On 1/9/21 1:59 AM, Otto Moerbeek wrote:
> 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
>
Hi Otto and William,
Thanks for confirming that this is a bug, and also for the fixes!
Regards,
Jordan
