Re: [PATCH] ARM: kprobes: Explicitly assign register for local variables

["Aiqun(Maria) Yu" <quic_aiquny@quicinc.com>]

On 9/27/2023 6:11 PM, Ard Biesheuvel wrote:
> On Wed, 27 Sept 2023 at 09:44, Aiqun(Maria) Yu <quic_aiquny@quicinc.com> wrote:
>>
>> On 9/27/2023 5:26 PM, Ard Biesheuvel wrote:
>>> Hello Maria,
>>>
>>> On Wed, 27 Sept 2023 at 06:00, Maria Yu <quic_aiquny@quicinc.com> wrote:
>>>>
>>>> Registers r7 is removed in clobber list, so compiler may choose r7 for
>>>> local variables usage, while r7 will be actually updated by the inline asm
>>>> code.
>>>
>>> The inline asm does not update R7, it preserves and restores it.
>> That is the asm is updating r7 purposely and compiler still choose r7
>> for the asm local varialbe and use it inside the asm code.
>> So the change is to fix the issue when "r7 is removed from the clobber
>> list of current asm code while actually r7 shouldn't be choosed for the
>> current asm local variables".
>> The issue is only reproducible when ftrace is not enabled, and r7 is
>> removed from the current clobber list.
>>
>> Let me have the assemble code that will make you understand better.
>>
>> --the original code:
>> "mov    r11, r7                 \n\t"
>> ...
>> "ldmia  %[regs], {r0-r7}        \n\t"
>> "blx    %[fn]                   \n\t"
>> ...
>> "mov    r7, r11                 \n\t"
>>
>> --After compile to choose register for [fn] and [regs].
>> mov     r11, r7
>> ldr  r7, [r1, #16] //r7 used for store asi->insn_fn
>> ...
>> ldmia.w  ip, {r0, r1, r2, r3, r4, r5, r6, r7}
>> blx r7
>> ...
>> mov r7,r11
>>
>> The current change is to avoid by fix the registers for local variable
>> usage and not choose r7 for [fn].
>>
> 
> OK, I understand now, thanks.
> 
> Would it help if we just always enabled frame pointers for this source file?
> 
> --- a/arch/arm/probes/kprobes/Makefile
> +++ b/arch/arm/probes/kprobes/Makefile
> @@ -9,6 +9,7 @@ test-kprobes-objs               := test-core.o
>   ifdef CONFIG_THUMB2_KERNEL
>   obj-$(CONFIG_KPROBES)          += actions-thumb.o checkers-thumb.o
>   test-kprobes-objs              += test-thumb.o
> +KBUILD_CFLAGS                  += -fno-omit-frame-pointer
I've tried this before, but it didn't work.
The compiler(clang 17 here I am using) will still complain for "inline 
asm clobber list contains reserved registers" when ftrace enabled.
>   else
>   obj-$(CONFIG_KPROBES)          += actions-arm.o checkers-arm.o
>   obj-$(CONFIG_OPTPROBES)                += opt-arm.o

-- 
Thx and BRs,
Aiqun(Maria) Yu