Hi Nicolai, On Wed, 03 Jan 2018 10:28:20 +0100 Nicolai Stange <nsta...@suse.de> wrote:
> Hi Stefano, > > Stefano Brivio <sbri...@redhat.com> writes: > > > On Tue, 2 Jan 2018 17:30:20 +0100 > > Nicolai Stange <nsta...@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); > > + > > Yes, this does also work. In fact, after having been lowered into SSA > form, it should be equivalent to what I posted. > > So, it's a matter of preference/style and I'd leave the decision on > this to the maintainers -- for me, either way is fine. > > I don't like the "sequence point" wording in the comment above though: > AFAICS, if taken in the meaning of C99, it's not any sequence point but > the volatile access in READ_ONCE() which ensures that there won't be any > reloads from ->hdrincl. If you don't mind, I'll adjust that comment if > asked to resend with your solution. Well, by "by adding a further sequence point" I refer to what we have to do to emulate READ_ONCE(), not to the reason why we need READ_ONCE(). However, this is a likely sign that my comment isn't that clear either. So unless you have better ideas, I would go with: + /* hdrincl should be READ_ONCE(inet->hdrincl) but READ_ONCE() doesn't + * work with bit fields. Doing this indirectly yields the same result. but I really hope you have a better idea. :) -- Stefano
