linux-ext4 - Re: [PATCH 5 2/4] Return 32/64-bit dir name hash according to usage type

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Date:	Tue, 24 Apr 2012 18:10:51 +0200
From:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
To:	Andreas Dilger <adilger@...mcloud.com>
CC:	Eric Sandeen <sandeen@...hat.com>, linux-nfs@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Fan Yong <yong.fan@...mcloud.com>, bfields@...hat.com
Subject: Re: [PATCH 5 2/4] Return 32/64-bit dir name hash according to usage
 type

On 04/24/2012 12:42 AM, Andreas Dilger wrote:
> On 2012-04-23, at 5:23 PM, Eric Sandeen wrote:
>> I'm curious about the above as well as:
>>
>>         case SEEK_END:
>>                 if (unlikely(offset>  0))
>>                         goto out_err; /* not supported for directories */
>>
>> The previous .llseek handler, and the generic handler for other filesystems, allow seeking past the end of the dir AFAICT.  (not sure why you'd want to, but I don't see that you'd get an error back).
>>
>> Is there a reason to uniquely exclude it in ext4?  Does that line up with POSIX?
>
> I don't know what the origin of this was...  I don't think there is a real reason for it except that it doesn't make any sense to do so.
>

I think I added that. According to pubs.opengroup.org:
(http://pubs.opengroup.org/onlinepubs/009695399/functions/seekdir.html)

void seekdir(DIR *dirp, long loc);

<quote>

If the value of loc was not obtained from an earlier call to telldir(), 
or if a call to rewinddir() occurred between the call to telldir() and 
the call to seekdir(), the results of subsequent calls to readdir() are 
unspecified.

</quote>


As telldir(), which should correlate to 'case SEEK_CUR' will not provide 
invalid values, the behaviour is undefined.


Also,


case SEEK_END:
[...]
                 if (dx_dir)
                         offset += ext4_get_htree_eof(file);
                 else
                         offset += inode->i_size;
[...]


         if (!dx_dir) {
                 if (offset > inode->i_sb->s_maxbytes)
                         goto out_err;
         } else if (offset > ext4_get_htree_eof(file))
                 goto out_err;




Hence, the additional:

          case SEEK_END:
                  if (unlikely(offset>  0))
                       goto out_err; /* not supported for directories */

	
is just a shortcut to avoid useless calculations.

Unless I missed something, it only remains the question if could break 
existing applications relying on undefined behaviour. However, I have no 
idea how an application might trigger that?


Thanks,
Bernd

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