Re: bpf: ability to attach freplace to multiple parents

Andrey Ignatov
2020-04-02T21:25:08Z

Alexei Starovoitov <alexei.starovoitov@gmail.com> [Thu, 2020-04-02 13:22 -0700]:
> On Fri, Mar 27, 2020 at 12:11:15PM +0100, Toke Høiland-Jørgensen wrote:
> >
> > Current code is in [0], for those following along. There are two bits of
> > kernel support missing before I can get it to where I want it for an
> > initial "release": Atomic replace of the dispatcher (this series), and
> > the ability to attach an freplace program to more than one "parent".
> > I'll try to get an RFC out for the latter during the merge window, but
> > I'll probably need some help in figuring out how to make it safe from
> > the verifier PoV.
>
> I have some thoughts on the second part "ability to attach an freplace
> to more than one 'parent'".
> I think the solution should be more generic than just freplace.
> fentry/fexit need to have the same feature.
> Few folks already said that they want to attach fentry to multiple
> kernel functions. It's similar to what people do with kprobe progs now.
> (attach to multiple and differentiate attach point based on parent IP)
> Similarly "bpftool profile" needs it to avoid creating new pair of fentry/fexit
> progs for every target bpf prog it's collecting stats about.
> I didn't add this ability to fentry/fexit/freplace only to simplify
> initial implementation ;) I think the time had come.
> Currently fentry/fexit/freplace progs have single prog->aux->linked_prog pointer.
> It just needs to become a linked list.
> The api extension could be like this:
> bpf_raw_tp_open(prog_fd, attach_prog_fd, attach_btf_id);
> (currently it's just bpf_raw_tp_open(prog_fd))
> The same pair of (attach_prog_fd, attach_btf_id) is already passed into prog_load
> to hold the linked_prog and its corresponding btf_id.
> I'm proposing to extend raw_tp_open with this pair as well to
> attach existing fentry/fexit/freplace prog to another target.
> Internally the kernel verify that btf of current linked_prog
> exactly matches to btf of another requested linked_prog and
> if they match it will attach the same prog to two target programs (in case of freplace)
> or two kernel functions (in case of fentry/fexit).
>
> Toke, Andrey,
> if above kinda makes sense from high level description
> I can prototype it quickly and then we can discuss details
> in the patches ?
> Or we can drill further into details and discuss corner cases.

That makes sense to me.

I've also been thinking of a way to "transition" ext prog from one
target program to another, but I had an impression that limiting number
of target progs to one for an ext prog is "by design" and hard to
change, and was looking at introducing a way to duplicate existing ext
prog by its fd but with different attach_prog_fd and attach_btf_id (smth
like BPF_PROG_DUP command) instead.

But since you're saying that there are actually many use-cases to be
able to attach freplace/fexit/fentry to multiple target programs, that
works as well. Happy to look at the prototype when it's available.

Thanks.

--
Andrey Ignatov