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

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Message-ID: <d0d5dbe3-d2bf-2284-b2a3-667c77487125@iogearbox.net>
Date:   Fri, 28 Jun 2019 00:08:56 +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>
[...]
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 183bf4d8e301..a6779e1cc1b8 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3605,17 +3605,13 @@ static int xdp_do_redirect_map(struct net_device *dev, struct xdp_buff *xdp,
>  			       struct bpf_redirect_info *ri)
>  {
>  	u32 index = ri->ifindex;
> -	void *fwd = NULL;
> +	void *fwd = ri->item;
>  	int err;
>  
>  	ri->ifindex = 0;
> +	ri->item = NULL;
>  	WRITE_ONCE(ri->map, NULL);
>  
> -	fwd = __xdp_map_lookup_elem(map, index);
> -	if (unlikely(!fwd)) {
> -		err = -EINVAL;
> -		goto err;
> -	}

If you look at the _trace_xdp_redirect{,_err}(), we should also get rid of the
extra NULL test in devmap_ifindex() which is not under tracepoint static key.