Re: [PATCH net-next] ipip: reject unsupported configurations in fill_forward_path

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> The ipip fill_forward_path callback currently does not check for
> configurations that cannot be offloaded to hardware:
> 
> - Collect metadata (flow-based) tunnels have no fixed destination
>   and rely on per-packet tunnel metadata, so the forward path
>   cannot be pre-computed.
> 
> - TOS inheritance (parms.iph.tos & 0x1) requires copying the outer
>   TOS from the inner packet at encapsulation time, which is not
>   known during forward path resolution.
> 
> Return -EOPNOTSUPP for both cases to fall back to the software
> forwarding path.
> 
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>  net/ipv4/ipip.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/net/ipv4/ipip.c b/net/ipv4/ipip.c
> index d1aa048a6099..0831f6b81717 100644
> --- a/net/ipv4/ipip.c
> +++ b/net/ipv4/ipip.c
> @@ -360,6 +360,12 @@ static int ipip_fill_forward_path(struct net_device_path_ctx *ctx,
>  	const struct iphdr *tiph = &tunnel->parms.iph;
>  	struct rtable *rt;
>  
> +	if (tunnel->collect_md)
> +		return -EOPNOTSUPP;
> +
> +	if (tunnel->parms.iph.tos & 0x1)
> +		return -EOPNOTSUPP;
> +
>  	rt = ip_route_output(dev_net(ctx->dev), tiph->daddr, tiph->saddr,
>  			     inet_dsfield_to_dscp(tiph->tos),
>  			     tunnel->parms.link, RT_SCOPE_UNIVERSE);
> 

commenting on sashiko's report:
https://sashiko.dev/#/patchset/20260725-ipip-fill-forward-path-fix-v1-1-bc69fd3127d5%40kernel.org

- This is a pre-existing issue, but should the IPv6 equivalent callback,
  ip6_tnl_fill_forward_path() in net/ipv6/ip6_tunnel.c, also receive similar
  checks?
  - already fixed in the following patch:
    https://lore.kernel.org/netdev/20260724-ip6ip6-route-lookup-fill_forward_path-v3-1-7b7991538614@kernel.org/

Regards,
Lorenzo

> ---
> base-commit: 04026c998c24ac47eb76886b9790c5710b603eb4
> change-id: 20260725-ipip-fill-forward-path-fix-543c41000a57
> 
> Best regards,
> -- 
> Lorenzo Bianconi <lorenzo@kernel.org>
> 

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