netdev - Re: [RFC bpf-next 09/16] bpf: Add BPF_TRAMPOLINE_BATCH

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Date:   Fri, 23 Oct 2020 16:31:10 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>, Daniel Xu <dxu@...uu.xyz>,
        Jesper Brouer <jbrouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Viktor Malik <vmalik@...hat.com>
Subject: Re: [RFC bpf-next 09/16] bpf: Add BPF_TRAMPOLINE_BATCH_ATTACH
 support

On Fri, 23 Oct 2020 13:03:22 -0700
Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:

> Basically, maybe ftrace subsystem could provide a set of APIs to
> prepare a set of functions to attach to. Then BPF subsystem would just
> do what it does today, except instead of attaching to a specific
> kernel function, it would attach to ftrace's placeholder. I don't know
> anything about ftrace implementation, so this might be far off. But I
> thought that looking at this problem from a bit of a different angle
> would benefit the discussion. Thoughts?

I probably understand bpf internals as much as you understand ftrace
internals ;-)

Anyway, what I'm currently working on, is a fast way to get to the
arguments of a function. For now, I'm just focused on x86_64, and only add
6 argments.

The main issue that Alexei had with using the ftrace trampoline, was that
the only way to get to the arguments was to set the "REGS" flag, which
would give a regs parameter that contained a full pt_regs. The problem with
this approach is that it required saving *all* regs for every function
traced. Alexei felt that this was too much overehead.

Looking at Jiri's patch, I took a look at the creation of the bpf
trampoline, and noticed that it's copying the regs on a stack (at least
what is used, which I think could be an issue).

For tracing a function, one must store all argument registers used, and
restore them, as that's how they are passed from caller to callee. And
since they are stored anyway, I figure, that should also be sent to the
function callbacks, so that they have access to them too.

I'm working on a set of patches to make this a reality.

-- Steve