Re: [RFC bpf-next 09/16] bpf: Add BPF_TRAMPOLINE_BATCH_ATTACH support

From: Steven Rostedt <rostedt@goodmis.org>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: "Jiri Olsa" <jolsa@kernel.org>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Andrii Nakryiko" <andriin@fb.com>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"John Fastabend" <john.fastabend@gmail.com>,
	"KP Singh" <kpsingh@chromium.org>, "Daniel Xu" <dxu@dxuuu.xyz>,
	"Jesper Brouer" <jbrouer@redhat.com>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>,
	"Viktor Malik" <vmalik@redhat.com>
Subject: Re: [RFC bpf-next 09/16] bpf: Add BPF_TRAMPOLINE_BATCH_ATTACH support
Date: Fri, 23 Oct 2020 16:31:10 -0400	[thread overview]
Message-ID: <20201023163110.54e4a202@gandalf.local.home> (raw)
In-Reply-To: <CAEf4Bzb_HPmGSoUX+9+LvSP2Yb95OqEQKtjpMiW1Um-rixAM8Q@mail.gmail.com>

On Fri, 23 Oct 2020 13:03:22 -0700
Andrii Nakryiko <andrii.nakryiko@gmail.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