Messages in this thread | | | Date | Sat, 02 Jun 2018 17:28:05 +0530 | From | "Naveen N. Rao" <> | Subject | Re: [PATCH -tip v4 24/27] bpf: error-inject: kprobes: Clear current_kprobe and enable preempt in kprobe |
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Masami Hiramatsu wrote: > On Thu, 31 May 2018 16:25:38 +0530 > "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> 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).
Agreed.
> > 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 clear.
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.
Thanks, Naveen
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