under linux for a line mode console ).
DASD's direct access storage devices ( otherwise known as hard disks ).
Tape Drives.
CTC ( Channel to Channel Adapters ),
ESCON or Paralell Cables used as a very high speed serial link
between 2 machines. We use 2 cables under linux to do a bi-directional serial link.


Debugging IO on s/390 & z/Architecture under VM
===============================================

Now we are ready to go on with IO tracing commands under VM

A few self explanatory queries:
Q OSA
Q CTC
Q DISK ( This command is CMS specific )
Q DASD






Q OSA on my machine returns
OSA  7C08 ON OSA   7C08 SUBCHANNEL = 0000
OSA  7C09 ON OSA   7C09 SUBCHANNEL = 0001
OSA  7C14 ON OSA   7C14 SUBCHANNEL = 0002
OSA  7C15 ON OSA   7C15 SUBCHANNEL = 0003

If you have a guest with certain priviliges you may be able to see devices
which don't belong to you to avoid this do add the option V.
e.g.
Q V OSA

Now using the device numbers returned by this command we will
Trace the io starting up on the first device 7c08 & 7c09
In our simplest case we can trace the 
start subchannels
like TR SSCH 7C08-7C09
or the halt subchannels
or TR HSCH 7C08-7C09
MSCH's ,STSCH's I think you can guess the rest

Ingo's favourite trick is tracing all the IO's & CCWS & spooling them into the reader of another
VM guest so he can ftp the logfile back to his own machine.I'll do a small bit of this & give you
 a look at the output.

1) Spool stdout to VM reader
SP PRT TO (another vm guest ) or * for the local vm guest
2) Fill the reader with the trace
TR IO 7c08-7c09 INST INT CCW PRT RUN
3) Start up linux 
i 00c  
4) Finish the trace
TR END
5) close the reader
C PRT
6) list reader contents
RDRLIST
7) copy it to linux4's minidisk 
RECEIVE / LOG TXT A1 ( replace
8)
filel & press F11 to look at it
You should see someting like.

00020942' SSCH  B2334000    0048813C    CC 0    SCH 0000    DEV 7C08
          CPA 000FFDF0   PARM 00E2C9C4    KEY 0  FPI C0  LPM 80
          CCW    000FFDF0  E4200100 00487FE8   0000  E4240100 ........
          IDAL                                      43D8AFE8
          IDAL                                      0FB76000
00020B0A'   I/O DEV 7C08 -> 000197BC'   SCH 0000   PARM 00E2C9C4
00021628' TSCH  B2354000 >> 00488164    CC 0    SCH 0000    DEV 7C08
          CCWA 000FFDF8   DEV STS 0C  SCH STS 00  CNT 00EC
           KEY 0   FPI C0  CC 0   CTLS 4007
00022238' STSCH B2344000 >> 00488108    CC 0    SCH 0000    DEV 7C08

If you don't like messing up your readed ( because you possibly booted from it )
you can alternatively spool it to another readers guest.


Other common VM device related commands
---------------------------------------------
These commands are listed only because they have
been of use to me in the past & may be of use to
you too. For more complete info on each of the commands
use type HELP <command> from CMS.
detaching devices
DET <devno range>
ATT <devno range> <guest> 
attach a device to guest * for your own guest
READY <devno> cause VM to issue a fake interrupt.

The VARY command is normally only available to VM administrators.
VARY ON PATH <path> TO <devno range>
VARY OFF PATH <PATH> FROM <devno range>
This is used to switch on or off channel paths to devices.

Q CHPID <channel path ID>
This displays state of devices using this channel path
D SCHIB <subchannel>
This displays the subchannel information SCHIB block for the device.
this I believe is also only available to administrators.
DEFINE CTC <devno>
defines a virtual CTC channel to channel connection
2 need to be defined on each guest for the CTC driver to use.
COUPLE  devno userid remote devno
Joins a local virtual device to a remote virtual device
( commonly used for the CTC driver ).

