linux-ext4 - Re: [PATCH 1/5] fscrypt: clean up and improve dentry revalidation

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Date:   Mon, 18 Mar 2019 13:29:49 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-fscrypt@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-mtd@...ts.infradead.org, linux-unionfs@...r.kernel.org,
        Sarthak Kukreti <sarthakkukreti@...omium.org>,
        Gao Xiang <gaoxiang25@...wei.com>
Subject: Re: [PATCH 1/5] fscrypt: clean up and improve dentry revalidation

On Sun, Mar 17, 2019 at 08:38:22PM +0000, Al Viro wrote:
> On Sun, Mar 17, 2019 at 01:04:40PM -0700, Eric Biggers wrote:
> > +	/*
> > +	 * Ciphertext name; valid if the directory's key is still unavailable.
> > +	 *
> > +	 * Note: since fscrypt forbids rename() on ciphertext names, it should
> > +	 * be safe to access ->d_parent directly here.
> 
> No, it is not.  Again, d_splice_alias() on buggered fs image picking a reference
> to your subdirectory when doing a lookup elsewhere.  It can relocate the
> damn thing, without rename() being allowed for _anything_.

You're talking about directory hard links, right?  E.g. if there's a directory
with two links a/dir and b/dir, and fscrypt_d_revalidate() is running on 'dir'
via a lookup in a/, it could be the case that b/dir is being concurrently looked
up.  Then the concurrent d_splice_alias() will move the whole dentry tree rooted
at 'dir' from a/ to b/ in the dcache.  Okay, it looks like you're right; I'll
update my comment to clarify that dget_parent() is still needed...

- Eric