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Subject: Re: [PATCH 10/12] powerpc/entry32: Blacklist exception entry points
 for kprobe.
To:     "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        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>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <7f367f35-1bb8-bbb6-f399-8e911f76e043@c-s.fr>
Date:   Mon, 30 Mar 2020 20:33:23 +0200
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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

Thanks
Christophe