Re: [RFC PATCH v2 3/6] fprobe: rethook: Use fprobe_regs in fprobe exit handler and rethook

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

On Wed, 9 Aug 2023 17:45:29 +0200
Florent Revest <revest@chromium.org> wrote:

> On Wed, Aug 9, 2023 at 4:43 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> > > I think there are two things that can be meant with "rethook uses ftrace_regs":
> > >
> > > - rethook callbacks receive a ftrace_regs (that's what you do further down)
> > > - rethook can hook to a traced function using a ftrace_regs (that's
> > > what you use in fprobe now)
> > >
> > > But I think the second proposition shouldn't imply that rethook_hook
> > > can _only_ hook to ftrace_regs. For the kprobe use case, I think there
> > > should also be a rethook_hook_pt_regs() that operates on a pt_regs. We
> > > could have a default implementation of rethook_hook that calls into
> > > the other (or vice versa) on HAVE_FTRACE_REGS_COMPATIBLE_WITH_PT_REGS
> > > but I think it's good to separate these two APIs
> >
> > Yeah, so for simplying the 2nd case, I added this dependency.
> >
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index aff2746c8af2..e321bdb8b22b 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -201,6 +201,7 @@ config KRETPROBE_ON_RETHOOK
> >         def_bool y
> >         depends on HAVE_RETHOOK
> >         depends on KRETPROBES
> > +       depends on HAVE_PT_REGS_COMPAT_FTRACE_REGS || !HAVE_DYNAMIC_FTRACE_WITH_ARGS
> >         select RETHOOK
> >
> > This is the point why I said that "do not remove kretprobe trampoline".
> > If there is arch dependent kretprobe trampoline, kretprobe does not use
> > the rethook for hooking return. And eventually I would like to remove
> > kretprobe itself (replace it with fprobe + rethook). If so, I don't want
> > to pay more efforts on this part, and keep kretprobe on rethook as it is.
> 
> What are your thoughts on kprobe + rethook though ?

Isn't it KRETPROBE_ON_RETHOOK?

> If that's something you think is worth having, then in this case, it
> seems that having a rethook_hook_pt_regs() API would help users.
> 
> If that's a frankenstein use case you don't want to support then I
> agree we can live without this API and get away with the cast
> protected by the depends on HAVE_PT_REGS_COMPAT_FTRACE_REGS...

Yeah, it needs to introduce arch_rethook_prepare_pt_regs() for each
arch too.

BTW, I found that I have to update the implementation of
arch_rethook_prepare() for x86. (Use ftrace_get_stack_pointer())

Thank you!

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