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Re: [Cluster-devel] [GFS2 Patch] GFS2: rename causes kernel Oops



Hi,

That ends the long unbroken run of there being no bugs in the dir
code :( It was good spotting to track that one down and the fix looks
good to me. I've pushed it into the -nmw tree. Thanks for fixing this,

Steve.

On Wed, 2010-07-14 at 18:12 -0400, Bob Peterson wrote:
> Hi,
> 
> This patch fixes a kernel Oops in the GFS2 rename code.
> 
> The problem was in the way the gfs2 directory code was trying
> to re-use sentinel directory entries.  
> 
> In the failing case, gfs2's rename function was renaming a
> file to another name that had the same non-trivial length.
> The file being renamed happened to be the first directory
> entry on the leaf block.
> 
> First, the rename code (gfs2_rename in ops_inode.c) found the
> original directory entry and decided it could do its job by
> simply replacing the directory entry with another.  Therefore
> it determined correctly that no block allocations were needed.
> 
> Next, the rename code deleted the old directory entry prior to
> replacing it with the new name.  Therefore, the soon-to-be
> replaced directory entry was temporarily made into a directory
> entry "sentinel" or a place holder at the start of a leaf block.
> 
> Lastly, it went to re-add the replacement directory entry in
> that leaf block.  However, when gfs2_dirent_find_space was
> looking for space in the leaf block, it used the wrong value
> for the sentinel.  That threw off its calculations so later
> it decides it can't really re-use the sentinel and therefore
> must allocate a new leaf block.  But because it previously decided
> to re-use the directory entry, it didn't waste the time to
> grab a new block allocation for the inode.  Therefore, the
> inode's i_alloc pointer was still NULL and it crashes trying to
> reference it.
> 
> In the case of sentinel directory entries, the entire dirent is
> reused, not just the "free space" portion of it, and therefore
> the function gfs2_dirent_find_space should use the value 0
> rather than GFS2_DIRENT_SIZE(0) for the actual dirent size.
> 
> Fixing this calculation enables the reproducer programs to work
> properly.
> 
> Regards,
> 
> Bob Peterson
> Red Hat GFS
> 
> Signed-off-by: Bob Peterson <rpeterso redhat com> 
> --
>  fs/gfs2/dir.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/gfs2/dir.c b/fs/gfs2/dir.c
> index 8295c5b..26ca336 100644
> --- a/fs/gfs2/dir.c
> +++ b/fs/gfs2/dir.c
> @@ -392,7 +392,7 @@ static int gfs2_dirent_find_space(const struct gfs2_dirent *dent,
>  	unsigned totlen = be16_to_cpu(dent->de_rec_len);
>  
>  	if (gfs2_dirent_sentinel(dent))
> -		actual = GFS2_DIRENT_SIZE(0);
> +		actual = 0;
>  	if (totlen - actual >= required)
>  		return 1;
>  	return 0;



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