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Date:   Mon, 18 Mar 2019 21:25:22 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     linux-fscrypt@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net,
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        Sarthak Kukreti <sarthakkukreti@chromium.org>,
        Gao Xiang <gaoxiang25@huawei.com>
Subject: Re: [PATCH 1/5] fscrypt: clean up and improve dentry revalidation
Message-ID: <20190318212522.GI2217@ZenIV.linux.org.uk>
References: <20190317200444.5967-1-ebiggers@kernel.org>
 <20190317200444.5967-2-ebiggers@kernel.org>
 <20190317203822.GH2217@ZenIV.linux.org.uk>
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On Mon, Mar 18, 2019 at 01:29:49PM -0700, Eric Biggers wrote:
> 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?

In case of corrupted fs image - yes; the same can happen if you have a network
filesystem with subdirectory moved around on server at the same time, but
there you'll need a lot more elaborate ->d_revalidate() anyway.

Directory hardlinks are certainly not allowed; however, we need the kernel
to survive when it runs into that kind of crap on disk...