Re: [PATCH -tip v4 24/27] bpf: error-inject: kprobes: Clear current_kprobe and enable preempt in kprobe

[Naveen N. Rao]

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).

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