cmp: fix -s (silent) when used with skip offsets

-s causes cmp to print nothing for differing files, for use when only
the exit status is of interest.

-z compares the file size first, for regular files, and fails the
comparison early if they do not match.

Prior to this change -s implied -z as an optimization, but this is not
valid when file offsets are specified.  Now, enable the -z optimization
for -s only if both skip arguments are not provided / 0.

Note that using -z with differing skip values will currently always
fail.  We may want to compare size1 - skip1 with size2 - skip2 instaead,
and in any case the man page should be clarified.

PR:		252542
Fixes:		3e6902efc8
Reported by:	William Ahern
Reviewed by:	markj
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D28071

usr.bin/cmp/cmp.c

@@ -92,7 +92,6 @@ main(int argc, char *argv[])
 			break;
 		case 's':		/* silent run */
 			sflag = true;
-			zflag = true;
 			break;
 		case 'x':		/* hex output */
 			lflag = true;
@@ -149,6 +148,9 @@ main(int argc, char *argv[])
 	skip1 = argc > 2 ? strtol(argv[2], NULL, 0) : 0;
 	skip2 = argc == 4 ? strtol(argv[3], NULL, 0) : 0;
 
+	if (sflag && skip1 == 0 && skip2 == 0)
+		zflag = true;
+
 	if (fd1 == -1) {
 		if (fd2 == -1) {
 			c_link(file1, skip1, file2, skip2);

usr.bin/cmp/tests/cmp_test2.sh

@@ -62,8 +62,22 @@ symlink_body() {
 	atf_check -s not-exit:0 -o ignore -e ignore cmp -h a.lnk adup.lnk
 }
 
+atf_test_case pr252542
+pr252542_head()
+{
+	atf_set "descr" "Test cmp(1) -s with file offset skips"
+}
+pr252542_body()
+{
+	echo -n '1234567890' > a
+	echo -n 'abc567890' > b
+	atf_check -s exit:0 cmp -s a b 4 3
+	atf_check -s exit:1 -o ignore cmp -z a b 4 3
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case special
 	atf_add_test_case symlink
+	atf_add_test_case pr252542
 }