From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stefano Brivio <sbrivio@redhat.com>
Subject: Re: [PATCH] net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field
 in raw_sendmsg()
Date: Tue, 2 Jan 2018 22:12:30 +0100
Message-ID: <20180102221230.452019a9@elisabeth>
References: <20180102163020.32473-1-nstange@suse.de>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Cc: "David S. Miller" <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Mohamed Ghannam <simo.ghannam@gmail.com>,
        Michal Kubecek <mkubecek@suse.cz>,
        Miroslav Benes <mbenes@suse.cz>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
To: Nicolai Stange <nstange@suse.de>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20180102163020.32473-1-nstange@suse.de>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: netdev.vger.kernel.org

Hi,

On Tue,  2 Jan 2018 17:30:20 +0100
Nicolai Stange <nstange@suse.de> wrote:

> [...]
>
> diff --git a/net/ipv4/raw.c b/net/ipv4/raw.c
> index 5b9bd5c33d9d..e84290c28c0c 100644
> --- a/net/ipv4/raw.c
> +++ b/net/ipv4/raw.c
> @@ -513,16 +513,18 @@ static int raw_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>  	int err;
>  	struct ip_options_data opt_copy;
>  	struct raw_frag_vec rfv;
> -	int hdrincl;
> +	int hdrincl, __hdrincl;
>  
>  	err = -EMSGSIZE;
>  	if (len > 0xFFFF)
>  		goto out;
>  
>  	/* hdrincl should be READ_ONCE(inet->hdrincl)
> -	 * but READ_ONCE() doesn't work with bit fields
> +	 * but READ_ONCE() doesn't work with bit fields.
> +	 * Emulate it by doing the READ_ONCE() from an intermediate int.
>  	 */
> -	hdrincl = inet->hdrincl;
> +	__hdrincl = inet->hdrincl;
> +	hdrincl = READ_ONCE(__hdrincl);

I guess you don't actually need to use a third variable. What about
doing READ_ONCE() on hdrincl itself after the first assignment?

Perhaps something like the patch below -- applies to net.git, yields
same binary output as your version with gcc 6, looks IMHO more
straightforward:

diff --git a/net/ipv4/raw.c b/net/ipv4/raw.c
index 125c1eab3eaa..8c2f783a95fc 100644
--- a/net/ipv4/raw.c
+++ b/net/ipv4/raw.c
@@ -519,10 +519,12 @@ static int raw_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	if (len > 0xFFFF)
 		goto out;
 
-	/* hdrincl should be READ_ONCE(inet->hdrincl)
-	 * but READ_ONCE() doesn't work with bit fields
+	/* hdrincl should be READ_ONCE(inet->hdrincl) but READ_ONCE() doesn't
+	 * work with bit fields. Emulate it by adding a further sequence point.
 	 */
 	hdrincl = inet->hdrincl;
+	hdrincl = READ_ONCE(hdrincl);
+
 	/*
 	 *	Check the flags.
 	 */


-- 
Stefano