netdev - [PATCH v2 bpf 3/3] libbpf: ignore return values of setsockopt for XDP rings.

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Date:   Fri, 26 Mar 2021 14:29:46 +0000
From:   Ciara Loftus <ciara.loftus@...el.com>
To:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        magnus.karlsson@...el.com, bjorn@...nel.org,
        magnus.karlsson@...il.com
Cc:     Ciara Loftus <ciara.loftus@...el.com>
Subject: [PATCH v2 bpf 3/3] libbpf: ignore return values of setsockopt for XDP rings.

During xsk_socket__create the XDP_RX_RING and XDP_TX_RING setsockopts
are called to create the rx and tx rings for the AF_XDP socket. If the ring
has already been set up, the setsockopt will return an error. However,
in the event of a failure during xsk_socket__create(_shared) after the
rings have been set up, the user may wish to retry the socket creation
using these pre-existing rings. In this case we can ignore the error
returned by the setsockopts. If there is a true error, the subsequent
call to mmap() will catch it.

Fixes: 1cad07884239 ("libbpf: add support for using AF_XDP sockets")

Acked-by: Magnus Karlsson <magnus.karlsson@...el.com>
Signed-off-by: Ciara Loftus <ciara.loftus@...el.com>
---
 tools/lib/bpf/xsk.c | 34 ++++++++++++++++------------------
 1 file changed, 16 insertions(+), 18 deletions(-)

diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
index d4991ddff05a..cfc4abf505c3 100644
--- a/tools/lib/bpf/xsk.c
+++ b/tools/lib/bpf/xsk.c
@@ -900,24 +900,22 @@ int xsk_socket__create_shared(struct xsk_socket **xsk_ptr,
 	}
 	xsk->ctx = ctx;
 
-	if (rx) {
-		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 (tx) {
-		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;
-		}
-	}
+	/* The return values of these setsockopt calls are intentionally not checked.
+	 * If the ring has already been set up setsockopt will return an error. However,
+	 * this scenario is acceptable as the user may be retrying the socket creation
+	 * with rings which were set up in a previous but ultimately unsuccessful call
+	 * to xsk_socket__create(_shared). The call later to mmap() will fail if there
+	 * is a real issue and we handle that return value appropriately there.
+	 */
+	if (rx)
+		setsockopt(xsk->fd, SOL_XDP, XDP_RX_RING,
+			   &xsk->config.rx_size,
+			   sizeof(xsk->config.rx_size));
+
+	if (tx)
+		setsockopt(xsk->fd, SOL_XDP, XDP_TX_RING,
+			   &xsk->config.tx_size,
+			   sizeof(xsk->config.tx_size));
 
 	err = xsk_get_mmap_offsets(xsk->fd, &off);
 	if (err) {
-- 
2.17.1