netdev - Re: [PATCH bpf 4/5] bpf: Set flow flag to allow any source IP in bpf_tunnel

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Date:   Fri, 15 Jul 2022 11:21:08 -0700
From:   Yonghong Song <yhs@...com>
To:     Paul Chaignon <paul@...valent.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        Kaixi Fan <fankaixi.li@...edance.com>,
        Nikolay Aleksandrov <razor@...ckwall.org>
Subject: Re: [PATCH bpf 4/5] bpf: Set flow flag to allow any source IP in
 bpf_tunnel_key



On 7/15/22 8:02 AM, Paul Chaignon wrote:
> Commit 26101f5ab6bd ("bpf: Add source ip in "struct bpf_tunnel_key"")
> added support for getting and setting the outer source IP of encapsulated
> packets via the bpf_skb_{get,set}_tunnel_key BPF helper. This change
> allows BPF programs to set any IP address as the source, including for
> example the IP address of a container running on the same host.
> 
> In that last case, however, the encapsulated packets are dropped when
> looking up the route because the source IP address isn't assigned to any
> interface on the host. To avoid this, we need to set the
> FLOWI_FLAG_ANYSRC flag.
> 
> Fixes: 26101f5ab6bd ("bpf: Add source ip in "struct bpf_tunnel_key"")
> Signed-off-by: Paul Chaignon <paul@...valent.com>
> ---
>   net/core/filter.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 5d16d66727fc..6d9c800cdab9 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4641,6 +4641,7 @@ BPF_CALL_4(bpf_skb_set_tunnel_key, struct sk_buff *, skb,
>   	info->key.tun_id = cpu_to_be64(from->tunnel_id);
>   	info->key.tos = from->tunnel_tos;
>   	info->key.ttl = from->tunnel_ttl;
> +	info->key.flow_flags = FLOWI_FLAG_ANYSRC;

Can we set FLOWI_FLAG_ANYSRC in all conditions?
In lwt_bpf.c, func bpf_lwt_xmit_reroute(), FLOWI_FLAG_ANYSRC
is set for ipv4 but not for ipv6. I am wondering whether
FLOWI_FLAG_ANYSRC needs to be set for ipv6 packet or not
in bpf_skb_set_tunnel_key().

>   
>   	if (flags & BPF_F_TUNINFO_IPV6) {
>   		info->mode |= IP_TUNNEL_INFO_IPV6;