io_uring/zcrx: warn on freelist violations

The freelist is appropriately sized to always be able to take a free
niov, but let's be more defensive and check the invariant with a
warning. That should help to catch any double-free issues.

Suggested-by: Kai Aizen <[email protected]>
Signed-off-by: Pavel Begunkov <[email protected]>
Link: https://patch.msgid.link/2f3cea363b04649755e3b6bb9ab66485a95936d5.1776760901.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <[email protected]>

Author: isilence

io_uring/zcrx.c

@@ -602,6 +602,8 @@ static void io_zcrx_return_niov_freelist(struct net_iov *niov)
 	struct io_zcrx_area *area = io_zcrx_iov_to_area(niov);
 
 	guard(spinlock_bh)(&area->freelist_lock);
+	if (WARN_ON_ONCE(area->free_count >= area->nia.num_niovs))
+		return;
 	area->freelist[area->free_count++] = net_iov_idx(niov);
 }