netdev - Re: [PATCH bpf-next v5 2/3] bpf_xdp_redirect_map: Perform map lookup in eBPF helper

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Date:   Thu, 27 Jun 2019 23:55:56 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        netdev@...r.kernel.org
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        David Miller <davem@...emloft.net>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: [PATCH bpf-next v5 2/3] bpf_xdp_redirect_map: Perform map lookup
 in eBPF helper

On 06/23/2019 04:17 AM, Toke Høiland-Jørgensen wrote:
> From: Toke Høiland-Jørgensen <toke@...hat.com>
> 
> The bpf_redirect_map() helper used by XDP programs doesn't return any
> indication of whether it can successfully redirect to the map index it was
> given. Instead, BPF programs have to track this themselves, leading to
> programs using duplicate maps to track which entries are populated in the
> devmap.
> 
> This patch fixes this by moving the map lookup into the bpf_redirect_map()
> helper, which makes it possible to return failure to the eBPF program. The
> lower bits of the flags argument is used as the return code, which means
> that existing users who pass a '0' flag argument will get XDP_ABORTED.
> 
> With this, a BPF program can check the return code from the helper call and
> react by, for instance, substituting a different redirect. This works for
> any type of map used for redirect.
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>

Overall series looks good to me. Just very small things inline here & in the
other two patches:

[...]
> @@ -3750,9 +3742,16 @@ BPF_CALL_3(bpf_xdp_redirect_map, struct bpf_map *, map, u32, ifindex,
>  {
>  	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>  
> -	if (unlikely(flags))
> +	/* Lower bits of the flags are used as return code on lookup failure */
> +	if (unlikely(flags > XDP_TX))
>  		return XDP_ABORTED;
>  
> +	ri->item = __xdp_map_lookup_elem(map, ifindex);
> +	if (unlikely(!ri->item)) {
> +		WRITE_ONCE(ri->map, NULL);

This WRITE_ONCE() is not needed. We never set it before at this point.

> +		return flags;
> +	}
> +
>  	ri->ifindex = ifindex;
>  	ri->flags = flags;
>  	WRITE_ONCE(ri->map, map);
>