io_uring: ignore POLLIN for recvmsg on MSG_ERRQUEUE

Currently, io_uring's recvmsg subscribes to both POLLERR and POLLIN. In
the context of TCP tx zero-copy, this is inefficient since we are only
reading the error queue and not using recvmsg to read POLLIN responses.

This patch was tested by using a simple sending program to call recvmsg
using io_uring with MSG_ERRQUEUE set and verifying with printks that the
POLLIN is correctly unset when the msg flags are MSG_ERRQUEUE.
Signed-off-by: Arjun Roy <arjunroy@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Luke Hsiao <lukehsiao@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

fs/io_uring.c

@@ -4898,6 +4898,12 @@ static bool io_arm_poll_handler(struct io_kiocb *req)
 		mask |= POLLIN | POLLRDNORM;
 	if (def->pollout)
 		mask |= POLLOUT | POLLWRNORM;
+
+	/* If reading from MSG_ERRQUEUE using recvmsg, ignore POLLIN */
+	if ((req->opcode == IORING_OP_RECVMSG) &&
+	    (req->sr_msg.msg_flags & MSG_ERRQUEUE))
+		mask &= ~POLLIN;
+
 	mask |= POLLERR | POLLPRI;
 
 	ipt.pt._qproc = io_async_queue_proc;