Re: [PATCH bpf-next v3 1/5] bpf: Support chain calling multiple BPF programs after each other

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, Oct 14, 2019 at 02:35:45PM +0200, Toke Høiland-Jørgensen wrote:
> Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
> 
> > On Wed, Oct 09, 2019 at 10:03:43AM +0200, Toke Høiland-Jørgensen wrote:
> >> Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
> >> 
> >> > Please implement proper indirect calls and jumps.
> >> 
> >> I am still not convinced this will actually solve our problem; but OK, I
> >> can give it a shot.
> >
> > If you're not convinced let's talk about it first.
> >
> > Indirect calls is a building block for debugpoints.
> > Let's not call them tracepoints, because Linus banned any discusion
> > that includes that name.
> > The debugpoints is a way for BPF program to insert points in its
> > code to let external facility to do tracing and debugging.
> >
> > void (*debugpoint1)(struct xdp_buff *, int code);
> > void (*debugpoint2)(struct xdp_buff *);
> > void (*debugpoint3)(int len);
> 
> So how would these work? Similar to global variables (i.e., the loader
> creates a single-entry PROG_ARRAY map for each one)? Presumably with
> some BTF to validate the argument types?
> 
> So what would it take to actually support this? It doesn't quite sound
> trivial to add?

Depends on definition of 'trivial' :)
The kernel has a luxury of waiting until clean solution is implemented
instead of resorting to hacks.

> > Essentially it's live debugging (tracing) of cooperative bpf programs
> > that added debugpoints to their code.
> 
> Yup, certainly not disputing that this would be useful for debugging;
> although it'll probably be a while before its use becomes widespread
> enough that it'll be a reliable tool for people deploying XDP programs...

same for any new api.

> > Obviously indirect calls can be used for a ton of other things
> > including proper chaing of progs, but I'm convinced that
> > you don't need chaining to solve your problem.
> > You need debugging.
> 
> Debugging is certainly also an area that I want to improve. However, I
> think that focusing on debugging as the driver for chaining programs was
> a mistake on my part; rudimentary debugging (using a tool such as
> xdpdump) is something that falls out of program chaining, but it's not
> the main driver for it.

xdpdump can be done already the way I suggested without adding new kernel
code and it will work on old-ish kernels. Aside from xdp itself
the other requirement is to have get_fd_by_id sys_bpf command.

> > If you disagree please explain _your_ problem again.
> > Saying that fb katran is a use case for chaining is, hrm, not correct.
> 
> I never said Katran was the driver for this. I just used Katran as one
> of the "prior art" examples for my "how are people solving running
> multiple programs on the same interface" survey.

and they solved it. that's the point.

> What I want to achieve is simply the ability to run multiple independent
> XDP programs on the same interface, without having to put any
> constraints on the programs themselves. I'm not disputing that this is
> *possible* to do completely in userspace, I just don't believe the
> resulting solution will be very good.

What makes me uneasy about the whole push for program chaining
is that tc cls_bpf supported multiple independent programs from day one.
Yet it doesn't help to run two firewalls hooked into tc ingress.
Similarly cgroup-bpf had a ton discussions on proper multi-prog api.
Everyone was eventually convinced that it's flexible and generic.
Yet people who started to use it complain that it's missing features
to make it truly usable in production.
Tracing is the only bit where multi-prog works.
Because kernel always runs all programs there.
If we could use PROG_RUN_ARRAY for XDP that could have been a solution.
But we cannot. Return codes matter for XDP.