Subject: Re: [PATCH] net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg()

From: Stefano Brivio <[email-masked]>
Date: Tue, 2 Jan 2018 22:12:30 +0100

Hi,

On Tue,  2 Jan 2018 17:30:20 +0100
Nicolai Stange <[email-masked]> 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