linux-kernel - Re: Bug report: unaligned access with ext4 encryption

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Message-ID: <20190110222928.GB22416@darkstar.musicnaut.iki.fi>
Date:   Fri, 11 Jan 2019 00:29:28 +0200
From:   Aaro Koskinen <aaro.koskinen@....fi>
To:     David Howells <dhowells@...hat.com>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        linux-fscrypt@...r.kernel.org, linux-kernel@...r.kernel.org,
        keyrings@...r.kernel.org
Subject: Re: Bug report: unaligned access with ext4 encryption

Hi,

On Fri, Jan 04, 2019 at 05:28:02PM +0000, David Howells wrote:
> Eric Biggers <ebiggers@...nel.org> wrote:
> > Hi Aaro, thanks for the bug report!  I think you're on the right track; it makes
> > much more sense to have the keyrings subsystem store the payload with better
> > alignment, than to work around the 2-byte alignment in fscrypt.
> > 
> > But how about '__aligned(__alignof__(u64))' instead?  4 bytes may not be enough.
> > 
> > David, what do you think?
> 
> Does that even work?

That should work.

> Might be better to just insert 6 bytes of padding with a comment, but yes I
> agree that it's probably better to align it to at least machine word size.

Padding is fragile, e.g. if struct rcu_head changes. Using __aligned should
make it always right automatically.

A.