Re: [PATCH -tip v4 24/27] bpf: error-inject: kprobes: Clear current_kprobe and enable preempt in kprobe
- Date: Mon, 4 Jun 2018 18:08:44 +0900
- From: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
- Subject: Re: [PATCH -tip v4 24/27] bpf: error-inject: kprobes: Clear current_kprobe and enable preempt in kprobe
On Sat, 02 Jun 2018 17:28:05 +0530
"Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:
> Masami Hiramatsu wrote:
> > On Thu, 31 May 2018 16:25:38 +0530
> > "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:
> >> Masami Hiramatsu wrote:
> >> > Clear current_kprobe and enable preemption in kprobe
> >> > even if pre_handler returns !0.
> >> >
> >> > This simplifies function override using kprobes.
> >> >
> >> > Jprobe used to require to keep the preemption disabled and
> >> > keep current_kprobe until it returned to original function
> >> > entry. For this reason kprobe_int3_handler() and similar
> >> > arch dependent kprobe handers checks pre_handler result
> >> > and exit without enabling preemption if the result is !0.
> >> >
> >> > After removing the jprobe, Kprobes does not need to
> >> > keep preempt disabled even if user handler returns !0
> >> > anymore.
> >> I think the reason jprobes did it that way is to address architecture
> >> specific requirements when changing a function. So, without that
> >> infrastructure, I am not sure if we will be able to claim support for
> >> over-riding functions with kprobes. I am not sure if we want to claim
> >> that, but this is something we need to be clear on.
> > Really? as far as I can see, there seems no such architecture.
> > The keeping preempt disabled is corresponding to keeping current_kprobe
> > since the current_kprobe is per-cpu.
> Right, and the reason for not resetting current_kprobe after kprobe
> handling is done is primarily for jprobes.
> > This means if it is preempted
> > before hitting break_handler and changed cpu core, we missed to
> > handle current_kprobe and goes to panic. But if we don't need
> > such "break back" (removing break_handler), we don't need to
> > keep current_kprobe (because it is not handled afterwards).
> > Anyway, changing function execution path is a "one-way" change.
> This is the problem. With jprobes, over-riding a function was not a
> "one-way" change because it involves more than just changing the [n]ip.
> That is the reason we had setjmp/longjmp (aka break_handler).
> > We don't have a chance to fixup that disabled preemption and current_kprobe
> > after returning to the new function. So current error-inject clears
> > current_kprobe and enable preemption before returning !0 from its
> > kprobe pre_handler.
> > This is just moving such needless operation from user-pre_handler to
> > kprobes itself.
> >> For powerpc, the current function override in error-inject works fine
> >> since the new function does nothing. But, if anyone wants to do more
> >> work in the replacement function, it won't work with the current
> >> approach.
> > If you are considering about TOC change etc. yes, it depends on
> > the archtecture. As far as I know IA64 and powerpc will not allow
> > to support changing execution path without special care.
> > Other "flat and simple" function call architectures like x86, arm
> > can change execution path without special care.
> Yes, that's the concern. As I stated earlier, the only user seems to be
> error-injection where this is not a concern. I wanted this to be made
> I've since noticed that you are updating Documentation/kprobes.txt to
> make this clear in patch 24/27 in this series. So, I'm ok with the
> changes in this series.
I see your concern. Yeah, that's why I added [24/27] for clearly stating it.
Masami Hiramatsu <mhiramat@xxxxxxxxxx>