[PATCH v2 2/2] io_uring: require 3 free CQ slots for 32b CQE in io_fill_nop_cqe()

[Hui Peng <benquike@gmail.com>]

When io_cqe_cache_refill() is called for a 32-byte CQE at the last slot
of an IORING_SETUP_CQE_MIXED ring (off + 1 == ctx->cq_entries),
io_fill_nop_cqe() writes an IORING_CQE_F_SKIP CQE at the last slot and
increments cached_cq_tail, consuming 1 free slot, after which the 32-byte
CQE itself requires 2 more contiguous free slots at index 0.

Currently, io_fill_nop_cqe() only checks io_cqring_queued(ctx) <
ctx->cq_entries (free >= 1). When free is 1 or 2, io_fill_nop_cqe()
succeeds and increments cached_cq_tail for the skip CQE, and then
io_cqe_cache_refill() computes len = min(free, ctx->cq_entries - off) < 2
and returns false because len < (cqe32 + 1). This leaves an orphan skip
CQE in the ring with cached_cq_tail already incremented while the 32-byte
CQE is deferred to cq_overflow_list.

Require at least 3 free slots (io_cqring_queued(ctx) + 3 <=
ctx->cq_entries) in io_fill_nop_cqe() before emitting the skip CQE.

Tested in QEMU against Linux 7.3.0-rc3 on a 4-entry IORING_SETUP_CQE_MIXED
ring with 2 queued CQEs (head = 1, tail = 3, free = 2, off = 3) followed
by a 32-byte NOP (IORING_NOP_CQE32): on the unfixed kernel cq_tail
advances to 4 due to the orphan skip CQE while the 32-byte CQE overflows,
whereas with the fix applied cq_tail remains at 3 and both slots remain
consistent.

Fixes: e26dca67fde1 ("io_uring: add support for IORING_SETUP_CQE_MIXED")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v2:
- Split out as patch 2/2 as requested by Jens Axboe.

 io_uring/io_uring.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index ae7c77158c58..b430301fead9 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -698,7 +698,12 @@ static unsigned int io_cqring_queued(struct io_ring_ctx *ctx)
  */
 static bool io_fill_nop_cqe(struct io_ring_ctx *ctx, unsigned int off)
 {
-	if (io_cqring_queued(ctx) < ctx->cq_entries) {
+	/*
+	 * Creating a skip CQE and posting a 32b CQE requires 3 free CQ slots
+	 * in total (1 for the skip CQE at the end of the ring and 2 for the
+	 * 32b CQE at the start of the ring).
+	 */
+	if (io_cqring_queued(ctx) + 3 <= ctx->cq_entries) {
 		struct io_uring_cqe *cqe = &ctx->rings->cqes[off];
 
 		cqe->user_data = 0;
-- 
2.55.0.1082.g2b9226bbc0-goog