[PATCH v2 01/21] powerpc: Create stackframe information on syscall

[Zack Weinberg]

On 26 Feb 2018, Adhemerval Zanella wrote:
> This patch adds a minimal stackframe creation on powerpc syscall
> implementation so backtrace works correctly on a signal handler.

I don't know powerpc well enough to know if this should be necessary.
I think one of the powerpc arch maintainers should comment.  I do have
a couple questions:

> +#ifdef __powerpc64__
> +	stdu r1, -FRAME_MIN_SIZE (r1)
> +	cfi_adjust_cfa_offset (FRAME_MIN_SIZE)
> +#else
> +	stwu r1,-16(1)
> +	cfi_def_cfa_offset (16)
> +#endif

Why does this use cfa_adjust_cfa_offset for 64-bit but _def_ for 32-bit?

> +#ifdef __powerpc64__
> +	addi r1, r1, FRAME_MIN_SIZE
> +#else
> +	addi r1,r1,16
> +#endif

If FRAME_MIN_SIZE were defined for ppc32, we could reduce the
ifdeffage here.

> +	cfi_def_cfa_offset (0)
>       sc
>       PSEUDO_RET

Shouldn't the stack adjustments be undone _after_ the 'sc'
instruction?  Actually, is it possible that a single
cfi_def_cfa_offset (0) at the beginning of the function is all that's
really needed here?  It seems like backtrace should be able to handle
leaf frames in general...

zw