[kvm-unit-tests PATCH v5 18/29] powerpc: Fix stack backtrace termination

[Thomas Huth]

On 16/12/2023 14.42, Nicholas Piggin wrote:
> The backtrace handler terminates when it sees a NULL caller address,
> but the powerpc stack setup does not keep such a NULL caller frame
> at the start of the stack.
> 
> This happens to work on pseries because the memory at 0 is mapped and
> it contains 0 at the location of the return address pointer if it
> were a stack frame. But this is fragile, and does not work with powernv
> where address 0 contains firmware instructions.
> 
> Use the existing dummy frame on stack as the NULL caller, and create a
> new frame on stack for the entry code.
> 
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> ---
>   powerpc/cstart64.S | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/powerpc/cstart64.S b/powerpc/cstart64.S
> index e18ae9a2..14ab0c6c 100644
> --- a/powerpc/cstart64.S
> +++ b/powerpc/cstart64.S
> @@ -46,8 +46,16 @@ start:
>   	add	r1, r1, r31
>   	add	r2, r2, r31
>   
> +	/* Zero backpointers in initial stack frame so backtrace() stops */
> +	li	r0,0
> +	std	r0,0(r1)
> +	std	r0,16(r1)
> +
> +	/* Create entry frame */
> +	stdu	r1,-INT_FRAME_SIZE(r1)

Shouldn't that rather be STACK_FRAME_OVERHEAD instead of INT_FRAME_SIZE...

>   	/* save DTB pointer */
> -	std	r3, 56(r1)
> +	SAVE_GPR(3,r1)

... since SAVE_GPR uses STACK_FRAME_OVERHEAD (via GPR0), too?

  Thomas