Re: [PATCH v2 2/2] ARM: Make the dumped instructions are consistent with the disassembled ones

From: "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 2/2] ARM: Make the dumped instructions are consistent with the disassembled ones
Date: Mon, 10 Oct 2022 19:29:59 +0800	[thread overview]
Message-ID: <9f2b2338-16a2-7889-a4cf-19ae83c82f2f@huawei.com> (raw)
In-Reply-To: <CAMj1kXHN=oa2bokjX6VJkoW2TC92hVF5pn5a4U40+bJ08k0kMA@mail.gmail.com>

On 2022/10/10 19:07, Ard Biesheuvel wrote:
> On Mon, 10 Oct 2022 at 12:46, Leizhen (ThunderTown)
> <thunder.leizhen@huawei.com> wrote:
>>
>>
>>
>> On 2022/10/10 18:10, Ard Biesheuvel wrote:
>>> On Mon, 10 Oct 2022 at 11:56, Zhen Lei <thunder.leizhen@huawei.com> wrote:
>>>>
>>>> In ARM, the mapping of instruction memory is always little-endian, except
>>>> some BE-32 supported ARM architectures. Such as ARMv7-R, its instruction
>>>> endianness may be BE-32. Of course, its data endianness will also be BE-32
>>>> mode. Due to two negatives make a positive, the instruction stored in the
>>>> register after reading is in little-endian format. But for the case of
>>>> BE-8, the instruction endianness is LE, the instruction stored in the
>>>> register after reading is in big-endian format, which is inconsistent
>>>> with the disassembled one.
>>>>
>>>> For example:
>>>> The content of disassembly:
>>>> c0429ee8:       e3500000        cmp     r0, #0
>>>> c0429eec:       159f2044        ldrne   r2, [pc, #68]
>>>> c0429ef0:       108f2002        addne   r2, pc, r2
>>>> c0429ef4:       1882000a        stmne   r2, {r1, r3}
>>>> c0429ef8:       e7f000f0        udf     #0
>>>>
>>>> The output of undefined instruction exception:
>>>> Internal error: Oops - undefined instruction: 0 [#1] SMP ARM
>>>> ... ...
>>>> Code: 000050e3 44209f15 02208f10 0a008218 (f000f0e7)
>>>>
>>>> This inconveniences the checking of instructions. What's worse is that,
>>>> for somebody who don't know about this, might think the instructions are
>>>> all broken.
>>>>
>>>> So, when CONFIG_CPU_ENDIAN_BE8=y, let's convert the instructions to
>>>> little-endian format before they are printed. The conversion result is
>>>> as follows:
>>>> Code: e3500000 159f2044 108f2002 1882000a (e7f000f0)
>>>>
>>>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>>>> ---
>>>>  arch/arm/kernel/traps.c | 7 +++++++
>>>>  1 file changed, 7 insertions(+)
>>>>
>>>> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
>>>> index 34aa80c09c508c1..50b00c9091f079d 100644
>>>> --- a/arch/arm/kernel/traps.c
>>>> +++ b/arch/arm/kernel/traps.c
>>>> @@ -193,6 +193,13 @@ static void dump_instr(const char *lvl, struct pt_regs *regs)
>>>>                                 bad = get_user(val, &((u32 __user *)addr)[i]);
>>>>                 }
>>>>
>>>> +               if (IS_ENABLED(CONFIG_CPU_ENDIAN_BE8)) {
>>>> +                       if (thumb)
>>>> +                               val = (__force unsigned int)cpu_to_le16(val);
>>>
>>> Better use swab16() here instead of the ugly __force cast, given that
>>> the swab is going to occur unconditionally here.
>>
>> Good idea.
>>
>>>
>>>
>>>> +                       else
>>>> +                               val = (__force unsigned int)cpu_to_le32(val);
>>>
>>> and swab32() here
>>
>> OK
>>
> 
> Actually, come to think of it, should this code perhaps be using the
> mem_to_opcode helpers that are being used elsewhere in the file?

Right, __mem_to_opcode_xxx is the correct solution. I need to test it.

> .
> 

-- 
Regards,
  Zhen Lei

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