[PATCH] libgcc: fix backtrace fallback on PowerPC Big-endian. [PR103004]

Segher Boessenkool segher@kernel.crashing.org
Thu Nov 11 13:43:45 GMT 2021


Hi!

On Wed, Nov 10, 2021 at 06:59:23PM -0300, Raphael Moreira Zinsly wrote:
> At the end of the backtrace stream _Unwind_Find_FDE() may not be able
> to find the frame unwind info and will later call the backtrace fallback
> instead of finishing. This occurs when using an old libc on ppc64 due to
> dl_iterate_phdr() not being able to set the fde in the last trace.
> When this occurs the cfa of the trace will be behind of context's cfa.
> Also, libgo’s probestackmaps() calls the backtrace with a null pointer
> and can get to the backchain fallback with the same problem, in this case
> we are only interested in find a stack map, we don't need nor can do a
> backchain.
> _Unwind_ForcedUnwind_Phase2() can hit the same issue as it uses
> uw_frame_state_for(), so we need to treat _URC_NORMAL_STOP.
> 
> libgcc/ChangeLog:
> 
>          * config/rs6000/linux-unwind.h (ppc_backchain_fallback): turn into
> 	 static to fix -Wmissing-prototypes. Check if it's called with a null
> 	 argument or at the end of the backtrace and return.
>          * unwind.inc (_Unwind_ForcedUnwind_Phase2): treat _URC_NORMAL_STOP.

Formatting is messed up.  Lines start with a capital.  Two spaces after
full stop, while you're at it.

> -void ppc_backchain_fallback (struct _Unwind_Context *context, void *a)
> +static void
> +ppc_backchain_fallback (struct _Unwind_Context *context, void *a)

This was already fixed in 75ef0353a2d3.

>  {
>    struct frame_layout *current;
>    struct trace_arg *arg = a;
>    int count;
>  
> -  /* Get the last address computed and start with the next.  */
> +  /* Get the last address computed.  */
>    current = context->cfa;

Empty line after here please.  Most of the time if you have a full-line
comment it means a new paragraph is starting.

> +  /* If the trace CFA is not the context CFA the backtrace is done.  */
> +  if (arg == NULL || arg->cfa != current)
> +	return;
> +
> +  /* Start with next address.  */
>    current = current->backchain;

Like you did here :-)

Do you have a testcase (that failed without this, but now doesn't)?

Looks okay, but please update and resend.


Segher


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