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

[Jeff Liu]

On 06/16/2013 08:46 AM, Richard Yao wrote:

> 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 switch...case 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) {
>         case SEEK_DATA:
>                 return ocfs2_seek_data(file, offset);
>         case SEEK_HOLE:
>                 return ocfs2_seek_hole(file, offset);
>         default:
>                 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.

Hmm??  Did you mean to say that an invalid whence(i.e, whence > SEEK_MAX)
can be passed into generic_file_llseek()?
If so, I don't think that is a problem because any invalid whence should
be rejected at the entrance of VFS lseek(2) with EINVAL.

Thanks,
-Jeff

 


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