linux-kernel - RE: [PATCH] KEYS: user: Align the payload buffer

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Date:   Wed, 20 Feb 2019 13:52:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'David Howells' <dhowells@...hat.com>,
        "jmorris@...ei.org" <jmorris@...ei.org>
CC:     Aaro Koskinen <aaro.koskinen@....fi>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Aaro Koskinen <aaro.koskinen@....fi>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] KEYS: user: Align the payload buffer

From: David Howells
> Sent: 20 February 2019 13:32
> 
> From: Eric Biggers <ebiggers@...gle.com>
> 
> Align the payload of "user" and "logon" keys so that users of the
> keyrings service can access it as a struct that requires more than
> 2-byte alignment.  fscrypt currently does this which results in the read
> of fscrypt_key::size being misaligned as it needs 4-byte alignment.
> 
> Align to __alignof__(u64) rather than __alignof__(long) since in the
> future it's conceivable that people would use structs beginning with
> u64, which on some platforms would require more than 'long' alignment.
...
>  include/keys/user-type.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/keys/user-type.h b/include/keys/user-type.h
> index e098cbe27db5..12babe991594 100644
> --- a/include/keys/user-type.h
> +++ b/include/keys/user-type.h
> @@ -31,7 +31,7 @@
>  struct user_key_payload {
>  	struct rcu_head	rcu;		/* RCU destructor */
>  	unsigned short	datalen;	/* length of this data */
> -	char		data[0];	/* actual data */
> +	char		data[0] __aligned(__alignof__(u64)); /* actual data */
>  };

I'd make the 'datalen' field 'unsigned int' at the same time.
It will use some of the hole you've made and generate better
code on most arches.

	David

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