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Date:   Tue, 31 Mar 2020 11:47:39 +0530
From:   "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [PATCH 10/12] powerpc/entry32: Blacklist exception entry points
 for kprobe.
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <dff05b59a161434a546010507000816750073f28.1585474724.git.christophe.leroy@c-s.fr>
        <aea027844b12fcbc29ea78d26c5848a6794d1688.1585474724.git.christophe.leroy@c-s.fr>
        <1585588031.jvow7mwq4x.naveen@linux.ibm.com>
        <7f367f35-1bb8-bbb6-f399-8e911f76e043@c-s.fr>
        <83053ddf-9ba6-d551-6711-890c3f3810b5@c-s.fr>
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Christophe Leroy wrote:
> 
> 
> Le 30/03/2020 à 20:33, Christophe Leroy a écrit :
>> 
>> 
>> Le 30/03/2020 à 19:08, Naveen N. Rao a écrit :
>>> Christophe Leroy wrote:
>>>> kprobe does not handle events happening in real mode.
>>>>
>>>> As exception entry points are running with MMU disabled,
>>>> blacklist them.
>>>>
>>>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>>>> ---
>>>>  arch/powerpc/kernel/entry_32.S | 7 +++++++
>>>>  1 file changed, 7 insertions(+)
>>>>
>>>> diff --git a/arch/powerpc/kernel/entry_32.S 
>>>> b/arch/powerpc/kernel/entry_32.S
>>>> index 94f78c03cb79..9a1a45d6038a 100644
>>>> --- a/arch/powerpc/kernel/entry_32.S
>>>> +++ b/arch/powerpc/kernel/entry_32.S
>>>> @@ -51,6 +51,7 @@ mcheck_transfer_to_handler:
>>>>      mfspr    r0,SPRN_DSRR1
>>>>      stw    r0,_DSRR1(r11)
>>>>      /* fall through */
>>>> +_ASM_NOKPROBE_SYMBOL(mcheck_transfer_to_handler)
>>>>
>>>>      .globl    debug_transfer_to_handler
>>>>  debug_transfer_to_handler:
>>>> @@ -59,6 +60,7 @@ debug_transfer_to_handler:
>>>>      mfspr    r0,SPRN_CSRR1
>>>>      stw    r0,_CSRR1(r11)
>>>>      /* fall through */
>>>> +_ASM_NOKPROBE_SYMBOL(debug_transfer_to_handler)
>>>>
>>>>      .globl    crit_transfer_to_handler
>>>>  crit_transfer_to_handler:
>>>> @@ -94,6 +96,7 @@ crit_transfer_to_handler:
>>>>      rlwinm    r0,r1,0,0,(31 - THREAD_SHIFT)
>>>>      stw    r0,KSP_LIMIT(r8)
>>>>      /* fall through */
>>>> +_ASM_NOKPROBE_SYMBOL(crit_transfer_to_handler)
>>>>  #endif
>>>>
>>>>  #ifdef CONFIG_40x
>>>> @@ -115,6 +118,7 @@ crit_transfer_to_handler:
>>>>      rlwinm    r0,r1,0,0,(31 - THREAD_SHIFT)
>>>>      stw    r0,KSP_LIMIT(r8)
>>>>      /* fall through */
>>>> +_ASM_NOKPROBE_SYMBOL(crit_transfer_to_handler)
>>>>  #endif
>>>>
>>>>  /*
>>>> @@ -127,6 +131,7 @@ crit_transfer_to_handler:
>>>>      .globl    transfer_to_handler_full
>>>>  transfer_to_handler_full:
>>>>      SAVE_NVGPRS(r11)
>>>> +_ASM_NOKPROBE_SYMBOL(transfer_to_handler_full)
>>>>      /* fall through */
>>>>
>>>>      .globl    transfer_to_handler
>>>> @@ -286,6 +291,8 @@ reenable_mmu:
>>>>      lwz    r2, GPR2(r11)
>>>>      b    fast_exception_return
>>>>  #endif
>>>> +_ASM_NOKPROBE_SYMBOL(transfer_to_handler)
>>>> +_ASM_NOKPROBE_SYMBOL(transfer_to_handler_cont)
>>>
>>> These are added after 'reenable_mmu', which is itself not blacklisted. 
>>> Is that intentional?
>> 
>> Yes I put it as the complete end of the entry part, ie just before 
>> stack_ovf which is a function by itself.
>> 
>> Note that reenable_mmu is inside an #ifdef CONFIG_TRACE_IRQFLAGS.
>> 
>> I'm not completely sure where to put the _ASM_NOKPROBE_SYMBOL()s, that's 
>> the reason why I put it close to the symbol itself in my first series.
>> 
>> Could you have a look at the code and tell me what looks the most 
>> appropriate as a location to you ?
>> 
>> https://elixir.bootlin.com/linux/v5.6/source/arch/powerpc/kernel/entry_32.S#L230 
> 
> Ok, thinking about it once more, I guess we have a problem as everything 
> after that reenable_mmu will be visible.

I see that we reach reenable_mmu through a 'rfi' with MSR_KERNEL, which 
seems safe to me. So, I figured it can be probed without issues?

- Naveen