Building a VM ramdisk under CMS which linux can use
def vfb-<blocksize> <subchannel> <number blocks>
blocksize is commonly 4096 for linux.
Formatting it
format <subchannel> <driver letter e.g. x> (blksize <blocksize>

Sharing a disk between multiple guests
LINK userid devno1 devno2 mode password



GDB on S390
===========
N.B. if compiling for debugging gdb works better without optimisation 
( see Compiling programs for debugging )

invocation
----------
gdb <victim program> <optional corefile>

Online help
-----------
help: gives help on commands
e.g.
help
help display
Note gdb's online help is very good use it.


Assembly
--------
info registers: displays registers other than floating point.
info all-registers: displays floating points as well.
disassemble: dissassembles
e.g.
disassemble without parameters will disassemble the current function
disassemble $pc $pc+10 

Viewing & modifying variables
-----------------------------
print or p: displays variable or register
e.g. p/x $sp will display the stack pointer

display: prints variable or register each time program stops
e.g.
display/x $pc will display the program counter
display argc

undisplay : undo's display's

info breakpoints: shows all current breakpoints

info stack: shows stack back trace ( if this dosent work too well, I'll show you the
stacktrace by hand below ).

info locals: displays local variables.

info args: display current procedure arguments.

set args: will set argc & argv each time the victim program is invoked.

set <variable>=value
set argc=100
set $pc=0



Modifying execution
-------------------
step: steps n lines of sourcecode
step steps 1 line.
step 100 steps 100 lines of code.

next: like step except this will not step into subroutines

stepi: steps a single machine code instruction.
e.g. stepi 100

nexti: steps a single machine code instruction but will not step into subroutines.

finish: will run until exit of the current routine

run: (re)starts a program

cont: continues a program

quit: exits gdb.


breakpoints
------------

break
sets a breakpoint
e.g.

break main

break *$pc

break *0x400618

heres a really useful one for large programs
rbr
Set a breakpoint for all functions matching REGEXP
e.g.
rbr 390
will set a breakpoint with all functions with 390 in their name.

info breakpoints
lists all breakpoints

delete: delete breakpoint by number or delete them all
e.g.
delete 1 will delete the first breakpoint
delete will delete them all

watch: This will set a watchpoint ( usually hardware assisted ),
This will watch a variable till it changes
e.g.
watch cnt, will watch the variable cnt till it changes.
As an aside unfortunately gdb's, architecture independent watchpoint code
is inconsistent & not very good, watchpoints usually work but not always.

info watchpoints: Display currently active watchpoints

condition: ( another useful one )
Specify breakpoint number N to break only if COND is true.
Usage is `condition N COND', where N is an integer and COND is an
expression to be evaluated whenever breakpoint N is reached.



User defined functions/macros
-----------------------------
define: ( Note this is very very useful,simple & powerful )
usage define <name> <list of commands> end

examples which you should consider putting into .gdbinit in your home directory
define d
stepi
disassemble $pc $pc+10
end

define e
nexti
disassemble $pc $pc+10
end


Other hard to classify stuff
----------------------------
signal n:
sends the victim program a signal.
e.g. signal 3 will send a SIGQUIT.

info signals:
what gdb does when the victim receives certain signals.

list:
e.g.
list lists current function source
list 1,10 list first 10 lines of curret file.
list test.c:1,10


directory:
Adds directories to be searched for source if gdb cannot find the source.
(note it is a bit sensititive about slashes ) 
e.g. To add the root of the filesystem to the searchpath do
directory //


call <function>
This calls a function in the victim program, this is pretty powerful
e.g.
(gdb) call printf("hello world")
outputs:
$1 = 11 

You might now be thinking that the line above didn't work, something extra had to be done.
(gdb) call fflush(stdout)
hello world$2 = 0
As an aside the debugger also calls malloc & free under the hood 
to make space for the "hello world" string.



hints
-----
1) command completion works just like bash 
( if you are a bad typist like me this really helps )
e.g. hit br <TAB> & cursor up & down :-).

