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Subject: Re: [PATCH v2 2/2] ARM: Make the dumped instructions are consistent
 with the disassembled ones
To:     Ard Biesheuvel <ardb@kernel.org>
CC:     Russell King <linux@armlinux.org.uk>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>
References: <20221010095346.1957-1-thunder.leizhen@huawei.com>
 <20221010095346.1957-3-thunder.leizhen@huawei.com>
 <CAMj1kXFHGPpoSvt3J2XN4tAf2QZ--Lp8KpsuyufSjx+HuRfz0A@mail.gmail.com>
From:   "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Message-ID: <08054412-06de-3c3e-48b8-1a7eb327a2d0@huawei.com>
Date:   Mon, 10 Oct 2022 18:46:12 +0800
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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

> 
> 
>> +               }
>> +
>>                 if (!bad)
>>                         p += sprintf(p, i == 0 ? "(%0*x) " : "%0*x ",
>>                                         width, val);
>> --
>> 2.25.1
>>
>>
>> _______________________________________________
>> linux-arm-kernel mailing list
>> linux-arm-kernel@lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> .
> 

-- 
Regards,
  Zhen Lei