sysfs: use generic_file_llseek() for sysfs_file_operations

13c589d5b0ac6 ("sysfs: use seq_file when reading regular files")
converted regular sysfs files to use seq_file.  The commit substituted
generic_file_llseek() with seq_lseek() for llseek implementation.

Before the change, all regular sysfs files were allowed to seek to any
position in [0, PAGE_SIZE] as the file size is always PAGE_SIZE and
generic_file_llseek() allows any seeking inside the range under file
size; however, seq_lseek()'s behavior is different.  It traverses the
output by repeatedly invoking ->show() until it reaches the target
offset or traversal indicates EOF.  As seq_files are fully dynamic and
may not end at all, it doesn't support seeking from the end
(SEEK_END).

Apparently, there are userland tools which uses SEEK_END to discover
the buffer size to use and the switch to seq_lseek() disturbs them as
SEEK_END fails with -EINVAL.

The only benefits of using seq_lseek() instead of
generic_file_llseek() are

* Early failure.  If traversing to certain file position should fail,
  seq_lseek() will report such failures on lseek(2) instead of the
  following read/write operations.

* EOF detection.  While SEEK_END is not supported, SEEK_SET/CUR +
  large offset can be used to detect eof - eof at the time of the seek
  anyway as the file size may change dynamically.

Both aren't necessary for sysfs or prospect kernfs users.  Revert to
genefic_file_llseek() and preserve the original behavior.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Link: https://lkml.kernel.org/r/20131031114358.GA5551@osiris
Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 382db3c045f37bdd9cd1b11e077e24d208997f45..79b5da2acbe184353475f53ccb03793404bd3563 100644 (file)
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -800,7 +800,7 @@ EXPORT_SYMBOL_GPL(sysfs_notify);
 const struct file_operations sysfs_file_operations = {
        .read           = seq_read,
        .write          = sysfs_write_file,
-       .llseek         = seq_lseek,
+       .llseek         = generic_file_llseek,
        .open           = sysfs_open_file,
        .release        = sysfs_release,
        .poll           = sysfs_poll,