2) if you have a debugging problem that takes a few steps to recreate
put the steps into a file called .gdbinit in your current working directory
if you have defined a few extra useful user defined commands put these in 
your home directory & they will be read each time gdb is launched.

A typical .gdbinit file might be.
break main
run
break runtime_exception
cont 


stack chaining in gdb by hand
-----------------------------
This is done using a the same trick described for VM 
p/x (*($sp+56))&0x7fffffff get the first backchain.

For z/Architecture
Replace 56 with 112 & ignore the &0x7fffffff
in the macros below & do nasty casts to longs like the following
as gdb unfortunately deals with printed arguments as ints which
messes up everything.
i.e. here is a 3rd backchain dereference
p/x *(long *)(***(long ***)$sp+112)


this outputs 
$5 = 0x528f18 
on my machine.
Now you can use 
info symbol (*($sp+56))&0x7fffffff 
you might see something like.
rl_getc + 36 in section .text  telling you what is located at address 0x528f18
Now do.
p/x (*(*$sp+56))&0x7fffffff 
This outputs
$6 = 0x528ed0
Now do.
info symbol (*(*$sp+56))&0x7fffffff
rl_read_key + 180 in section .text
now do
p/x (*(**$sp+56))&0x7fffffff
& so on.

Disassembling instructions without debug info
---------------------------------------------
gdb typically compains if there is a lack of debugging
symbols in  the disassemble command with 
"No function contains specified address." to get around
this do 
x/<number lines to disassemble>xi <address>
e.g.
x/20xi 0x400730



Note: Remember gdb has history just like bash you don't need to retype the
whole line just use the up & down arrows.



For more info
-------------
From your linuxbox do 
man gdb or info gdb.

core dumps
----------
What a core dump ?,
A core dump is a file generated by the kernel ( if allowed ) which contains the registers,
& all active pages of the program which has crashed.
From this file gdb will allow you to look at the registers & stack trace & memory of the
program as if it just crashed on your system, it is usually called core & created in the
current working directory.
This is very useful in that a customer can mail a core dump to a technical support department
& the technical support department can reconstruct what happened.
Provided the have an identical copy of this program with debugging symbols compiled in & 
the source base of this build is available.
In short it is far more useful than something like a crash log could ever hope to be.

In theory all that is missing to restart a core dumped program is a kernel patch which
will do the following.
1) Make a new kernel task structure
2) Reload all the dumped pages back into the kernel's memory management structures.
3) Do the required clock fixups
4) Get all files & network connections for the process back into an identical state ( really difficult ).
5) A few more difficult things I haven't thought of.



Why have I never seen one ?.
Probably because you haven't used the command 
ulimit -c unlimited in bash
to allow core dumps, now do 
ulimit -a 
to verify that the limit was accepted.

A sample core dump
To create this I'm going to do
ulimit -c unlimited
gdb 
to launch gdb (my victim app. ) now be bad & do the following from another 
telnet/xterm session to the same machine
ps -aux | grep gdb
kill -SIGSEGV <gdb's pid>
or alternatively use killall -SIGSEGV gdb if you have the killall command.
Now look at the core dump.
./gdb ./gdb core
Displays the following
GNU gdb 4.18
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "s390-ibm-linux"...
Core was generated by `./gdb'.
Program terminated with signal 11, Segmentation fault.
Reading symbols from /usr/lib/libncurses.so.4...done.
Reading symbols from /lib/libm.so.6...done.
Reading symbols from /lib/libc.so.6...done.
Reading symbols from /lib/ld-linux.so.2...done.
#0  0x40126d1a in read () from /lib/libc.so.6
Setting up the environment for debugging gdb.
Breakpoint 1 at 0x4dc6f8: file utils.c, line 471.
Breakpoint 2 at 0x4d87a4: file top.c, line 2609.
(top-gdb) info stack
#0  0x40126d1a in read () from /lib/libc.so.6
#1  0x528f26 in rl_getc (stream=0x7ffffde8) at input.c:402
#2  0x528ed0 in rl_read_key () at input.c:381
#3  0x5167e6 in readline_internal_char () at readline.c:454
#4  0x5168ee in readline_internal_charloop () at readline.c:507
#5  0x51692c in readline_internal () at readline.c:521
#6  0x5164fe in readline (prompt=0x7ffff810 "\177����x\177������\177����x��")
    at readline.c:349
#7  0x4d7a8a in command_line_input (prrompt=0x564420 "(gdb) ", repeat=1,
    annotation_suffix=0x4d6b44 "prompt") at top.c:2091
#8  0x4d6cf0 in command_loop () at top.c:1345
#9  0x4e25bc in main (argc=1, argv=0x7ffffdf4) at main.c:635


