[Pdbg] [PATCH v2] stack: guess endian for stack frame walking

[Alistair Popple]

Nice! Must admit I haven't read the code in depth but what condition does the
haphazard endian detection use to detemine endianess?

- Alistair

On Friday, 21 September 2018 2:34:17 PM AEST Nicholas Piggin wrote:
> The stack unwinder currently does not do any endian conversion, which
> means it won't work correctly if the stack does not match pdbg endian.
> 
> This patch attempts an endian flip if the stack looks wrong, and goes
> with that if it's an improvement. This was nice for debugging skiboot.
> 
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> ---
> v2: Improve ugly code slightly
> 
>  src/thread.c | 60 +++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 55 insertions(+), 5 deletions(-)
> 
> diff --git a/src/thread.c b/src/thread.c
> index d282307..b53dccd 100644
> --- a/src/thread.c
> +++ b/src/thread.c
> @@ -105,6 +105,15 @@ static int load8(struct pdbg_target *target, uint64_t addr, uint64_t *value)
>  	return 1;
>  }
>  
> +uint64_t flip_endian(uint64_t v)
> +{
> +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> +	return be64toh(v);
> +#else
> +	return le64toh(v);
> +#endif
> +}
> +
>  static int dump_stack(struct thread_regs *regs)
>  {
>  	struct pdbg_target *target;
> @@ -117,24 +126,65 @@ static int dump_stack(struct thread_regs *regs)
>  		break;
>  	}
>  
> -	printf("STACK:\n");
> +	printf("STACK:           SP                NIA\n");
>  	if (!target)
>  		pdbg_log(PDBG_ERROR, "Unable to read memory (no ADU found)\n");
>  
>  	if (sp && is_real_address(regs, sp)) {
> -		if (!load8(target, sp, &sp))
> +		uint64_t tmp;
> +		bool flip = false;
> +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> +		bool be = false;
> +#else
> +		bool be = true;
> +#endif
> +
> +		if (!load8(target, sp, &tmp))
>  			return 1;
> -		while (sp && is_real_address(regs, sp)) {
> +
> +		if (!tmp) {
> +badstack:
> +			printf("SP:0x%016" PRIx64 " points to 0x%016" PRIx64 ", not decoding\n", sp, tmp);
> +			return 0;
> +		}
> +
> +		/* Haphazard endian detection */
> +		if (tmp < sp || (tmp - sp > (1024*1024*1024))) {
> +			uint64_t tmp2;
> +			tmp2 = flip_endian(tmp);
> +			if (tmp2 < sp || (tmp2 - sp > (1024*1024*1024)))
> +				goto badstack;
> +			sp = tmp2;
> +			flip = true;
> +			be = !be;
> +		} else {
> +			sp = tmp;
> +		}
> +
> +		if (be)
> +			printf("Looks like big-endian\n");
> +		else
> +			printf("Looks like little-endian\n");
> +
> +		while (sp) {
> +			if (!is_real_address(regs, sp))
> +				break;
> +
>  			if (!load8(target, sp + 16, &pc))
>  				return 1;
> +			if (flip)
> +				pc = flip_endian(pc);
>  
> -			printf(" 0x%016" PRIx64 " 0x%16" PRIx64 "\n", sp, pc);
> +			printf(" 0x%016" PRIx64 " 0x%016" PRIx64 "\n", sp, pc);
>  
>  			if (!load8(target, sp, &sp))
>  				return 1;
> +			if (flip)
> +				sp = flip_endian(sp);
>  		}
> +	} else {
> +		printf("SP:0x%016" PRIx64 " does not appear to be a stack\n", sp);
>  	}
> -
>  	return 0;
>  }
>  
>