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

On Fri, May 6, 2022 at 8:05 AM Yichao Yu <[email protected]> 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.

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