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, 30 insertions, 0 deletions

diff --git a/io_uring/register.c b/io_uring/register.c
index eca26d4884d9..52b2f9b74af8 100644
--- a/io_uring/register.c
+++ b/io_uring/register.c
@@ -28,6 +28,7 @@
 #include "kbuf.h"
 #include "napi.h"
 #include "eventfd.h"
+#include "msg_ring.h"
 
 #define IORING_MAX_RESTRICTIONS	(IORING_RESTRICTION_LAST + \
 				 IORING_REGISTER_LAST + IORING_OP_LAST)
@@ -588,6 +589,32 @@ struct file *io_uring_register_get_file(unsigned int fd, bool registered)
 	return ERR_PTR(-EOPNOTSUPP);
 }
 
+/*
+ * "blind" registration opcodes are ones where there's no ring given, and
+ * hence the source fd must be -1.
+ */
+static int io_uring_register_blind(unsigned int opcode, void __user *arg,
+				   unsigned int nr_args)
+{
+	switch (opcode) {
+	case IORING_REGISTER_SEND_MSG_RING: {
+		struct io_uring_sqe sqe;
+
+		if (!arg || nr_args != 1)
+			return -EINVAL;
+		if (copy_from_user(&sqe, arg, sizeof(sqe)))
+			return -EFAULT;
+		/* no flags supported */
+		if (sqe.flags)
+			return -EINVAL;
+		if (sqe.opcode == IORING_OP_MSG_RING)
+			return io_uring_sync_msg_ring(&sqe);
+		}
+	}
+
+	return -EINVAL;
+}
+
 SYSCALL_DEFINE4(io_uring_register, unsigned int, fd, unsigned int, opcode,
 		void __user *, arg, unsigned int, nr_args)
 {
@@ -602,6 +629,9 @@ SYSCALL_DEFINE4(io_uring_register, unsigned int, fd, unsigned int, opcode,
 	if (opcode >= IORING_REGISTER_LAST)
 		return -EINVAL;
 
+	if (fd == -1)
+		return io_uring_register_blind(opcode, arg, nr_args);
+
 	file = io_uring_register_get_file(fd, use_registered_ring);
 	if (IS_ERR(file))
 		return PTR_ERR(file);