Add io_uring_register_sync_msg() helper

This helpers helps issue a sync MSG_RING request, as done via
IORING_REGISTER_SEND_MSG_RING. It requires the SQE to be cleared
and initialized with io_uring_prep_msg_ring(), just like a normal
async MSG_RING request would be. It does not require a source ring,
as no source ring is used - the result of the operation (what would've
been cqe->res if the async approach would've been used) is returned
directly from io_uring_register_sync_msg().

Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

src/include/liburing.h

@@ -286,6 +286,7 @@ int io_uring_unregister_buf_ring(struct io_uring *ring, int bgid);
 int io_uring_buf_ring_head(struct io_uring *ring, int buf_group, uint16_t *head);
 int io_uring_register_sync_cancel(struct io_uring *ring,
 				 struct io_uring_sync_cancel_reg *reg);
+int io_uring_register_sync_msg(struct io_uring_sqe *sqe);
 
 int io_uring_register_file_alloc_range(struct io_uring *ring,
 					unsigned off, unsigned len);

src/liburing-ffi.map

@@ -242,4 +242,5 @@ LIBURING_2.11 {
 	global:
 		io_uring_memory_size;
 		io_uring_memory_size_params;
+		io_uring_register_sync_msg;
 } LIBURING_2.10;

src/liburing.map

@@ -124,4 +124,5 @@ LIBURING_2.11 {
 	global:
 		io_uring_memory_size;
 		io_uring_memory_size_params;
+		io_uring_register_sync_msg;
 } LIBURING_2.10;

src/register.c

@@ -368,6 +368,11 @@ int io_uring_register_sync_cancel(struct io_uring *ring,
 	return do_register(ring, IORING_REGISTER_SYNC_CANCEL, reg, 1);
 }
 
+int io_uring_register_sync_msg(struct io_uring_sqe *sqe)
+{
+	return __sys_io_uring_register(-1, IORING_REGISTER_SEND_MSG_RING, sqe, 1);
+}
+
 int io_uring_register_file_alloc_range(struct io_uring *ring,
 					unsigned off, unsigned len)
 {