Re: [PATCH net-next] net: do not write to msg_get_inq in caller

[Jens Axboe <axboe@kernel.dk>]

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@gmail.com> wrote:
>>>
>>> From: Willem de Bruijn <willemb@google.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@google.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.

-- 
Jens Axboe