io_uring: limit size of provided buffer ring

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>
Link: https://lore.kernel.org/r/20220613101157.3687-4-dylany@fb.comReviewed-by: Hao Xu <howeyxu@tencent.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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)