io_uring: abstract out provided buffer list selection

In preparation for providing another way to select a buffer, move the
existing logic into a helper.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3448,29 +3448,41 @@ static int io_buffer_add_list(struct io_ring_ctx *ctx,
 	return xa_err(xa_store(&ctx->io_bl_xa, bgid, bl, GFP_KERNEL));
 }
 
+static void __user *io_provided_buffer_select(struct io_kiocb *req, size_t *len,
+					      struct io_buffer_list *bl,
+					      unsigned int issue_flags)
+{
+	struct io_buffer *kbuf;
+
+	if (list_empty(&bl->buf_list))
+		return ERR_PTR(-ENOBUFS);
+
+	kbuf = list_first_entry(&bl->buf_list, struct io_buffer, list);
+	list_del(&kbuf->list);
+	if (*len > kbuf->len)
+		*len = kbuf->len;
+	req->flags |= REQ_F_BUFFER_SELECTED;
+	req->kbuf = kbuf;
+	io_ring_submit_unlock(req->ctx, issue_flags);
+	return u64_to_user_ptr(kbuf->addr);
+}
+
 static void __user *io_buffer_select(struct io_kiocb *req, size_t *len,
 				     unsigned int issue_flags)
 {
-	struct io_buffer *kbuf = req->kbuf;
 	struct io_ring_ctx *ctx = req->ctx;
 	struct io_buffer_list *bl;
 
 	io_ring_submit_lock(req->ctx, issue_flags);
 
 	bl = io_buffer_get_list(ctx, req->buf_index);
-	if (bl && !list_empty(&bl->buf_list)) {
-		kbuf = list_first_entry(&bl->buf_list, struct io_buffer, list);
-		list_del(&kbuf->list);
-		if (*len > kbuf->len)
-			*len = kbuf->len;
-		req->flags |= REQ_F_BUFFER_SELECTED;
-		req->kbuf = kbuf;
+	if (unlikely(!bl)) {
 		io_ring_submit_unlock(req->ctx, issue_flags);
-		return u64_to_user_ptr(kbuf->addr);
+		return ERR_PTR(-ENOBUFS);
 	}
 
-	io_ring_submit_unlock(req->ctx, issue_flags);
-	return ERR_PTR(-ENOBUFS);
+	/* selection helpers drop the submit lock again, if needed */
+	return io_provided_buffer_select(req, len, bl, issue_flags);
 }
 
 #ifdef CONFIG_COMPAT