io_uring/net: fix zc fixed buf lifetime

Notifications usually outlive requests, so we need to pin buffers with
it by assigning a rsrc to it instead of the request.

Fixed: b48c312be05e8 ("io_uring/net: simplify zerocopy send user API")
Signed-off-by: Pavel Begunkov <[e-mail]>
Link: https://lore.kernel.org/r/dd64[e-mail]
Reviewed-by: Stefan Metzmacher <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

io_uring/net.c

···
 	if (zc->flags & ~(IORING_RECVSEND_POLL_FIRST |
 			  IORING_RECVSEND_FIXED_BUF))
 		return -EINVAL;
-	if (zc->flags & IORING_RECVSEND_FIXED_BUF) {
-		unsigned idx = READ_ONCE(sqe->buf_index);
-
-		if (unlikely(idx >= ctx->nr_user_bufs))
-			return -EFAULT;
-		idx = array_index_nospec(idx, ctx->nr_user_bufs);
-		req->imu = READ_ONCE(ctx->user_bufs[idx]);
-		io_req_set_rsrc_node(req, ctx, 0);
-	}
 	notif = zc->notif = io_alloc_notif(ctx);
 	if (!notif)
 		return -ENOMEM;
···
 	notif->cqe.res = 0;
 	notif->cqe.flags = IORING_CQE_F_NOTIF;
 	req->flags |= REQ_F_NEED_CLEANUP;
+	if (zc->flags & IORING_RECVSEND_FIXED_BUF) {
+		unsigned idx = READ_ONCE(sqe->buf_index);
+
+		if (unlikely(idx >= ctx->nr_user_bufs))
+			return -EFAULT;
+		idx = array_index_nospec(idx, ctx->nr_user_bufs);
+		req->imu = READ_ONCE(ctx->user_bufs[idx]);
+		io_req_set_rsrc_node(notif, ctx, 0);
+	}
 
 	zc->buf = u64_to_user_ptr(READ_ONCE(sqe->addr));
 	zc->len = READ_ONCE(sqe->len);