Re: [PATCH 1/2] vfs: stop d_splice_alias creating directory aliases

["J. Bruce Fields" <bfields@fieldses.org>]

These two patches are a resend.  I think they're appropriate for 3.5.

--b.

On Wed, May 23, 2012 at 06:05:45PM -0400, J. Bruce Fields wrote:
> From: "J. Bruce Fields" <bfields@redhat.com>
> 
> A directory should never have more than one dentry pointing to it.
> 
> But d_splice_alias() will add one if it finds a directory with an
> already-existing non-DISCONNECTED dentry.
> 
> I can't find an obvious reproducer, but I also can't see what prevents
> d_splice_alias() from encountering such a case.
> 
> It therefore seems safest to allow d_splice_alias to use any dentry it
> finds.
> 
> (Prior to the removal of dentry_unhash() from vfs_rmdir(), around v3.0,
> this could cause an nfsd deadlock like this:
> 
> 	- Somebody attempts to remove a non-empty directory.
> 	- The dentry_unhash() in vfs_rmdir() unhashes the dentry
> 	  pointing to the non-empty directory.
> 	- ->rmdir() then fails with -ENOTEMPTY
> 	- Before the vfs_rmdir() caller reaches dput(), an nfsd process
> 	  in rename looks up the directory by filehandle; at the end of
> 	  that lookup, this dentry is found by d_alloc_anon(), and a
> 	  reference is taken on it, preventing dput() from removing it.
> 	- A regular lookup of the directory calls d_splice_alias(),
> 	  finds only an unhashed (not a DISCONNECTED) dentry, and
> 	  insteads adds a new one, so the directory now has two
> 	  dentries.
> 	- The nfsd process in rename, which was previously looking up
> 	  the source directory of the rename, now looks up the target
> 	  directory (which is the same), and gets the dentry newly
> 	  created by the previous lookup.
> 	- The rename, seeing two different dentries, assumes this is a
> 	  cross-directory rename and attempts to take the i_mutex on the
> 	  directory twice.
> 
> That reproducer no longer exists, but I don't think there was anything
> fundamentally incorrect about the vfs_rmdir() behavior there, so I think
> the real fault was here in d_splice_alias().)
> 
> Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> ---
>  fs/dcache.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index b60ddc4..2434c1e 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1606,9 +1606,8 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
>  
>  	if (inode && S_ISDIR(inode->i_mode)) {
>  		spin_lock(&inode->i_lock);
> -		new = __d_find_alias(inode, 1);
> +		new = __d_find_any_alias(inode);
>  		if (new) {
> -			BUG_ON(!(new->d_flags & DCACHE_DISCONNECTED));
>  			spin_unlock(&inode->i_lock);
>  			security_d_instantiate(new, inode);
>  			d_move(new, dentry);
> -- 
> 1.7.9.5
> 
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