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

Stefano Brivio <sbrivio@redhat.com> writes:

> 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);
> +

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.

Thanks,

Nicolai

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