Re: [PATCH RFC v2 bpf-next 1/3] bpf: add bpf_link support for BPF_NETFILTER programs

Stanislav Fomichev
2023-03-23T18:31:29Z

On Wed, Mar 22, 2023 at 5:41 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> Hi Florian, Stan,
>
> On Fri, Mar 03, 2023 at 01:27:52AM +0100, Florian Westphal wrote:
> > Stanislav Fomichev <sdf@google.com> wrote:
> > > On 03/02, Florian Westphal wrote:
> > > > + struct {
> > > > + __u32 pf;
> > > > + __u32 hooknum;
> > > > + __s32 prio;
> > > > + } netfilter;
> > >
> > > For recent tc BPF program extensions, we've discussed that it might be
> > > better
> > > to have an option to attach program before/after another one in the chain.
> > > So the API essentially would receive a before/after flag + fd/id of the
> > >
> > > Should we do something similar here? See [0] for the original
> > > discussion.
> > >
> > > 0: https://lore.kernel.org/bpf/YzzWDqAmN5DRTupQ@google.com/
> >
> > Thanks for the pointer, I will have a look.
> >
> > The above exposes the "prio" of netfilter hooks, so someone
> > that needs their hook to run early on, say, before netfilters
> > nat engine, could just use INT_MIN.
> >
> > We could -- for nf bpf -- make the bpf_link fail if a hook
> > with the same priority already exists to avoid the "undefined
> > behaviour" here (same prio means register order decides what
> > hook function runs first ...).
> >
> > This could be relevant if you have e.g. one bpf program collecting
> > statistics vs. one doing drops.
> >
> > I'll dig though the thread and would try to mimic the tc link
> > mechanism as close as possible.
>
> While I think the direction the TC link discussion took is totally fine,
> TC has the advantage (IIUC) of being a somewhat isolated hook. Meaning
> it does not make sense for a user to mix priority values && before/after
> semantics.
>
> Netfilter is different in that there is by default modules active with
> fixed priority values. So mixing in before/after semantics here could
> get confusing.

I don't remember the details, so pls correct me, but last time I
looked, this priority was basically an ordering within a hook?
And there were a bunch of kernel-hardcoded values. So either that
whole story has to become a UAPI (so the bpf program knows
before/after which kernel hook it has to run), or we need some other
ordering mechanism. (I'm not sure what's the story with bpf vs kernel
hooks interop, so maybe it's all moot?)
Am I missing something? Can you share more about why those fixed
priorities are fine?

> Thanks,
> Daniel