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Date:	Tue, 24 Apr 2012 18:10:51 +0200
From:	Bernd Schubert <>
To:	Andreas Dilger <>
CC:	Eric Sandeen <>,,,,
	Fan Yong <>,
Subject: Re: [PATCH 5 2/4] Return 32/64-bit dir name hash according to usage

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

void seekdir(DIR *dirp, long loc);


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 


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


case SEEK_END:
                 if (dx_dir)
                         offset += ext4_get_htree_eof(file);
                         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?


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