[<prev] [next>] [thread-next>] [day] [month] [year] [list]
Message-ID: <4B2B156D.9040604@byu.net>
Date: Thu, 17 Dec 2009 22:38:53 -0700
From: Eric Blake <ebb9@....net>
To: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: utimensat fails to update ctime
POSIX requires that utimensat/futimens must update ctime in all cases
where any change is made (it only exempts when both atime and mtime were
requested as UTIME_OMIT, where the file must exist but no change is made).
Unfortunately, when atime is specified and mtime is UTIME_OMIT, the
kernel mistakenly behaves like read(), by updating atime but not ctime.
This in turn caused a regression in coreutils 8.2, visible through 'touch -a':
http://lists.gnu.org/archive/html/bug-coreutils/2009-12/msg00171.html
Here is a simple program demonstrating the failure:
$ cat foo.c
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
int
main ()
{
int fd = creat ("file", 0600);
struct stat st1, st2;
struct timespec t[2] = { { 1000000000, 0 }, { 0, UTIME_OMIT } };
fstat (fd, &st1);
sleep (1);
futimens (fd, t);
fstat (fd, &st2);
return st1.st_ctime == st2.st_ctime;
}
$ gcc -o foo foo.c -D_GNU_SOURCE
$ ./foo; echo $?
1
The exit status should have been 0.
GNU coreutils will end up working around the bug by calling fstat/[l]stat
prior to futimens/utimensat, and populating the mtime field with the
desired value rather than using UTIME_OMIT. But this is a pointless stat
call, which could be avoided if the kernel were fixed to comply with POSIX
by updating ctime even when mtime is UTIME_OMIT.
--
Don't work too hard, make some time for fun as well!
Eric Blake ebb9@....net
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists