[PATCH 3/3] io_uring: limit size of provided buffer ring

[Dylan Yudaken <dylany@fb.com>]

The type of head and tail do not allow more than 2^15 entries in a
provided buffer ring, so do not allow this.
At 2^16 while each entry can be indexed, there is no way to
disambiguate full vs empty.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
---
 fs/io_uring.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 6eea18e8330c..85b116ddfd2a 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -13002,6 +13002,10 @@ static int io_register_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
 	if (!is_power_of_2(reg.ring_entries))
 		return -EINVAL;
 
+	/* cannot disambiguate full vs empty due to head/tail size */
+	if (reg.ring_entries >= 65536)
+		return -EINVAL;
+
 	if (unlikely(reg.bgid < BGID_ARRAY && !ctx->io_bl)) {
 		int ret = io_init_bl_list(ctx);
 		if (ret)
-- 
2.30.2