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

Raphael Moreira Zinsly rzinsly@linux.ibm.com
Wed Nov 10 21:59:23 GMT 2021


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(-)

diff --git a/libgcc/config/rs6000/linux-unwind.h b/libgcc/config/rs6000/linux-unwind.h
index 8deccc1d650..131d2d9cb7e 100644
--- a/libgcc/config/rs6000/linux-unwind.h
+++ b/libgcc/config/rs6000/linux-unwind.h
@@ -395,14 +395,20 @@ struct frame_layout
 };
 
 
-void ppc_backchain_fallback (struct _Unwind_Context *context, void *a)
+static void
+ppc_backchain_fallback (struct _Unwind_Context *context, void *a)
 {
   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;
+  /* 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



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