On Fri, Jan 08, 2021 at 07:09:01PM -0800, Jordan Geoghegan wrote:
> Hey folks,
>
> I've noticed some surprising behaviour from cmp(1) when using the '-s'
> flag.
>
> It appears that cmp -s is ignoring the byte offset arguments I'm giving
> it.
<snip>
> Not sure what to make of this, I noticed this same behaviour on
> DragonflyBSD and FreeBSD, so maybe I'm just missing something obvious.
> This certainly caused some frustration before I figured out what was going
> on.
The bug seems to be in the short-circuit optimization for regular files[1]:
void
c_regular(int fd1, char *file1, off_t skip1, off_t len1,
int fd2, char *file2, off_t skip2, off_t len2)
{
u_char ch, *p1, *p2;
off_t byte, length, line;
int dfound;
if (sflag && len1 != len2)
exit(1);
if (skip1 > len1)
eofmsg(file1);
len1 -= skip1;
if (skip2 > len2)
eofmsg(file2);
len2 -= skip2;
The short-circuit should probably be moved below the subsequent chunk of
code (i.e. below `len2 -= skip2`). The eofmsg function already obeys sflag,
so it'll be quiet.[2] Doing this works for me. See patch at end of message.
Interestingly, DragonflyBSD and FreeBSD already do it this way[3][4], yet I
can confirm FreeBSD still has the problem. (DragonflyBSD has nearly
identical code.) But that implementation duplicates the short-circuit, along
with the bug of not accounting for skip1 and skip2, in cmp.c as part of
implementing the -z flag[5]:
if (special)
c_special(fd1, file1, skip1, fd2, file2, skip2);
else {
if (zflag && sb1.st_size != sb2.st_size) {
if (!sflag)
(void) printf("%s %s differ: size\n",
file1, file2);
exit(DIFF_EXIT);
}
c_regular(fd1, file1, skip1, sb1.st_size,
fd2, file2, skip2, sb2.st_size);
}
exit(0);
It appears that the June 20, 2000 fix to the short-circuit in regular.c
wasn't recognized during the July 14, 2000 -z feature addition.[6][7]
[1] https://cvsweb.openbsd.org/src/usr.bin/cmp/regular.c?rev=1.12
[2] https://cvsweb.openbsd.org/src/usr.bin/cmp/misc.c?rev=1.7
[3]
https://gitweb.dragonflybsd.org/dragonfly.git/blob/4d4f84f:/usr.bin/cmp/regular.c
[4] https://svnweb.freebsd.org/base/head/usr.bin/cmp/regular.c?revision=344551
[5]
https://svnweb.freebsd.org/base/head/usr.bin/cmp/cmp.c?revision=344551&view=markup#l193
[6]
https://svnweb.freebsd.org/base/head/usr.bin/cmp/regular.c?revision=61883&view=markup
[7]
https://svnweb.freebsd.org/base/head/usr.bin/cmp/cmp.c?view=markup&pathrev=63157
--- regular.c 6 Feb 2015 23:21:59 -0000 1.12
+++ regular.c 9 Jan 2021 07:51:13 -0000
@@ -51,15 +51,15 @@ c_regular(int fd1, char *file1, off_t sk
off_t byte, length, line;
int dfound;
- if (sflag && len1 != len2)
- exit(1);
-
if (skip1 > len1)
eofmsg(file1);
len1 -= skip1;
if (skip2 > len2)
eofmsg(file2);
len2 -= skip2;
+
+ if (sflag && len1 != len2)
+ exit(1);
length = MINIMUM(len1, len2);
if (length > SIZE_MAX) {
