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

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Date:   Fri, 26 Mar 2021 10:14:11 +0100
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     Ciara Loftus <ciara.loftus@...el.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn@...nel.org>
Subject: Re: [PATCH bpf 3/3] libbpf: ignore return values of setsockopt for
 XDP rings.

On Wed, Mar 24, 2021 at 3:46 PM Ciara Loftus <ciara.loftus@...el.com> wrote:
>
> 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")
>
> 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 ec3c23299329..1f1c4c11c292 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -904,24 +904,22 @@ 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));

Thanks Ciara!

This is a pragmatic solution, but I do not see any better way around
it since these operations are irreversible. And it works without any
fix to the kernel which is good and you have a comment explaining
things clearly. With that said, it would be nice as a follow up to
bpf-next to actually return a unique error value (among the ones that
this function can return) when the rings have already been mapped.
This way the user can react to this in a more informed way in the
future.

Acked-by: Magnus Karlsson <magnus.karlsson@...el.com>

>
>         err = xsk_get_mmap_offsets(xsk->fd, &off);
>         if (err) {
> --
> 2.17.1
>