netdev - Re: [PATCH v2] af_unix: don't post cmsg for SO

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Message-ID: <460fe33a-bf6d-4966-be04-abb6d89b9f9e@kernel.dk>
Date: Fri, 19 Dec 2025 13:24:29 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 netdev <netdev@...r.kernel.org>
Cc: io-uring <io-uring@...r.kernel.org>, Jakub Kicinski <kuba@...nel.org>,
 Willem de Bruijn <willemb@...gle.com>, Kuniyuki Iwashima
 <kuniyu@...gle.com>, Julian Orth <ju.orth@...il.com>
Subject: Re: [PATCH v2] af_unix: don't post cmsg for SO_INQ unless explicitly
 asked for

On 12/19/25 1:08 PM, Willem de Bruijn wrote:
> [PATCH net v2] assuming this is intended to go through the net tree.

Assuming this will hit -rc3 then, as netdev PRs usually go out on
thursdays?

> Jens Axboe wrote:
>> On 12/19/25 12:02 PM, Willem de Bruijn wrote:
>>> Jens Axboe wrote:
>>>> A previous commit added SO_INQ support for AF_UNIX (SOCK_STREAM), but it
>>>> posts a SCM_INQ cmsg even if just msg->msg_get_inq is set. This is
>>>> incorrect, as ->msg_get_inq is just the caller asking for the remainder
>>>> to be passed back in msg->msg_inq, it has nothing to do with cmsg. The
>>>> original commit states that this is done to make sockets
>>>> io_uring-friendly", but it's actually incorrect as io_uring doesn't use
>>>> cmsg headers internally at all, and it's actively wrong as this means
>>>> that cmsg's are always posted if someone does recvmsg via io_uring.
>>>>
>>>> Fix that up by only posting a cmsg if u->recvmsg_inq is set.
>>>>
>>>> Additionally, mirror how TCP handles inquiry handling in that it should
>>>> only be done for a successful return. This makes the logic for the two
>>>> identical.
>>>>
>>>> Cc: stable@...r.kernel.org
>>>> Fixes: df30285b3670 ("af_unix: Introduce SO_INQ.")
>>>> Reported-by: Julian Orth <ju.orth@...il.com>
>>>> Link: https://github.com/axboe/liburing/issues/1509
>>>> Signed-off-by: Jens Axboe <axboe@...nel.dk>
>>>>
>>>> ---
>>>>
>>>> V2:
>>>> - Unify logic with tcp
>>>> - Squash the two patches into one
>>>>
>>>> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
>>>> index 55cdebfa0da0..a7ca74653d94 100644
>>>> --- a/net/unix/af_unix.c
>>>> +++ b/net/unix/af_unix.c
>>>> @@ -2904,6 +2904,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
>>>>  	unsigned int last_len;
>>>>  	struct unix_sock *u;
>>>>  	int copied = 0;
>>>> +	bool do_cmsg;
>>>>  	int err = 0;
>>>>  	long timeo;
>>>>  	int target;
>>>> @@ -2929,6 +2930,9 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
>>>>  
>>>>  	u = unix_sk(sk);
>>>>  
>>>> +	do_cmsg = READ_ONCE(u->recvmsg_inq);
>>>> +	if (do_cmsg)
>>>> +		msg->msg_get_inq = 1;
>>>
>>> I would avoid overwriting user written fields if it's easy to do so.
>>>
>>> In this case it probably is harmless. But we've learned the hard way
>>> that applications can even get confused by recvmsg setting msg_flags.
>>> I've seen multiple reports of applications failing to scrub that field
>>> inbetween calls.
>>>
>>> Also just more similar to tcp:
>>>
>>>        do_cmsg = READ_ONCE(u->recvmsg_inq);
>>>        if ((do_cmsg || msg->msg_get_inq) && (copied ?: err) >= 0) {
>>
>> I think you need to look closer, because this is actually what the tcp
>> path does:
>>
>> if (tp->recvmsg_inq) {
>> 	[...]
>> 	msg->msg_get_inq = 1;
>> }
> 
> I indeed missed that TCP does the same. Ack. Indeed consistency was
> what I asked for.

FWIW, I don't disagree with you, but sorting that out should then be a
followup patch that would then touch both tcp and streams.

> Reviewed-by: Willem de Bruijn <willemb@...gle.com>

Thanks!

-- 
Jens Axboe