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

  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.
---
 libgcc/config/rs6000/linux-unwind.h | 10 ++++++++--
 libgcc/unwind.inc                   |  5 +++--
 2 files changed, 11 insertions(+), 4 deletions(-)
  

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
  

Hi Segher,

On 11/11/2021 10:43, Segher Boessenkool wrote:
> 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.
>

Ok.

>> -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.

Ops, missed that.

> 
>>   {
>>     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.
> 

Ok.

>> +  /* 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)?
> 

I don't have a simple testcase for that, but many of the asan and go 
tests catch that.

> Looks okay, but please update and resend.
> 
> 
> Segher
> 

Thanks,