io_uring/register: provide helper to get io_ring_ctx from 'fd'

Can be done in one of two ways:

1) Regular file descriptor, just fget()
2) Registered ring, index our own table for that

In preparation for adding another register use of needing to get a ctx
from a file descriptor, abstract out this helper and use it in the main
register syscall as well.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/register.c

@@ -550,21 +550,16 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 	return ret;
 }
 
-SYSCALL_DEFINE4(io_uring_register, unsigned int, fd, unsigned int, opcode,
-		void __user *, arg, unsigned int, nr_args)
+/*
+ * Given an 'fd' value, return the ctx associated with if. If 'registered' is
+ * true, then the registered index is used. Otherwise, the normal fd table.
+ * Caller must call fput() on the returned file, unless it's an ERR_PTR.
+ */
+struct file *io_uring_register_get_file(int fd, bool registered)
 {
-	struct io_ring_ctx *ctx;
-	long ret = -EBADF;
 	struct file *file;
-	bool use_registered_ring;
 
-	use_registered_ring = !!(opcode & IORING_REGISTER_USE_REGISTERED_RING);
-	opcode &= ~IORING_REGISTER_USE_REGISTERED_RING;
-
-	if (opcode >= IORING_REGISTER_LAST)
-		return -EINVAL;
-
-	if (use_registered_ring) {
+	if (registered) {
 		/*
 		 * Ring fd has been registered via IORING_REGISTER_RING_FDS, we
 		 * need only dereference our task private array to find it.
@@ -572,27 +567,44 @@ SYSCALL_DEFINE4(io_uring_register, unsigned int, fd, unsigned int, opcode,
 		struct io_uring_task *tctx = current->io_uring;
 
 		if (unlikely(!tctx || fd >= IO_RINGFD_REG_MAX))
-			return -EINVAL;
+			return ERR_PTR(-EINVAL);
 		fd = array_index_nospec(fd, IO_RINGFD_REG_MAX);
 		file = tctx->registered_rings[fd];
-		if (unlikely(!file))
-			return -EBADF;
 	} else {
 		file = fget(fd);
-		if (unlikely(!file))
-			return -EBADF;
-		ret = -EOPNOTSUPP;
-		if (!io_is_uring_fops(file))
-			goto out_fput;
 	}
 
+	if (unlikely(!file))
+		return ERR_PTR(-EBADF);
+	if (io_is_uring_fops(file))
+		return file;
+	fput(file);
+	return ERR_PTR(-EOPNOTSUPP);
+}
+
+SYSCALL_DEFINE4(io_uring_register, unsigned int, fd, unsigned int, opcode,
+		void __user *, arg, unsigned int, nr_args)
+{
+	struct io_ring_ctx *ctx;
+	long ret = -EBADF;
+	struct file *file;
+	bool use_registered_ring;
+
+	use_registered_ring = !!(opcode & IORING_REGISTER_USE_REGISTERED_RING);
+	opcode &= ~IORING_REGISTER_USE_REGISTERED_RING;
+
+	if (opcode >= IORING_REGISTER_LAST)
+		return -EINVAL;
+
+	file = io_uring_register_get_file(fd, use_registered_ring);
+	if (IS_ERR(file))
+		return PTR_ERR(file);
 	ctx = file->private_data;
 
 	mutex_lock(&ctx->uring_lock);
 	ret = __io_uring_register(ctx, opcode, arg, nr_args);
 	mutex_unlock(&ctx->uring_lock);
 	trace_io_uring_register(ctx, opcode, ctx->nr_user_files, ctx->nr_user_bufs, ret);
-out_fput:
 	if (!use_registered_ring)
 		fput(file);
 	return ret;

io_uring/register.h

@@ -4,5 +4,6 @@
 
 int io_eventfd_unregister(struct io_ring_ctx *ctx);
 int io_unregister_personality(struct io_ring_ctx *ctx, unsigned id);
+struct file *io_uring_register_get_file(int fd, bool registered);
 
 #endif