Our mmap(2) has a limitation where the `offset' parameter must be

page-aligned.  cmp(1) should know about this flaw, and work around it.

While i was at it, fixed an uninitialized variable as reported by
-Wall.

1 files changed, 12 insertions, 2 deletions

diff --git a/usr.bin/cmp/regular.c b/usr.bin/cmp/regular.c
index 76f2387..5d301fa 100644
--- a/usr.bin/cmp/regular.c
+++ b/usr.bin/cmp/regular.c
@@ -44,9 +44,12 @@ static char sccsid[] = "@(#)regular.c	8.3 (Berkeley) 4/2/94";
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
+#include <unistd.h>
 
 #include "extern.h"
 
+#define ROUNDPAGE(i) ((i) & ~pagemask)
+
 void
 c_regular(fd1, file1, skip1, len1, fd2, file2, skip2, len2)
 	int fd1, fd2;
@@ -56,6 +59,7 @@ c_regular(fd1, file1, skip1, len1, fd2, file2, skip2, len2)
 	u_char ch, *p1, *p2;
 	off_t byte, length, line;
 	int dfound;
+	off_t pagemask, off1, off2;
 
 	if (sflag && len1 != len2)
 		exit(1);
@@ -67,21 +71,27 @@ c_regular(fd1, file1, skip1, len1, fd2, file2, skip2, len2)
 		eofmsg(file2);
 	len2 -= skip2;
 
+	pagemask = (off_t)getpagesize() - 1;
+	off1 = ROUNDPAGE(skip1);
+	off2 = ROUNDPAGE(skip2);
+
 	length = MIN(len1, len2);
 	if (length > SIZE_T_MAX)
 		return (c_special(fd1, file1, skip1, fd2, file2, skip2));
 
 	if ((p1 = (u_char *)mmap(NULL,
-	    (size_t)length, PROT_READ, 0, fd1, skip1)) == (u_char *)-1)
+	    (size_t)length, PROT_READ, 0, fd1, off1)) == (u_char *)-1)
 		err(ERR_EXIT, "%s", file1);
 
 	madvise(p1, length, MADV_SEQUENTIAL);
 	if ((p2 = (u_char *)mmap(NULL,
-	    (size_t)length, PROT_READ, 0, fd2, skip2)) == (u_char *)-1)
+	    (size_t)length, PROT_READ, 0, fd2, off2)) == (u_char *)-1)
 		err(ERR_EXIT, "%s", file2);
 	madvise(p2, length, MADV_SEQUENTIAL);
 
 	dfound = 0;
+	p1 += skip1 - off1;
+	p2 += skip2 - off2;
 	for (byte = line = 1; length--; ++p1, ++p2, ++byte) {
 		if ((ch = *p1) != *p2)
 			if (lflag) {