io_uring/msg_ring: add support for sending a sync message

Normally MSG_RING requires both a source and a destination ring. But
some users don't always have a ring avilable to send a message from, yet
they still need to notify a target ring.

Add support for using io_uring_register(2) without having a source ring,
using a file descriptor of -1 for that. Internally those are called
blind registration opcodes. Implement IORING_REGISTER_SEND_MSG_RING as a
blind opcode, which simply takes an sqe that the application can put on
the stack and use the normal liburing helpers to initialize it. Then the
app can call:

io_uring_register(-1, IORING_REGISTER_SEND_MSG_RING, &sqe, 1);

and get the same behavior in terms of the target, where a CQE is posted
with the details given in the sqe.

For now this takes a single sqe pointer argument, and hence arg must
be set to that, and nr_args must be 1. Could easily be extended to take
an array of sqes, but for now let's keep it simple.

Link: https://lore.kernel.org/r/20240924115932.116167-3-axboe@kernel.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 29 insertions, 0 deletions

diff --git a/io_uring/msg_ring.c b/io_uring/msg_ring.c
index b8c527f08cd5..edea1ffd501c 100644
--- a/io_uring/msg_ring.c
+++ b/io_uring/msg_ring.c
@@ -331,6 +331,35 @@ done:
 	return IOU_OK;
 }
 
+int io_uring_sync_msg_ring(struct io_uring_sqe *sqe)
+{
+	struct io_msg io_msg = { };
+	struct fd f;
+	int ret;
+
+	ret = __io_msg_ring_prep(&io_msg, sqe);
+	if (unlikely(ret))
+		return ret;
+
+	/*
+	 * Only data sending supported, not IORING_MSG_SEND_FD as that one
+	 * doesn't make sense without a source ring to send files from.
+	 */
+	if (io_msg.cmd != IORING_MSG_DATA)
+		return -EINVAL;
+
+	ret = -EBADF;
+	f = fdget(sqe->fd);
+	if (fd_file(f)) {
+		ret = -EBADFD;
+		if (io_is_uring_fops(fd_file(f)))
+			ret = __io_msg_ring_data(fd_file(f)->private_data,
+						 &io_msg, IO_URING_F_UNLOCKED);
+		fdput(f);
+	}
+	return ret;
+}
+
 void io_msg_cache_free(const void *entry)
 {
 	struct io_kiocb *req = (struct io_kiocb *) entry;