Re: Restoring pc to a different value than lr on aarch64

[Luis Machado <luis.machado@arm.com>]

On 5/6/22 13:46, Yichao Yu via Gdb wrote:
> On Fri, May 6, 2022 at 8:05 AM Yichao Yu <yyc1992@gmail.com> wrote:
>>
>> I have a case in my code where I want to restore the value of lr (x30)
>> during unwinding, to a different value than the return address of the
>> code. However, it seems that for aarch64,
>> (aarch64_dwarf2_frame_init_reg among other functions) hardcode x30 and
>> pc to be exactly the same value after unwinding.
> 
> Actually I misspoke for that. It seems that sp is probably fine and
> the only thing missing causing pc to not work is that
> aarch64_dwarf_reg_to_regnum doesn't understand the PC dwarf reg
> number. It seems that the only thing needed is to add a
> 
> +  if (reg == AARCH64_DWARF_PC)
> +    return AARCH64_PC_REGNUM;
> 
> to that function.
> 

Yes, GDB always assumes the PC from the previous frame is the LR from 
the current frame. That is what GCC generates.

If a different setup is needed, GDB needs to be taught about it.

>>
>> According to aadwarf64[1],
>>
>>> having both LR and PC columns is useful for describing asynchronously created stack frames. A DWARF expression may use this register to restore the context in case of a signal context.
>>
>> so assume the intention is that if I explicitly unwind the pc in
>> addition to lr, it should work. I tried to do that, and also to set
>> return address column to 32, as well as trying to mark the frame as
>> signal frame but none of them seems to work. Is there any way for gdb
>> to honer the explicit unwinding of pc?
>>
>> Also it seems that the sp is also card coded to be cfa. My code also
>> contains explicit saving and restoring of that as well so if that's
>> the case (haven't tested yet) it would be a problem too...
>>
>> Would it be possible to not use this hard-coded logic if the frame
>> contains explicit override of the pc value?
>>
>> Yichao
>>
>> A bit more about the actual code. This is done as part of runtime
>> patching code. The actual restoration of lr is done by returning to a
>> runtime allocated stub that restores lr and directly branch back to
>> the return location. After returning, all registers values are
>> restored back to their previous one. The stack pointer is also
>> switched out since we cannot rely on how much stack space the call
>> site has available.

This seems to work in a similar way as signal handler. GDB needs to be 
taught where to find the registers so it can properly unwind things.

>>
>> [1] https://github.com/ARM-software/abi-aa/blob/8a7b266879c60ca1c76e94ebb279b2dac60ed6a5/aadwarf64/aadwarf64.rst#note-9