On Sat, Jan 09, 2021 at 12:05:31AM -0800, William Ahern wrote:
> 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) {
>
I came to the same diff independently. In the meantime it has been committed.
-Otto