LDD
===
This is a program which lists the shared libraries which a library needs,
Note you also get the relocations of the shared library text segments which
help when using objdump --source.
e.g.
 ldd ./gdb
outputs
libncurses.so.4 => /usr/lib/libncurses.so.4 (0x40018000)
libm.so.6 => /lib/libm.so.6 (0x4005e000)
libc.so.6 => /lib/libc.so.6 (0x40084000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)


Debugging shared libraries
==========================
Most programs use shared libraries, however it can be very painful
when you single step instruction into a function like printf for the 
first time & you end up in functions like _dl_runtime_resolve this is
the ld.so doing lazy binding, lazy binding is a concept in ELF where 
shared library functions are not loaded into memory unless they are 
actually used, great for saving memory but a pain to debug.
To get around this either relink the program -static or exit gdb type 
export LD_BIND_NOW=true this will stop lazy binding & restart the gdb'ing 
the program in question.
 


Debugging modules
=================
As modules are dynamically loaded into the kernel their address can be
anywhere to get around this use the -m option with insmod to emit a load
map which can be piped into a file if required.

The proc file system
====================
What is it ?.
It is a filesystem created by the kernel with files which are created on demand
by the kernel if read, or can be used to modify kernel parameters,
it is a powerful concept.

e.g.

cat /proc/sys/net/ipv4/ip_forward 
On my machine outputs 
0 
telling me ip_forwarding is not on to switch it on I can do
echo 1 >  /proc/sys/net/ipv4/ip_forward
cat it again
cat /proc/sys/net/ipv4/ip_forward 
On my machine now outputs
1
IP forwarding is on.
There is a lot of useful info in here best found by going in & having a look around,
so I'll take you through some entries I consider important.

All the processes running on the machine have there own entry defined by
/proc/<pid>
So lets have a look at the init process
cd /proc/1

cat cmdline
emits
init [2]

cd /proc/1/fd
This contains numerical entries of all the open files,
some of these you can cat e.g. stdout (2)

cat /proc/29/maps
on my machine emits

00400000-00478000 r-xp 00000000 5f:00 4103       /bin/bash
00478000-0047e000 rw-p 00077000 5f:00 4103       /bin/bash
0047e000-00492000 rwxp 00000000 00:00 0
40000000-40015000 r-xp 00000000 5f:00 14382      /lib/ld-2.1.2.so
40015000-40016000 rw-p 00014000 5f:00 14382      /lib/ld-2.1.2.so
40016000-40017000 rwxp 00000000 00:00 0
40017000-40018000 rw-p 00000000 00:00 0
40018000-4001b000 r-xp 00000000 5f:00 14435      /lib/libtermcap.so.2.0.8
4001b000-4001c000 rw-p 00002000 5f:00 14435      /lib/libtermcap.so.2.0.8
4001c000-4010d000 r-xp 00000000 5f:00 14387      /lib/libc-2.1.2.so
4010d000-40111000 rw-p 000f0000 5f:00 14387      /lib/libc-2.1.2.so
40111000-40114000 rw-p 00000000 00:00 0
40114000-4011e000 r-xp 00000000 5f:00 14408      /lib/libnss_files-2.1.2.so
4011e000-4011f000 rw-p 00009000 5f:00 14408      /lib/libnss_files-2.1.2.so
7fffd000-80000000 rwxp ffffe000 00:00 0


Showing us the shared libraries init uses where they are in memory
& memory access permissions for each virtual memory area.

