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

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Message-ID: <ca5fe77f-f7a8-4886-b874-0e7063622ab7@kernel.dk>
Date: Wed, 17 Dec 2025 19:19:07 -0700
From: Jens Axboe <axboe@...nel.dk>
To: netdev <netdev@...r.kernel.org>
Cc: Jakub Kicinski <kuba@...nel.org>, Kuniyuki Iwashima <kuniyu@...gle.com>,
 Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH] af_unix: don't post cmsg for SO_INQ unless explicitly asked
 for

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@...r.kernel.org
Fixes: df30285b3670 ("af_unix: Introduce SO_INQ.")
Signed-off-by: Jens Axboe <axboe@...nel.dk>

---

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);
 		}
 	} else {
 		scm_destroy(&scm);

-- 
Jens Axboe