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On Fri, Mar 26, 2021 at 02:29:46PM +0000, Ciara Loftus wrote:

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During xsk_socket__create the XDP_RX_RING and XDP_TX_RING

setsockopts

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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@intel.com>
Signed-off-by: Ciara Loftus <redacted>
---
 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

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**xsk_ptr,

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    }
    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.

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+    * If the ring has already been set up setsockopt will return an error.

However,

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+    * this scenario is acceptable as the user may be retrying the socket

creation

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+    * with rings which were set up in a previous but ultimately

unsuccessful call

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+    * to xsk_socket__create(_shared). The call later to mmap() will fail if

there

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+    * 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));

Instead of ignoring the error can you remember that setsockopt was

done

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in struct xsk_socket and don't do it the second time?

Ideally we don't have to ignore the error. However in the event of failure

struct xsk_socket is freed at the end of xsk_socket__create so we can't use it
to remember state between subsequent calls to __create().

but umem is not, right? and fd is taken from there.

Yes, got it. We can add a new field to struct xsk_umem. It's much better than ignoring the return values.
I'll add this in the v3. Thanks for your suggestion!

Ciara