Messages in this thread Patch in this message | | | Date | Tue, 2 Jan 2018 22:12:30 +0100 | From | Stefano Brivio <> | Subject | Re: [PATCH] net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg() |
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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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