Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a struct/union

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 2 Aug 2023 17:47:03 +0200
Florent Revest <revest@chromium.org> wrote:

> On Wed, Aug 2, 2023 at 3:56 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> > On Tue, 1 Aug 2023 20:40:54 -0400
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > > On Wed, 2 Aug 2023 09:21:46 +0900
> > > Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> > >
> > > > > Then use kprobes. When I asked Masami what the difference between fprobes
> > > > > and kprobes was, he told me that it would be that it would no longer rely
> > > > > on the slower FTRACE_WITH_REGS. But currently, it still does.
> > > >
> > > > kprobes needs to keep using pt_regs because software-breakpoint exception
> > > > handler gets that. And fprobe is used for bpf multi-kprobe interface,
> > > > but I think it can be optional.
> > > >
> > > > So until user-land tool supports the ftrace_regs, you can just disable
> > > > using fprobes if CONFIG_DYNAMIC_FTRACE_WITH_REGS=n
> > >
> > > I'm confused. I asked about the difference between kprobes on ftrace
> > > and fprobes, and you said it was to get rid of the requirement of
> > > FTRACE_WITH_REGS.
> > >
> > >  https://lore.kernel.org/all/20230120205535.98998636329ca4d5f8325bc3@kernel.org/
> >
> > Yes, it is for enabling fprobe (and fprobe-event) on more architectures.
> > I don't think it's possible to change everything at once. So, it will be
> > changed step by step. At the first step, I will replace pt_regs with
> > ftrace_regs, and make bpf_trace.c and fprobe_event depends on
> > FTRACE_WITH_REGS.
> >
> > At this point, we can split the problem into two, how to move bpf on
> > ftrace_regs and how to move fprobe-event on ftrace_regs. fprobe-event
> > change is not hard because it is closing in the kernel and I can do it.
> > But for BPF, I need to ask BPF user-land tools to support ftrace_regs.
> 
> Ah! I finally found the branch where I had pushed my proof of concept
> of fprobe with ftrace_regs... it's a few months old and I didn't get
> it in a state such that it could be sent to the list but maybe this
> can save you a little bit of lead time Masami :) (especially the bpf
> and arm64 specific bits)
> 
> https://github.com/FlorentRevest/linux/commits/bpf-arm-complete
> 
> 08afb628c6e1 ("ftrace: Add a macro to forge an incomplete pt_regs from
> a ftrace_regs")
> 203e96fe1790 ("fprobe, rethook: Use struct ftrace_regs instead of
> struct pt_regs")
> 1a9e280b9b16 ("arm64,rethook,kprobes: Replace kretprobe with rethook on arm64")
> 7751c6db9f9d ("bpf: Fix bpf get_func_ip() on arm64 multi-kprobe programs")
> a10c49c0d717 ("selftests/bpf: Update the tests deny list on aarch64")

Thanks for the work! I also pushed my patches on 

https://kernel.googlesource.com/pub/scm/linux/kernel/git/mhiramat/linux/+/refs/heads/topic/fprobe-ftrace-regs

628e6c19d7dc ("tracing/fprobe: Enable fprobe events with CONFIG_DYNAMIC_FTRACE_WITH_ARGS")
311c98c29cfd ("fprobe: Use fprobe_regs in fprobe entry handler")

This doesn't cover arm64 and rethook, but provides ftrace_regs optimized
fprobe-event code, which uses a correct APIs for ftrace_regs.

For the rethook we still need to provide 2 version for kretprobe(pt_regs)
and fprobe(ftrace_regs).
I think eventually we should replace the kretprobe with fprobe, but
current rethook is tightly coupled with kretprobe and the kretprobe
needs pt_regs. So, I would like to keep arm64 kretprobe impl, and add
new rethook with ftrace_regs.

Or, maybe we need these 2 configs intermediately.
CONFIG_RETHOOK_WITH_REGS - in this case, kretprobe uses rethook
CONFIG_RETHOOK_WITH_ARGS - in this case, kretprobe uses its own stack

The problem is ftrace_regs only depends on CONFIG_DYNAMIC_FTRACE_WITH_*.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>