[PATCH v2] libgcc: fix backtrace fallback on PowerPC Big-endian. [PR103004]

[PATCH v2] libgcc: fix backtrace fallback on PowerPC Big-endian. [PR103004]

[Raphael Moreira Zinsly]

Changes since v1:
	- Removed -Wmissing-prototypes fix.
	- Fixed formatting of Changelog and patch.

--->8---

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): 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 | 8 +++++++-
 libgcc/unwind.inc                   | 5 +++--
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/libgcc/config/rs6000/linux-unwind.h b/libgcc/config/rs6000/linux-unwind.h
index 8deccc1d650..ad1ab286a2f 100644
--- a/libgcc/config/rs6000/linux-unwind.h
+++ b/libgcc/config/rs6000/linux-unwind.h
@@ -401,8 +401,14 @@ void ppc_backchain_fallback (struct _Unwind_Context *context, void *a)
   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;
+
+  /* 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;
 
   for (count = arg->count; current != NULL; current = current->backchain)
diff --git a/libgcc/unwind.inc b/libgcc/unwind.inc
index 456a5ee682f..dc2f9c13e97 100644
--- a/libgcc/unwind.inc
+++ b/libgcc/unwind.inc
@@ -160,12 +160,13 @@ _Unwind_ForcedUnwind_Phase2 (struct _Unwind_Exception *exc,
 
       /* Set up fs to describe the FDE for the caller of cur_context.  */
       code = uw_frame_state_for (context, &fs);
-      if (code != _URC_NO_REASON && code != _URC_END_OF_STACK)
+      if (code != _URC_NO_REASON && code != _URC_END_OF_STACK
+	  && code != _URC_NORMAL_STOP)
 	return _URC_FATAL_PHASE2_ERROR;
 
       /* Unwind successful.  */
       action = _UA_FORCE_UNWIND | _UA_CLEANUP_PHASE;
-      if (code == _URC_END_OF_STACK)
+      if (code == _URC_END_OF_STACK || code == _URC_NORMAL_STOP)
 	action |= _UA_END_OF_STACK;
       stop_code = (*stop) (1, action, exc->exception_class, exc,
 			   context, stop_argument);
-- 
2.31.1

Re: [PATCH v2] libgcc: fix backtrace fallback on PowerPC Big-endian. [PR103004]

[Segher Boessenkool]

Hi!

On Thu, Nov 11, 2021 at 11:40:10AM -0300, Raphael Moreira Zinsly wrote:
> Changes since v1:
> 	- Removed -Wmissing-prototypes fix.
> 	- Fixed formatting of Changelog and patch.
> 
> --->8---
> 
> 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): 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.

Committed, thanks!  Please note the changelog formatting fixes I had
to do, for later patches :-)


Segher