Re: [PATCH 1/2] ocfs2: Fix llseek() semantics and do some cleanup

From: Richard Yao
Date: Sat Jun 15 2013 - 20:47:39 EST

On 06/15/2013 01:09 AM, Jeff Liu wrote:
> [Add ocfs2-devel to CC-list]
> Hello Richard,
> Thanks for your patch.
> On 06/15/2013 03:23 AM, Richard Yao wrote:
>> There are multiple issues with the custom llseek implemented in ocfs2 for
>> implementing SEEK_HOLE/SEEK_DATA.
>> 1. It takes the inode->i_mutex lock before calling generic_file_llseek(), which
>> is unnecessary.
> Agree, but please see my comments below.
>> 2. It fails to take the filp->f_lock spinlock before modifying filp->f_pos and
>> filp->f_version, which differs from generic_file_llseek().
>> 3. It does a offset > inode->i_sb->s_maxbytes check that permits seeking up to
>> the maximum file size possible on the ocfs2 filesystem, even when it is past
>> the end of the file. Seeking beyond that (if possible), would return EINVAL
>> instead of ENXIO.
>> 4. The switch statement tries to cover all whence values when in reality it
>> should only care about SEEK_HOLE/SEEK_DATA. Any other cases should be passsed
>> to generic_file_llseek().
> I have another patch set for refactoring ocfs2_file_llseek() but not yet found time
> to run a comprehensive tests. It can solve the existing issues but also improved the
> SEEK_DATA/SEEK_HOLE for unwritten extents, i.e. OCFS2_EXT_UNWRITTEN.
> With this change, SEEK_DATA/SEEK_HOLE will go into separate function with a little code
> duplication instead of the current mix-ups in ocfs2_seek_data_hole_offset(), i.e,
> loff_t ocfs2_file_llseek()
> {
> switch (origin) {
> case SEEK_END:
> case SEEK_CUR:
> case SEEK_SET:
> return generic_file_llseek(file, offset, origin);
> case SEEK_DATA:
> return ocfs2_seek_data(file, offset);
> case SEEK_HOLE:
> return ocfs2_seek_hole(file, offset);
> default:
> return -EINVAL;
> }
> }
> I personally like keeping SEEK_DATA/SEEK_HOLE in style rather
> than dealing with them in a condition check block.

I would prefer to see the code structured like this:

loff_t ocfs2_file_llseek()
switch (origin) {
return ocfs2_seek_data(file, offset);
return ocfs2_seek_hole(file, offset);
return generic_file_llseek(file, offset, origin);

Unfortunately, I just noticed that this code has a problem. In specific,
generic_file_llseek() calls generic_file_llseek_size(), which has a
switch statement for whence that fails to distinguish between SEEK_SET
and invalid whence values. Invalid whence values are mapped to SEEK_SET
instead of returning EINVAL, which is wrong. That issue affects all
filesystems that do not specify a custom llseek() function and it would
affect ocfs2 if my version of the function is used.

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