Re: [PATCH net] net: clear the dst when changing skb protocol

[Jakub Kicinski <kuba@kernel.org>]

On Thu, 5 Jun 2025 15:50:31 +0200 Maciej Żenczykowski wrote:
> On Thu, Jun 5, 2025 at 3:22 PM Jakub Kicinski <kuba@kernel.org> wrote:
> > > I wonder if this shouldn't drop dst even when doing ipv4->ipv4 or
> > > ipv6->ipv6 -- it's encapping, presumably old dst is irrelevant...  
> >
> > I keep going back and forth on this. You definitely have a point,
> > but I feel like there are levels to how BPF prog can make the dst
> > irrelevant:
> >  - change proto
> >  - encap
> >  - adjust room but not set any encap flag
> >  - overwrite the addrs without calling any helpers
> > First case we have to cover for safety, last we can't possibly cover.
> > So the question is whether we should draw the line somewhere in
> > the middle, or leave this patch as is and if the actual use case arrives
> > - let BPF call skb_dst_drop() as a kfunc. Right now I'm leaning towards
> > the latter.
> >
> > Does that make sense? Does anyone else have an opinion?  
> 
> It does make a fair bit of sense.
> Question: does calling it as a kfunc require kernel BTF?
> Specifically some ram limited devices want to disable CONFIG_DEBUG_INFO_BTF...
> I know normal bpf helpers don't need that...
> I guess you could always convert ipv4 -> ipv6 -> ipv4 ;-)

Not sure how BPF folks feel about that, but technically we could
also add a flag to bpf_skb_adjust_room() or bpf_skb_change_proto().