io_uring: add support for IORING_OP_PIPE

This works just like pipe2(2), except it also supports fixed file
descriptors. Used in a similar fashion as for other fd instantiating
opcodes (like accept, socket, open, etc), where sqe->file_slot is set
appropriately if two direct descriptors are desired rather than a set
of normal file descriptors.

sqe->addr must be set to a pointer to an array of 2 integers, which
is where the fixed/normal file descriptors are copied to.

sqe->pipe_flags contains flags, same as what is allowed for pipe2(2).

Future expansion of per-op private flags can go in sqe->ioprio,
like we do for other opcodes that take both a "syscall" flag set and
an io_uring opcode specific flag set.

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

include/uapi/linux/io_uring.h

@@ -73,6 +73,7 @@ struct io_uring_sqe {
 		__u32		futex_flags;
 		__u32		install_fd_flags;
 		__u32		nop_flags;
+		__u32		pipe_flags;
 	};
 	__u64	user_data;	/* data to be passed back at completion time */
 	/* pack this to avoid bogus arm OABI complaints */
@@ -283,6 +284,7 @@ enum io_uring_op {
 	IORING_OP_EPOLL_WAIT,
 	IORING_OP_READV_FIXED,
 	IORING_OP_WRITEV_FIXED,
+	IORING_OP_PIPE,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,