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

[Ard Biesheuvel]

On Mon, 10 Oct 2022 at 11:56, Zhen Lei <thunder.leizhen@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  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.


> +                       else
> +                               val = (__force unsigned int)cpu_to_le32(val);

and swab32() here


> +               }
> +
>                 if (!bad)
>                         p += sprintf(p, i == 0 ? "(%0*x) " : "%0*x ",
>                                         width, val);
> --
> 2.25.1
>
>
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