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

[Florent Revest]

On Wed, Aug 9, 2023 at 4:43 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> 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 ?

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...