Re: [patch] vfs: fix vfs_rename_dir for FS_RENAME_DOES_D_MOVE filesystems

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Jul 21, 2008 at 01:41:47PM +0200, Miklos Szeredi wrote:
> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> vfs_rename_dir() doesn't properly account for filesystems with
> FS_RENAME_DOES_D_MOVE.  If new_dentry has a target inode attached, it
> unhashes the new_dentry prior to the rename() iop and rehashes it
> after, but doesn't account for the possibility that rename() may have
> swapped {old,new}_dentry.  For FS_RENAME_DOES_D_MOVE filesystems, it
> rehashes new_dentry (now the old renamed-from name, which d_move()
> expected to go away), such that a subsequent lookup will find it.
> 
> This was caught by the recently posted POSIX fstest suite, rename/10.t 
> test 62 (and others) on ceph.
> 
> Fix by not rehashing the new dentry.  Rehashing would only make sense
> if the rename failed (which should happen extremely rarely), but we
> cannot handle that case correctly 100% of the time anyway, so...

Lovely.  AFAICS, that's a fallout from
commit 349457ccf2592c14bdf13b6706170ae2e94931b1
Author: Mark Fasheh <mark.fasheh@oracle.com>
Date:   Fri Sep 8 14:22:21 2006 -0700

    [PATCH] Allow file systems to manually d_move() inside of ->rename()

that had allowed that crap for directories.  Note that d_rehash() used
to be needed (d_move() would unhash the source otherwise) and d_move()
used to be unconditional until the changeset above.

It's _probably_ OK now, but I'd really like to think about NFS behaviour.
There are subtle traps in that area.

BTW, failing rename() is trivial - just have a non-empty target...