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Date:   Tue, 31 Mar 2020 11:43:18 +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>
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Christophe Leroy wrote:
> 
> 
> 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.

Ok, I see what you mean. 'reenable_mmu' can probably be moved after the 
end of 'transfer_to_handler_cont' (as also removing what looks to be an 
unused label '1' for the branch to trace_hardirqs_off), but that's a 
minor thing. From the blacklisting point, this is not an issue.

- Naveen