io_uring/tctx: work around xa_store() allocation error issue

syzbot triggered the following WARN_ON:

WARNING: CPU: 0 PID: 16 at io_uring/tctx.c:51 __io_uring_free+0xfa/0x140 io_uring/tctx.c:51

which is the

WARN_ON_ONCE(!xa_empty(&tctx->xa));

sanity check in __io_uring_free() when a io_uring_task is going through
its final put. The syzbot test case includes injecting memory allocation
failures, and it very much looks like xa_store() can fail one of its
memory allocations and end up with ->head being non-NULL even though no
entries exist in the xarray.

Until this issue gets sorted out, work around it by attempting to
iterate entries in our xarray, and WARN_ON_ONCE() if one is found.

Reported-by: syzbot+cc36d44ec9f368e443d3@syzkaller.appspotmail.com
Link: https://lore.kernel.org/io-uring/673c1643.050a0220.87769.0066.GAE@google.com/
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/tctx.c

@@ -47,8 +47,19 @@ static struct io_wq *io_init_wq_offload(struct io_ring_ctx *ctx,
 void __io_uring_free(struct task_struct *tsk)
 {
 	struct io_uring_task *tctx = tsk->io_uring;
+	struct io_tctx_node *node;
+	unsigned long index;
 
-	WARN_ON_ONCE(!xa_empty(&tctx->xa));
+	/*
+	 * Fault injection forcing allocation errors in the xa_store() path
+	 * can lead to xa_empty() returning false, even though no actual
+	 * node is stored in the xarray. Until that gets sorted out, attempt
+	 * an iteration here and warn if any entries are found.
+	 */
+	xa_for_each(&tctx->xa, index, node) {
+		WARN_ON_ONCE(1);
+		break;
+	}
 	WARN_ON_ONCE(tctx->io_wq);
 	WARN_ON_ONCE(tctx->cached_refs);