/proc/1/cwd is a softlink to the current working directory.
/proc/1/root is the root of the filesystem for this process. 

/proc/1/mem is the current running processes memory which you
can read & write to like a file.
strace uses this sometimes as it is a bit faster than the
rather inefficent ptrace interface for peeking at DATA.


cat status 

Name:   init
State:  S (sleeping)
Pid:    1
PPid:   0
Uid:    0       0       0       0
Gid:    0       0       0       0
Groups:
VmSize:      408 kB
VmLck:         0 kB
VmRSS:       208 kB
VmData:       24 kB
VmStk:         8 kB
VmExe:       368 kB
VmLib:         0 kB
SigPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 7fffffffd7f0d8fc
SigCgt: 00000000280b2603
CapInh: 00000000fffffeff
CapPrm: 00000000ffffffff
CapEff: 00000000fffffeff

User PSW:    070de000 80414146
task: 004b6000 tss: 004b62d8 ksp: 004b7ca8 pt_regs: 004b7f68
User GPRS:
00000400  00000000  0000000b  7ffffa90
00000000  00000000  00000000  0045d9f4
0045cafc  7ffffa90  7fffff18  0045cb08
00010400  804039e8  80403af8  7ffff8b0
User ACRS:
00000000  00000000  00000000  00000000
00000001  00000000  00000000  00000000
00000000  00000000  00000000  00000000
00000000  00000000  00000000  00000000
Kernel BackChain  CallChain    BackChain  CallChain
       004b7ca8   8002bd0c     004b7d18   8002b92c
       004b7db8   8005cd50     004b7e38   8005d12a
       004b7f08   80019114                     
Showing among other things memory usage & status of some signals &
the processes'es registers from the kernel task_structure
as well as a backchain which may be useful if a process crashes
in the kernel for some unknown reason.

Some driver debugging techniques
================================
debug feature
-------------
Some of our drivers now support a "debug feature" in
/proc/s390dbf see s390dbf.txt in the linux/Documentation directory
for more info.
e.g. 
to switch on the lcs "debug feature"
echo 5 > /proc/s390dbf/lcs/level
& then after the error occurred.
cat /proc/s390dbf/lcs/sprintf >/logfile
the logfile now contains some information which may help
tech support resolve a problem in the field.



high level debugging network drivers
------------------------------------
ifconfig is a quite useful command
it gives the current state of network drivers.

If you suspect your network device driver is dead
one way to check is type 
ifconfig <network device> 
e.g. tr0
You should see something like
tr0       Link encap:16/4 Mbps Token Ring (New)  HWaddr 00:04:AC:20:8E:48
          inet addr:9.164.185.132  Bcast:9.164.191.255  Mask:255.255.224.0
          UP BROADCAST RUNNING MULTICAST  MTU:2000  Metric:1
          RX packets:246134 errors:0 dropped:0 overruns:0 frame:0
          TX packets:5 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100

if the device doesn't say up
try
/etc/rc.d/init.d/network start 
( this starts the network stack & hopefully calls ifconfig tr0 up ).
ifconfig looks at the output of /proc/net/dev & presents it in a more presentable form
Now ping the device from a machine in the same subnet.
if the RX packets count & TX packets counts don't increment you probably
have problems.
next 
cat /proc/net/arp
Do you see any hardware addresses in the cache if not you may have problems.
Next try
ping -c 5 <broadcast_addr> i.e. the Bcast field above in the output of
ifconfig. Do you see any replies from machines other than the local machine
if not you may have problems. also if the TX packets count in ifconfig
hasn't incremented either you have serious problems in your driver 
(e.g. the txbusy field of the network device being stuck on ) 
or you may have multiple network devices connected.


chandev
-------
There is a new device layer for channel devices, some
drivers e.g. lcs are registered with this layer.
If the device uses the channel device layer you'll be
able to find what interrupts it uses & the current state 
of the device.
See the manpage chandev.8 &type cat /proc/chandev for more info.



