seq_file: fix mishandling of consecutive pread() invocations.
authorEarl Chew <echew@ixiacom.com>
Wed, 21 Mar 2012 23:33:43 +0000 (16:33 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Thu, 22 Mar 2012 00:54:54 +0000 (17:54 -0700)
commit7904ac84244b59f536c2a5d1066a10f46df07b08
tree82d9c44c26b52954884a70f1ae2b7fa78ea8e633
parentdc716e96f5a467835e8121e1caaf25d66a901cb3
seq_file: fix mishandling of consecutive pread() invocations.

The following program illustrates the problem:

    char buf[8192];

    int fd = open("/proc/self/maps", O_RDONLY);

    n = pread(fd, buf, sizeof(buf), 0);
    printf("%d\n", n);

    /* lseek(fd, 0, SEEK_CUR); */ /* Uncomment to work around */

    n = pread(fd, buf, sizeof(buf), 0);
    printf("%d\n", n);

The second printf() prints zero, but uncommenting the lseek() corrects its
behaviour.

To fix, make seq_read() mirror seq_lseek() when processing changes in
*ppos.  Restore m->version first, then if required traverse and update
read_pos on success.

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=11856

Signed-off-by: Earl Chew <echew@ixiacom.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
fs/seq_file.c