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

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