io_uring: Add io_uring_setup flag to pre-register ring fd and never install it

With IORING_REGISTER_USE_REGISTERED_RING, an application can register
the ring fd and use it via registered index rather than installed fd.
This allows using a registered ring for everything *except* the initial
mmap.

With IORING_SETUP_NO_MMAP, io_uring_setup uses buffers allocated by the
user, rather than requiring a subsequent mmap.

The combination of the two allows a user to operate *entirely* via a
registered ring fd, making it unnecessary to ever install the fd in the
first place. So, add a flag IORING_SETUP_REGISTERED_FD_ONLY to make
io_uring_setup register the fd and return a registered index, without
installing the fd.

This allows an application to avoid touching the fd table at all, and
allows a library to never even momentarily install a file descriptor.

This splits out an io_ring_add_registered_file helper from
io_ring_add_registered_fd, for use by io_uring_setup.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Link: https://lore.kernel.org/r/bc8f431bada371c183b95a83399628b605e978a3.1682699803.git.josh@joshtriplett.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 3 insertions, 0 deletions

diff --git a/io_uring/io_uring.h b/io_uring/io_uring.h
index 259bf798a390..9b8dfb3bb2b4 100644
--- a/io_uring/io_uring.h
+++ b/io_uring/io_uring.h
@@ -75,6 +75,9 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd);
 int io_uring_alloc_task_context(struct task_struct *task,
 				struct io_ring_ctx *ctx);
 
+int io_ring_add_registered_file(struct io_uring_task *tctx, struct file *file,
+				     int start, int end);
+
 int io_poll_issue(struct io_kiocb *req, struct io_tw_state *ts);
 int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr);
 int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin);