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

[Nicolai Stange]

Hi Stefano,

Stefano Brivio <sbrivio@xxxxxxxxxx> writes:

> On Tue,  2 Jan 2018 17:30:20 +0100
> Nicolai Stange <nstange@xxxxxxx> 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