linux-kernel - Re: overlayfs vs. fscrypt

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Message-ID: <1852545.qrIQg0rEWx@blindfold>
Date:   Wed, 13 Mar 2019 13:47:40 +0100
From:   Richard Weinberger <richard@....at>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     linux-fsdevel@...r.kernel.org, linux-fscrypt@...r.kernel.org,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: overlayfs vs. fscrypt

Am Mittwoch, 13. März 2019, 13:36:02 CET schrieb Miklos Szeredi: 
> I don't get it.  Does fscrypt try to check permissions via
> ->d_revalidate?  Why is it not doing that via ->permission()?

Please let me explain. Suppose we have a fscrypto directory /mnt and
I *don't* have the key.

When reading the directory contents of /mnt will return an encrypted filename.
e.g.
# ls /mnt
+mcQ46ne5Y8U6JMV9Wdq2C

As soon I load my key the real name is shown and I can read the file contents too.
That's why fscrypt has ->d_revalidate(). It checks for the key, if the key is
still not here -> stay with the old encrypted name. If the key is present
-> reveal the real name.

Same happens on the other direction if I unlink my key from the keyring.

> >
> > 2. Teach overlayfs to deal with a upper that has ->d_revalidate().
> > Given the complexity of overlayfs I'm not sure how feasible this is.
> > But I'm no overlayfs expert, maybe I miss something.
> 
> I don't think it would be too complex.  But first I'd like to
> understand exactly why fscrypt is (ab) using d_revalidate().

I hope my answer makes things more clear.

Thanks,
//richard