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

[Ard Biesheuvel]

On Mon, 10 Oct 2022 at 12:46, Leizhen (ThunderTown)
<thunder.leizhen@xxxxxxxxxx> wrote:
>
>
>
> On 2022/10/10 18:10, Ard Biesheuvel wrote:
> > 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.
>
> 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?