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Date:   Mon, 17 Feb 2020 10:03:22 +0100
From:   Christophe Leroy <>
To:     Masami Hiramatsu <>
Cc:     Benjamin Herrenschmidt <>,
        Paul Mackerras <>,
        Michael Ellerman <>,
        Larry Finger <>,
        "Naveen N. Rao" <>,,,,
        Anil S Keshavamurthy <>,
        "David S. Miller" <>
Subject: Re: [PATCH] powerpc/kprobes: Fix trap address when trap happened in
 real mode

Le 16/02/2020 à 13:34, Masami Hiramatsu a écrit :
> On Sat, 15 Feb 2020 11:28:49 +0100
> Christophe Leroy <> wrote:
>> Hi,
>> Le 14/02/2020 à 14:54, Masami Hiramatsu a écrit :
>>> Hi,
>>> On Fri, 14 Feb 2020 12:47:49 +0000 (UTC)
>>> Christophe Leroy <> wrote:
>>>> When a program check exception happens while MMU translation is
>>>> disabled, following Oops happens in kprobe_handler() in the following
>>>> test:
>>>> 		} else if (*addr != BREAKPOINT_INSTRUCTION) {
>>> Thanks for the report and patch. I'm not so sure about powerpc implementation
>>> but at where the MMU translation is disabled, can the handler work correctly?
>>> (And where did you put the probe on?)
>>> Your fix may fix this Oops, but if the handler needs special care, it is an
>>> option to blacklist such place (if possible).
>> I guess that's another story. Here we are not talking about a place
>> where kprobe has been illegitimately activated, but a place where there
>> is a valid trap, which generated a valid 'program check exception'. And
>> kprobe was off at that time.
> Ah, I got it. It is not a kprobe breakpoint, but to check that correctly,
> it has to know the address where the breakpoint happens. OK.
>> As any 'program check exception' due to a trap (ie a BUG_ON, a WARN_ON,
>> a debugger breakpoint, a perf breakpoint, etc...) calls
>> kprobe_handler(), kprobe_handler() must be prepared to handle the case
>> where the MMU translation is disabled, even if probes are not supposed
>> to be set for functions running with MMU translation disabled.
> Can't we check the MMU is disabled there (as same as checking the exception
> happened in user space or not)?

What do you mean by 'there' ? At the entry of kprobe_handler() ?

That's what my patch does, it checks whether MMU is disabled or not. If 
it is, it converts the address to a virtual address.

Do you mean kprobe_handler() should bail out early as it does when the 
trap happens in user mode ? Of course we can do that, I don't know 
enough about kprobe to know if kprobe_handler() should manage events 
that happened in real-mode or just ignore them. But I tested adding an 
event on a function that runs in real-mode, and it (now) works.

So, what should we do really ?


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