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Subject: Re: [PATCH] powerpc/kprobes: Fix trap address when trap happened in
 real mode
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Larry Finger <Larry.Finger@lwfinger.net>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        stable@kernel.vger.org,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        "David S. Miller" <davem@davemloft.net>
References: <b1451438f7148ad0e03306a1f1409f4ad1d6ec7c.1581684263.git.christophe.leroy@c-s.fr>
 <20200214225434.464ec467ad9094961abb8ddc@kernel.org>
 <e09d3c42-542e-48c1-2f1e-cfe605b05bec@c-s.fr>
 <20200216213411.824295a321d8fa979dedbbbe@kernel.org>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <baee8186-549a-f6cf-3619-884b6d708185@c-s.fr>
Date:   Mon, 17 Feb 2020 10:03:22 +0100
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Le 16/02/2020 à 13:34, Masami Hiramatsu a écrit :
> On Sat, 15 Feb 2020 11:28:49 +0100
> Christophe Leroy <christophe.leroy@c-s.fr> 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 <christophe.leroy@c-s.fr> 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 ?

Christophe