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

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Message-ID: <20210327022729.cgizt5xnhkerbrmy@ast-mbp>
Date:   Fri, 26 Mar 2021 19:27:29 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Ciara Loftus <ciara.loftus@...el.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        magnus.karlsson@...el.com, bjorn@...nel.org,
        magnus.karlsson@...il.com
Subject: Re: [PATCH v2 bpf 3/3] libbpf: ignore return values of setsockopt
 for XDP rings.

On Fri, Mar 26, 2021 at 02:29:46PM +0000, Ciara Loftus 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")
> 
> 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));

Instead of ignoring the error can you remember that setsockopt was done
in struct xsk_socket and don't do it the second time?