Starting points for debugging scripting languages etc.
======================================================

bash/sh

bash -x <scriptname>
e.g. bash -x /usr/bin/bashbug
displays the following lines as it executes them.
+ MACHINE=i586
+ OS=linux-gnu
+ CC=gcc
+ CFLAGS= -DPROGRAM='bash' -DHOSTTYPE='i586' -DOSTYPE='linux-gnu' -DMACHTYPE='i586-pc-linux-gnu' -DSHELL -DHAVE_CONFIG_H   -I. -I. -I./lib -O2 -pipe
+ RELEASE=2.01
+ PATCHLEVEL=1
+ RELSTATUS=release
+ MACHTYPE=i586-pc-linux-gnu   

perl -d <scriptname> runs the perlscript in a fully intercative debugger
<like gdb>.
Type 'h' in the debugger for help.

for debugging java type
jdb <filename> another fully interactive gdb style debugger.
& type ? in the debugger for help.



Dumptool & Lcrash ( lkcd )
==========================
Michael Holzheu & others here at IBM have a fairly mature port of 
SGI's lcrash tool which allows one to look at kernel structures in a
running kernel.

It also complements a tool called dumptool which dumps all the kernel's
memory pages & registers to either a tape or a disk.
This can be used by tech support or an ambitious end user do
post mortem debugging of a machine like gdb core dumps.

Going into how to use this tool in detail will be explained
in other documentation supplied by IBM with the patches & the 
lcrash homepage http://oss.sgi.com/projects/lkcd/ & the lcrash manpage.

How they work
-------------
Lcrash is a perfectly normal program,however, it requires 2 
additional files, Kerntypes which is built using a patch to the 
linux kernel sources in the linux root directory & the System.map.

Kerntypes is an an objectfile whose sole purpose in life
is to provide stabs debug info to lcrash, to do this
Kerntypes is built from kerntypes.c which just includes the most commonly
referenced header files used when debugging, lcrash can then read the
.stabs section of this file.

Debugging a live system it uses /dev/mem
alternatively for post mortem debugging it uses the data 
collected by dumptool.



SysRq
=====
This is now supported by linux for s/390 & z/Architecture.
To enable it do compile the kernel with 
Kernel Hacking -> Magic SysRq Key Enabled
echo "1" > /proc/sys/kernel/sysrq
also type
echo "8" >/proc/sys/kernel/printk
To make printk output go to console.
On 390 all commands are prefixed with
^-
e.g.
^-t will show tasks.
^-? or some unknown command will display help.
The sysrq key reading is very picky ( I have to type the keys in an
 xterm session & paste them  into the x3270 console )
& it may be wise to predefine the keys as described in the VM hints above

This is particularly useful for syncing disks unmounting & rebooting
if the machine gets partially hung.

Read Documentation/sysrq.txt for more info

References:
===========
Enterprise Systems Architecture Reference Summary
Enterprise Systems Architecture Principles of Operation
Hartmut Penners s390 stack frame sheet.
IBM Mainframe Channel Attachment a technology brief from a CISCO webpage
Various bits of man & info pages of Linux.
Linux & GDB source.
Various info & man pages.
CMS Help on tracing commands.
Linux for s/390 Elf Application Binary Interface
Linux for z/Series Elf Application Binary Interface ( Both Highly Recommended )
z/Architecture Principles of Operation SA22-7832-00
Enterprise Systems Architecture/390 Reference Summary SA22-7209-01 & the
Enterprise Systems Architecture/390 Principles of Operation SA22-7201-05

Special Thanks
==============
Special thanks to Neale Ferguson who maintains a much
prettier HTML version of this page at
http://penguinvm.princeton.edu/notes.html#Debug390
Bob Grainger Stefan Bader & others for reporting bugs
OpenPOWER on IntegriCloud