netdev - Re: [PATCH net-next] net: do not write to msg_get

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Message-ID: <fb772a80-7034-42b1-a80a-117a7339d371@kernel.dk>
Date: Mon, 5 Jan 2026 10:58:44 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
 pabeni@...hat.com, horms@...nel.org, kuniyu@...gle.com,
 Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] net: do not write to msg_get_inq in caller

On 1/5/26 10:57 AM, Willem de Bruijn wrote:
> Jens Axboe wrote:
>> On 1/5/26 10:42 AM, Willem de Bruijn wrote:
>>> Eric Dumazet wrote:
>>>> On Mon, Jan 5, 2026 at 5:33?PM Willem de Bruijn
>>>> <willemdebruijn.kernel@...il.com> wrote:
>>>>>
>>>>> From: Willem de Bruijn <willemb@...gle.com>
>>>>>
>>>>> msg_get_inq is an input field from caller to callee. Don't set it in
>>>>> the callee, as the caller may not clear it on struct reuse.
>>>>>
>>>>> This is a kernel-internal variant of msghdr only, and the only user
>>>>> does reinitialize the field. So this is not critical.
>>>>>
>>>>> But it is more robust to avoid the write, and slightly simpler code.
>>>>>
>>>>> Callers set msg_get_inq to request the input queue length to be
>>>>> returned in msg_inq. This is equivalent to but independent from the
>>>>> SO_INQ request to return that same info as a cmsg (tp->recvmsg_inq).
>>>>> To reduce branching in the hot path the second also sets the msg_inq.
>>>>> That is WAI.
>>>>>
>>>>> This is a small follow-on to commit 4d1442979e4a ("af_unix: don't
>>>>> post cmsg for SO_INQ unless explicitly asked for"), which fixed the
>>>>> inverse.
>>>>>
>>>>> Also collapse two branches using a bitwise or.
>>>>>
>>>>> Link: https://lore.kernel.org/netdev/willemdebruijn.kernel.24d8030f7a3de@gmail.com/
>>>>> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>>>>> ---
>>>>
>>>> Patch looks sane to me, but the title is a bit confusing, I guess you meant
>>>>
>>>> "net: do not write to msg_get_inq in callee" ?
>>>
>>> Indeed, thanks. Will fix.
>>>
>>>>
>>>> Also, unix_stream_read_generic() is currently potentially adding a NULL deref
>>>> if u->recvmsg_inq is non zero, but msg is NULL ?
>>>>
>>>> If this is the case  we need a Fixes: tag.
>>>
>>> Oh good point. state->msg can be NULL as of commit 2b514574f7e8 ("net:
>>> af_unix: implement splice for stream af_unix sockets"). That commit
>>> mentions "we mostly have to deal with a non-existing struct msghdr
>>> argument".
>>
>> Worth noting that this is currently not possible, as io_uring should
>> be the only one setting ->recvmsg_inq and it would not do that via
>> splice. Should still be fixed of course.
> 
> recvmsg_inq is written from setsockopt SO_INQ. Do you mean
> msg_get_inq?
> 
> I think this is reachable with a setsockopt + splice:
> 
>         do_cmsg = READ_ONCE(u->recvmsg_inq);
>         if (do_cmsg)
>                 msg->msg_get_inq = 1;

Indeed you are right, I mixed up the two...

-- 
Jens Axboe