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

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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 unspecified.


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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