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Date:   Wed, 19 Jul 2023 23:41:03 -0700
From:   Eric Biggers <ebiggers@kernel.org>
To:     Gabriel Krisman Bertazi <krisman@suse.de>
Cc:     brauner@kernel.org, tytso@mit.edu,
        linux-f2fs-devel@lists.sourceforge.net, viro@zeniv.linux.org.uk,
        linux-fsdevel@vger.kernel.org, jaegeuk@kernel.org,
        linux-ext4@vger.kernel.org,
        Gabriel Krisman Bertazi <krisman@collabora.com>
Subject: Re: [PATCH v3 3/7] libfs: Validate negative dentries in
 case-insensitive directories
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 <20230720060657.GB2607@sol.localdomain>
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On Wed, Jul 19, 2023 at 11:06:57PM -0700, Eric Biggers wrote:
> 
> I'm also having trouble understanding exactly when ->d_name is stable here.
> AFAICS, unfortunately the VFS has an edge case where a dentry can be moved
> without its parent's ->i_rwsem being held.  It happens when a subdirectory is
> "found" under multiple names.  The VFS doesn't support directory hard links, so
> if it finds a second link to a directory, it just moves the whole dentry tree to
> the new location.  This can happen if a filesystem image is corrupted and
> contains directory hard links.  Coincidentally, it can also happen in an
> encrypted directory due to the no-key name => normal name transition...

Sorry, I think I got this slightly wrong.  The move does happen with the
parent's ->i_rwsem held, but it's for read, not for write.  First, before
->lookup is called, the ->i_rwsem of the parent directory is taken for read.
->lookup() calls d_splice_alias() which can call __d_unalias() which does the
__d_move().  If the old alias is in a different directory (which cannot happen
in that fscrypt case, but can happen in the general "directory hard links"
case), __d_unalias() takes that directory's ->i_rwsem for read too.

So it looks like the parent's ->i_rwsem does indeed exclude moves of child
dentries, but only if it's taken for *write*.  So I guess you can rely on that;
it's just a bit more subtle than it first appears.  Though, some of your
explanation seems to assume that a read lock is sufficient ("In __lookup_slow,
either the parent inode is locked by the caller (lookup_slow) ..."), so maybe
there is still a problem.

- Eric