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Subject: Re: [PATCH v2] powerpc/kprobes: Fix Oops by passing ppc_inst as a
 pointer to emulate_step() on ppc32
To:     "Naveen N. Rao" <naveen.n.rao@linux.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: <5bdc8cbc9a95d0779e27c9ddbf42b40f51f883c0.1624425798.git.christophe.leroy@csgroup.eu>
 <1624531892.3vdz8ibfty.naveen@linux.ibm.com>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <b44b66f4-3e46-425b-3c8e-d5a7a180494a@csgroup.eu>
Date:   Thu, 24 Jun 2021 13:09:38 +0200
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Le 24/06/2021 à 12:59, Naveen N. Rao a écrit :
> Christophe Leroy wrote:
>> From: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>>
>> Trying to use a kprobe on ppc32 results in the below splat:
>>     BUG: Unable to handle kernel data access on read at 0x7c0802a6
>>     Faulting instruction address: 0xc002e9f0
>>     Oops: Kernel access of bad area, sig: 11 [#1]
>>     BE PAGE_SIZE=4K PowerPC 44x Platform
>>     Modules linked in:
>>     CPU: 0 PID: 89 Comm: sh Not tainted 5.13.0-rc1-01824-g3a81c0495fdb #7
>>     NIP:  c002e9f0 LR: c0011858 CTR: 00008a47
>>     REGS: c292fd50 TRAP: 0300   Not tainted  (5.13.0-rc1-01824-g3a81c0495fdb)
>>     MSR:  00009000 <EE,ME>  CR: 24002002  XER: 20000000
>>     DEAR: 7c0802a6 ESR: 00000000
>>     <snip>
>>     NIP [c002e9f0] emulate_step+0x28/0x324
>>     LR [c0011858] optinsn_slot+0x128/0x10000
>>     Call Trace:
>>      opt_pre_handler+0x7c/0xb4 (unreliable)
>>      optinsn_slot+0x128/0x10000
>>      ret_from_syscall+0x0/0x28
>>
>> The offending instruction is:
>>     81 24 00 00     lwz     r9,0(r4)
>>
>> Here, we are trying to load the second argument to emulate_step():
>> struct ppc_inst, which is the instruction to be emulated. On ppc64,
>> structures are passed in registers when passed by value. However, per
>> the ppc32 ABI, structures are always passed to functions as pointers.
>> This isn't being adhered to when setting up the call to emulate_step()
>> in the optprobe trampoline. Fix the same.
>>
>> Fixes: eacf4c0202654a ("powerpc: Enable OPTPROBES on PPC32")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>> ---
>> v2: Rebased on powerpc/merge 7f030e9d57b8
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> 
> Thanks for rebasing this!
> 
> I think git am drops everything after three dashes, so applying this patch will drop your SOB. The 
> recommended style (*) for adding a changelog is to include it within [] before the second SOB.
> 

Yes, I saw that after sending the mail. Usually I add a signed-off-by with 'git --amend -s' when I 
add the history, so it goes before the '---' I'm adding.

This time I must have forgotten the '-s' so it was added by the 'git format-patch -s' which is in my 
submit script, and so it was added at the end.

It's not really a big deal, up to Michael to either move it at the right place or discard it, I 
don't really mind. Do the easiest for you.

Thanks
Christophe