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, 20 insertions, 11 deletions

diff --git a/io_uring/tctx.c b/io_uring/tctx.c
index 3a8d1dd97e1b..c043fe93a3f2 100644
--- a/io_uring/tctx.c
+++ b/io_uring/tctx.c
@@ -208,31 +208,40 @@ void io_uring_unreg_ringfd(void)
 	}
 }
 
-static int io_ring_add_registered_fd(struct io_uring_task *tctx, int fd,
+int io_ring_add_registered_file(struct io_uring_task *tctx, struct file *file,
 				     int start, int end)
 {
-	struct file *file;
 	int offset;
-
 	for (offset = start; offset < end; offset++) {
 		offset = array_index_nospec(offset, IO_RINGFD_REG_MAX);
 		if (tctx->registered_rings[offset])
 			continue;
 
-		file = fget(fd);
-		if (!file) {
-			return -EBADF;
-		} else if (!io_is_uring_fops(file)) {
-			fput(file);
-			return -EOPNOTSUPP;
-		}
 		tctx->registered_rings[offset] = file;
 		return offset;
 	}
-
 	return -EBUSY;
 }
 
+static int io_ring_add_registered_fd(struct io_uring_task *tctx, int fd,
+				     int start, int end)
+{
+	struct file *file;
+	int offset;
+
+	file = fget(fd);
+	if (!file) {
+		return -EBADF;
+	} else if (!io_is_uring_fops(file)) {
+		fput(file);
+		return -EOPNOTSUPP;
+	}
+	offset = io_ring_add_registered_file(tctx, file, start, end);
+	if (offset < 0)
+		fput(file);
+	return offset;
+}
+
 /*
  * Register a ring fd to avoid fdget/fdput for each io_uring_enter()
  * invocation. User passes in an array of struct io_uring_rsrc_update