The file length comparison short-circuit in cmp.c for when the -s flag, regular files, and SKIP1 are specified is broken. c_regular in regular.c had a similar buggy short-circuit that was fixed in June 2000. But in July 2000 the -z flag feature was added which accidentally reintroduced the same bug. The bug is comparing the file sizes without accounting for skip1 and skip2. See https://svnweb.freebsd.org/base/head/usr.bin/cmp/cmp.c?revision=344551&view=markup#l193 Note that both sflag and zflag are set when -s is specified. Discovered when investigating a bug report on the OpenBSD mailing-list. More detailed dissection at https://marc.info/?l=openbsd-misc&m=161017967207304&w=2
In the man page, -z For regular files compare file sizes first, and fail the comparison if they are not equal. so I think this is expected behaviour with -z; do you agree? For -s this is certainly a bug; I'm considering this as a fix: diff --git a/usr.bin/cmp/cmp.c b/usr.bin/cmp/cmp.c index c762f1346abf..47f9b671985c 100644 --- a/usr.bin/cmp/cmp.c +++ b/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);
Please see the review in https://reviews.freebsd.org/D28071
A commit in branch main references this bug: URL: https://cgit.FreeBSD.org/src/commit/?id=80445b7a3f738e0b0a33ee7a11905a275346a6de commit 80445b7a3f738e0b0a33ee7a11905a275346a6de Author: Ed Maste <emaste@FreeBSD.org> AuthorDate: 2021-01-11 00:02:56 +0000 Commit: Ed Maste <emaste@FreeBSD.org> CommitDate: 2021-01-11 00:02:56 +0000 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: 3e6902efc802ab57fc4e9bf798f2d271b152e7f9 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 | 4 +++- usr.bin/cmp/tests/cmp_test2.sh | 14 ++++++++++++++ 2 files changed, 17 insertions(+), 1 deletion(-)
A commit in branch stable/12 references this bug: URL: https://cgit.FreeBSD.org/src/commit/?id=23b5fa56a182574fef416caeb55e6c735087e411 commit 23b5fa56a182574fef416caeb55e6c735087e411 Author: Ed Maste <emaste@FreeBSD.org> AuthorDate: 2021-01-11 00:02:56 +0000 Commit: Ed Maste <emaste@FreeBSD.org> CommitDate: 2021-01-15 14:25:36 +0000 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: 3e6902efc802ab57fc4e9bf798f2d271b152e7f9 Reported by: William Ahern Reviewed by: markj Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D28071 (cherry picked from commit 80445b7a3f738e0b0a33ee7a11905a275346a6de) usr.bin/cmp/cmp.c | 4 +++- usr.bin/cmp/tests/cmp_test2.sh | 14 ++++++++++++++ 2 files changed, 17 insertions(+), 1 deletion(-)