Re: [PATCH 2/2] ArmPkg/DefaultExceptionHandlerLib AARCH64: add minimal backtrace to crash dump

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 7 September 2016 at 12:32, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> On 7 September 2016 at 12:25, Michael Zimmermann
> <sigmaepsilon92@gmail.com> wrote:
>>> However, looking at this
>>> code, this is still not sufficient to find the *next* frame pointer on
>>> the stack.
>> are you sure about that? this code looks like it does just that:
>> https://github.com/torvalds/linux/blob/master/arch/arm/kernel/stacktrace.c
>>
>
> First of all, that comment makes little sense:
>
> * With framepointer enabled, a simple function prologue looks like this:
> * mov ip, sp
> * stmdb sp!, {fp, ip, lr, pc}
>
> I assume this should be
>
> stmdb sp!, {fp, ip, lr}
>
> * sub fp, ip, #4
> *
> * A simple function epilogue looks like this:
> * ldm sp, {fp, sp, pc}
>

OK, I spoke a bit too soon. The 'pop' does not actually increment the
stack pointer, so while the push does make sense (and pushing the pc
in addition is just informational), the pop should actually be
implemented as

ldm sp, {fp, sp, lr}

... do other stuff including incrementing the sp to its original value ...

bx lr

In any case, unless you have any information that suggests that GCC in
Thumb2 mode can be coerced into managing the frame pointer in a way
that allows us to do something similar in EDK2, I am going to stick
with my assertion that this is not currently possible on ARM.

Thanks,
Ard.