On 12/18/25 2:15 PM, Willem de Bruijn wrote:

Jens Axboe wrote:

quoted

On 12/18/25 1:35 PM, Willem de Bruijn wrote:

quoted

Jens Axboe wrote:

quoted

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 cmsg if u->recvmsg_inq is set.

Cc: stable@vger.kernel.org
Fixes: df30285b3670 ("af_unix: Introduce SO_INQ.")
Reported-by: Julian Orth <redacted>
Link: https://github.com/axboe/liburing/issues/1509
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 net/unix/af_unix.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 55cdebfa0da0..110d716087b5 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c

@@ -3086,12 +3086,16 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 
 	mutex_unlock(&u->iolock);
 	if (msg) {
+		bool do_cmsg;
+
 		scm_recv_unix(sock, msg, &scm, flags);
 
-		if (READ_ONCE(u->recvmsg_inq) || msg->msg_get_inq) {
+		do_cmsg = READ_ONCE(u->recvmsg_inq);
+		if (do_cmsg || msg->msg_get_inq) {
 			msg->msg_inq = READ_ONCE(u->inq_len);
-			put_cmsg(msg, SOL_SOCKET, SCM_INQ,
-				 sizeof(msg->msg_inq), &msg->msg_inq);
+			if (do_cmsg)
+				put_cmsg(msg, SOL_SOCKET, SCM_INQ,
+					 sizeof(msg->msg_inq), &msg->msg_inq);

Is it intentional that msg_inq is set also if msg_get_inq is not set,
but do_cmsg is?

It doesn't really matter, what matters is the actual cmsg posting be
guarded. The msg_inq should only be used for a successful return anyway,
I think we're better off reading it unconditionally than having multiple
branches.

Not really important, if you prefer to keep them consistent, that's fine
with me too.

quoted

It just seems a bit surprising behavior.

That is an entangling of two separate things.
- msg_get_inq sets msg_inq, and
- cmsg_flags & TCP_CMSG_INQ inserts TCP_CM_INQ cmsg

The original TCP patch also entangles them, but in another way.
The cmsg is written only if msg_get_inq is requested.

The cmsg is written iff TCP_CMSG_INQ is set, not if ->msg_get_inq is the
only thing set. That part is important.

But yes, both need the data left.

I see, writing msg_inq if not requested is benign indeed. The inverse
is not true.

Ok. I do think it would be good to have the protocols consistent.
Simpler to reason about the behavior and intent long term.

Sure, I can do that. Would you prefer patch 1 and 2 folded as well, or
keep them separate? If we're mirroring the logic, seems like 1 patch
would be better.

-- 
Jens Axboe