linux-kernel - Re: [PATCH v2] powerpc/kprobes: Fix Oops by passing ppc_inst as a pointer to emulate

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Message-Id: <1624531892.3vdz8ibfty.naveen@linux.ibm.com>
Date:   Thu, 24 Jun 2021 16:29:30 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] powerpc/kprobes: Fix Oops by passing ppc_inst as a
 pointer to emulate_step() on ppc32

Christophe Leroy wrote:
> From: Naveen N. Rao <naveen.n.rao@...ux.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@...r.kernel.org
> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> ---
> v2: Rebased on powerpc/merge 7f030e9d57b8
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.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.


- Naveen

(*) https://www.kernel.org/doc/html/latest/maintainer/modifying-patches.html