(dd_out): Fix handling of all-zeroes block at end of input with

conv=sparse.

This change fixes two separate issues observed when the last output
block is all zeroes, and conv=sparse is in use. In this case, care
must be taken to roll back the last seek and write the entire last zero
block at the original offset where it should have occurred: when the
destination file is a block device, it is not possible to roll back
by just one character as the write would then not be properly aligned.

Furthermore, the buffer used to write this last all-zeroes block
needs to be properly zeroed-out. This was not the case previously,
resulting in a junk data byte appearing instead of a zero in the
output stream.

PR:		bin/189174
PR:		bin/189284
Reviewed by:	kib
MFC after:	2 weeks

1 files changed, 43 insertions, 24 deletions

diff --git a/bin/dd/dd.c b/bin/dd/dd.c
index 8048b6a..6b4b4f0 100644
--- a/bin/dd/dd.c
+++ b/bin/dd/dd.c
@@ -51,6 +51,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/disklabel.h>
 #include <sys/filio.h>
 
+#include <assert.h>
 #include <ctype.h>
 #include <err.h>
 #include <errno.h>
@@ -77,6 +78,7 @@ STAT	st;			/* statistics */
 void	(*cfunc)(void);		/* conversion function */
 uintmax_t cpy_cnt;		/* # of blocks to copy */
 static off_t	pending = 0;	/* pending seek if sparse */
+static off_t	last_sp = 0;	/* size of last added sparse block */
 u_int	ddflags = 0;		/* conversion options */
 size_t	cbsz;			/* conversion block size */
 uintmax_t files_cnt = 1;	/* # of files to copy */
@@ -174,6 +176,8 @@ setup(void)
 	} else if ((in.db = malloc(MAX(in.dbsz, cbsz) + cbsz)) == NULL ||
 	    (out.db = malloc(out.dbsz + cbsz)) == NULL)
 		err(1, "output buffer");
+
+	/* dbp is the first free position in each buffer. */
 	in.dbp = in.db;
 	out.dbp = out.db;
 
@@ -436,8 +440,15 @@ dd_out(int force)
 	 * we play games with the buffer size, and it's usually a partial write.
 	 */
 	outp = out.db;
+
+	/*
+	 * If force, first try to write all pending data, else try to write
+	 * just one block. Subsequently always write data one full block at
+	 * a time at most.
+	 */
 	for (n = force ? out.dbcnt : out.dbsz;; n = out.dbsz) {
-		for (cnt = n;; cnt -= nw) {
+		cnt = n;
+		do {
 			sparse = 0;
 			if (ddflags & C_SPARSE) {
 				sparse = 1;	/* Is buffer sparse? */
@@ -449,18 +460,24 @@ dd_out(int force)
 			}
 			if (sparse && !force) {
 				pending += cnt;
+				last_sp = cnt;
 				nw = cnt;
 			} else {
 				if (pending != 0) {
-					if (force)
-						pending--;
+					/* If forced to write, and we have no
+					 * data left, we need to write the last
+					 * sparse block explicitly.
+					 */
+					if (force && cnt == 0) {
+						pending -= last_sp;
+						assert(outp == out.db);
+						memset(outp, 0, cnt);
+					}
 					if (lseek(out.fd, pending, SEEK_CUR) ==
 					    -1)
 						err(2, "%s: seek error creating sparse file",
 						    out.name);
-					if (force)
-						write(out.fd, outp, 1);
-					pending = 0;
+					pending = last_sp = 0;
 				}
 				if (cnt)
 					nw = write(out.fd, outp, cnt);
@@ -475,27 +492,29 @@ dd_out(int force)
 					err(1, "%s", out.name);
 				nw = 0;
 			}
+
 			outp += nw;
 			st.bytes += nw;
-			if ((size_t)nw == n) {
-				if (n != out.dbsz)
-					++st.out_part;
-				else
-					++st.out_full;
-				break;
-			}
-			++st.out_part;
-			if ((size_t)nw == cnt)
-				break;
-			if (out.flags & ISTAPE)
-				errx(1, "%s: short write on tape device",
-				    out.name);
-			if (out.flags & ISCHR && !warned) {
-				warned = 1;
-				warnx("%s: short write on character device",
-				    out.name);
+
+			if ((size_t)nw == n && n == out.dbsz)
+				++st.out_full;
+			else
+				++st.out_part;
+
+			if ((size_t) nw != cnt) {
+				if (out.flags & ISTAPE)
+					errx(1, "%s: short write on tape device",
+				    	out.name);
+				if (out.flags & ISCHR && !warned) {
+					warned = 1;
+					warnx("%s: short write on character device",
+				    	out.name);
+				}
 			}
-		}
+
+			cnt -= nw;
+		} while (cnt != 0);
+
 		if ((out.dbcnt -= n) < out.dbsz)
 			break;
 	}