netdev - Re: [PATCH v2 bpf-next 0/3] bpf: Program extensions or dynamic re-linking

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Date:   Tue, 21 Jan 2020 09:00:01 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>, davem@...emloft.net,
        daniel@...earbox.net, netdev@...r.kernel.org, bpf@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH v2 bpf-next 0/3] bpf: Program extensions or dynamic
 re-linking

On Tue, Jan 21, 2020 at 04:37:31PM +0100, Toke Høiland-Jørgensen wrote:
> Alexei Starovoitov <ast@...nel.org> writes:
> 
> > The last few month BPF community has been discussing an approach to call
> > chaining, since exiting bpt_tail_call() mechanism used in production XDP
> > programs has plenty of downsides. The outcome of these discussion was a
> > conclusion to implement dynamic re-linking of BPF programs. Where rootlet XDP
> > program attached to a netdevice can programmatically define a policy of
> > execution of other XDP programs. Such rootlet would be compiled as normal XDP
> > program and provide a number of placeholder global functions which later can be
> > replaced with future XDP programs. BPF trampoline, function by function
> > verification were building blocks towards that goal. The patch 1 is a final
> > building block. It introduces dynamic program extensions. A number of
> > improvements like more flexible function by function verification and better
> > libbpf api will be implemented in future patches.
> 
> This is great, thank you! I'll go play around with it; couldn't spot
> anything obvious from eye-balling the code, except that yeah, it does
> need a more flexible libbpf api :)
> 
> One thing that's not obvious to me: How can userspace tell which
> programs replace which functions after they are loaded? Is this put into
> prog_tags in struct bpf_prog_info, or?

good point. Would be good to extend bpf_prog_info. Since prog-to-prog
connection is unidirectional the bpf_prog_info of extension prog will be able
to say which original program it's replacing. bpftool prog show will be able to
print all this data. I think fenry/fexit progs would need the same
bpf_prog_info extension. attach_prog_id + attach_btf_id would be enough.
In the mean time I can try to hack drgn script to do the same.