libbpf: Only create rx and tx XDP rings when necessary

Prior to this commit xsk_socket__create(_shared) always attempted to create
the rx and tx rings for the socket. However this causes an issue when the
socket being setup is that which shares the fd with the UMEM. If a
previous call to this function failed with this socket after the rings were
set up, a subsequent call would always fail because the rings are not torn
down after the first call and when we try to set them up again we encounter
an error because they already exist. Solve this by remembering whether the
rings were set up by introducing new bools to struct xsk_umem which
represent the ring setup status and using them to determine whether or
not to set up the rings.

Fixes: 1cad078 ("libbpf: add support for using AF_XDP sockets")
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210331061218.1647-4-ciara.loftus@intel.com

tools/lib/bpf/xsk.c

@@ -59,6 +59,8 @@ struct xsk_umem {
 	int fd;
 	int refcount;
 	struct list_head ctx_list;
+	bool rx_ring_setup_done;
+	bool tx_ring_setup_done;
 };
 
 struct xsk_ctx {
@@ -857,6 +859,7 @@ int xsk_socket__create_shared(struct xsk_socket **xsk_ptr,
 	struct xsk_ctx *ctx;
 	int err, ifindex;
 	bool unmap = umem->fill_save != fill;
+	bool rx_setup_done = false, tx_setup_done = false;
 
 	if (!umem || !xsk_ptr || !(rx || tx))
 		return -EFAULT;
@@ -884,6 +887,8 @@ int xsk_socket__create_shared(struct xsk_socket **xsk_ptr,
 		}
 	} else {
 		xsk->fd = umem->fd;
+		rx_setup_done = umem->rx_ring_setup_done;
+		tx_setup_done = umem->tx_ring_setup_done;
 	}
 
 	ctx = xsk_get_ctx(umem, ifindex, queue_id);
@@ -902,23 +907,27 @@ int xsk_socket__create_shared(struct xsk_socket **xsk_ptr,
 	}
 	xsk->ctx = ctx;
 
-	if (rx) {
+	if (rx && !rx_setup_done) {
 		err = setsockopt(xsk->fd, SOL_XDP, XDP_RX_RING,
 				 &xsk->config.rx_size,
 				 sizeof(xsk->config.rx_size));
 		if (err) {
 			err = -errno;
 			goto out_put_ctx;
 		}
+		if (xsk->fd == umem->fd)
+			umem->rx_ring_setup_done = true;
 	}
-	if (tx) {
+	if (tx && !tx_setup_done) {
 		err = setsockopt(xsk->fd, SOL_XDP, XDP_TX_RING,
 				 &xsk->config.tx_size,
 				 sizeof(xsk->config.tx_size));
 		if (err) {
 			err = -errno;
 			goto out_put_ctx;
 		}
+		if (xsk->fd == umem->fd)
+			umem->rx_ring_setup_done = true;
 	}
 
 	err = xsk_get_mmap_offsets(xsk->fd, &off);