Src addr code review (Was: Source IP incorrect on multi homed systems)

[曹煜]

Hi all,
I've hacked that source code myself months ago, and it works well on
my use case (I have 4 dual stack pppoe wan set on my openwrt router,
and seted a wireguard sever on it), my hack will pickup the dst_addr
from incoming handshake packet in kernel sk_buff, and then use that
addr as src_addr to reply.
I'm not good at source code, and I know that my hack may be ugly, but
it works, hope this patch can help:
https://github.com/openwrt/packages/issues/9538#issuecomment-1150592803

Daniel Gröber <dxld at darkboxed.org> 于2023年2月20日周一 06:42写道：
>
> Hi,
>
> I though it might be useful to do some quick and dirty code review instead
> of speculating wildly to figure out where these source IP selection
> problems could be coming from ;)
>
> From previous code deep dives I know the udp_tunnel_xmit_skb function is
> where tunnel packets get handed off to the kernel. So in
> net/wireguard/socket.c:send4 we have:
>
>         udp_tunnel_xmit_skb(rt, sock, skb, fl.saddr, fl.daddr, ds,
>                             ip4_dst_hoplimit(&rt->dst), 0, fl.fl4_sport,
>                             fl.fl4_dport, false, false);
>
> Where fl.saddr is the source address that's supposedly wrong (sometimes? I
> guess?) Where does that come from?
>
> Let's look at the code (heavily culled):
>
>         struct flowi4 fl = {
>                 .saddr = endpoint->src4.s_addr,
>         };
>         if (cache)
>                 rt = dst_cache_get_ip4(cache, &fl.saddr);
>         if (!rt) {
>                 if (unlikely(!inet_confirm_addr(sock_net(sock), NULL, 0,
>                                                 fl.saddr, RT_SCOPE_HOST)))
>                         fl.saddr = 0;
>                 if (unlikely(endpoint->src_if4 && ((IS_ERR(rt) &&
>                              PTR_ERR(rt) == -EINVAL) || (!IS_ERR(rt) &&
>                              rt->dst.dev->ifindex != endpoint->src_if4))))
>                         fl.saddr = 0;
>
> Well it's initialized from endpoint->src4.s_addr, overwritten with zero in
> some cases, which I believe lets the kernel do it's regular source addr
> selection, and populated from something called dst_cache at some callsites.
>
> @Nico could it perhaps simply be that you're hitting one of these zero'ing
> cases and that's why it's using regular kernel src addr selection instead
> of the cached endpoint src4 address?
>
> The first case !inet_confirm_addr(..., RT_SCOPE_HOST) ought to confirm that
> the saddr is actually still a local address. Makes sens if the address we
> remembered was removed from the interface we can't use it anymore.
>
> The second case looks like it's checking if the (sometimes cached) src_if4
> interface index is still what the route we're about to use points to.
>
> If neither of those seem likely we can keep reading :)
>
> --Daniel
>
>
>