linux-kernel - Re: [PATCH -tip v4 24/27] bpf: error-inject: kprobes: Clear current

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Message-Id: <1527937081.8b4s5988lk.naveen@linux.ibm.com>
Date:   Sat, 02 Jun 2018 17:28:05 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Arnd Bergmann <arnd@...db.de>,
        Alexei Starovoitov <ast@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>, Josef Bacik <jbacik@...com>,
        James Hogan <jhogan@...nel.org>,
        Laura Abbott <labbott@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Will Deacon <will.deacon@....com>, x86@...nel.org
Subject: Re: [PATCH -tip v4 24/27] bpf: error-inject: kprobes: Clear
 current_kprobe and enable preempt in kprobe

Masami Hiramatsu wrote:
> On Thu, 31 May 2018 16:25:38 +0530
> "Naveen N. Rao" <naveen.n.rao@...ux.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