Re: [PATCH][gdb] Fix internal-error in process_event_stop_test

[Tom de Vries <tdevries@suse.de>]

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On 1/7/21 9:10 PM, Tom Tromey wrote:
>>>>>> "Tom" == Tom de Vries <tdevries@suse.de> writes:
> 
> Tom> Fix this by only trying to install the master exception breakpoint in
> Tom> libgcc.debug using the _Unwind_DebugHook method, if the install using probes
> Tom> in libgcc failed.
> 
> This seems like a reasonable idea.
> 
> Tom> Any comments?
> 
> I suspect this would be a bit shorter if the loop only examined
> "primary" objfiles (ones that don't have a backlink) and then delegated
> to a helper function that did the search.  What do you think?

I've made the loop now over "primary" objfiles.  Also, I've split up the
helper function into one using probes, and the other using the hook,
which hopefully helps to make the code a bit clearer.

I'll put this through testing and commit, unless there are further comments.

Thanks,
- Tom


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[gdb] Fix internal-error in process_event_stop_test

The function create_exception_master_breakpoint in gdb/breakpoint.c attempts
to set a master exception breakpoint in each objfile.  It tries this using
a libgcc/unwind probe, and if that fails then using the
_Unwind_DebugHook symbol:
...
   for (objfile *objfile : current_program_space->objfiles ())
     {
        /* Try using probes.  */
        if (/* successful */)
          continue;

        /* Try using _Unwind_DebugHook */
     }
...

The preference scheme works ok both if the objfile has debug info, and if it's
stripped.

But it doesn't work when the objfile has a .gnu_debuglink to a .debug file
(and the .debug file is present).  What happens is that:
- we first encounter objfile libgcc.debug
- we try using probes, and this fails
- so we try _Unwind_DebugHook, which succeeds
- next we encounter objfile libgcc
- we try using probes, and this succeeds.
So, we end up with a master exception breakpoint in both libgcc (using probes)
and libgcc.debug (using _Unwind_DebugHook).

This eventually causes:
...
(gdb) PASS: gdb.cp/nextoverthrow.exp: post-check - next over a throw 3
next^M
src/gdb/infrun.c:6384: internal-error: \
  void process_event_stop_test(execution_control_state*): \
  Assertion `ecs->event_thread->control.exception_resume_breakpoint != NULL' \
  failed.^M
A problem internal to GDB has been detected,^M
further debugging may prove unreliable.^M
Quit this debugging session? (y or n) FAIL: gdb.cp/nextoverthrow.exp: next
past catch (GDB internal error)
...

To trigger this internal-error, we need to use gcc-10 or later to compile the
test-case, such that it contains the fix for gcc PR97774 - "Incorrect line
info for try/catch".

Fix this by only trying to install the master exception breakpoint in
libgcc.debug using the _Unwind_DebugHook method, if the install using probes
in libgcc failed.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-01-05  Tom de Vries  <tdevries@suse.de>

	PR gdb/26881
	* breakpoint.c (create_exception_master_breakpoint_probe)
	(create_exception_master_breakpoint_hook): Factor out
	of ...
	(create_exception_master_breakpoint): ... here.  Only try to install
	the master exception breakpoint in objfile.debug using the
	_Unwind_DebugHook method, if the install using probes in objfile
	failed.

---
 gdb/breakpoint.c | 164 +++++++++++++++++++++++++++++++++----------------------
 1 file changed, 99 insertions(+), 65 deletions(-)

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 69e452993f1..70a4d0e1690 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -3477,92 +3477,126 @@ create_std_terminate_master_breakpoint (void)
     }
 }
 
-/* Install a master breakpoint on the unwinder's debug hook.  */
+/* Install a master breakpoint on the unwinder's debug hook for OBJFILE using a
+   probe.  Return true if a breakpoint was installed.  */
 
-static void
-create_exception_master_breakpoint (void)
+static bool
+create_exception_master_breakpoint_probe (objfile *objfile)
 {
-  const char *const func_name = "_Unwind_DebugHook";
+  struct breakpoint *b;
+  struct gdbarch *gdbarch;
+  struct breakpoint_objfile_data *bp_objfile_data;
 
-  for (objfile *objfile : current_program_space->objfiles ())
-    {
-      struct breakpoint *b;
-      struct gdbarch *gdbarch;
-      struct breakpoint_objfile_data *bp_objfile_data;
-      CORE_ADDR addr;
-      struct explicit_location explicit_loc;
+  bp_objfile_data = get_breakpoint_objfile_data (objfile);
 
-      bp_objfile_data = get_breakpoint_objfile_data (objfile);
+  /* We prefer the SystemTap probe point if it exists.  */
+  if (!bp_objfile_data->exception_searched)
+    {
+      std::vector<probe *> ret
+	= find_probes_in_objfile (objfile, "libgcc", "unwind");
 
-      /* We prefer the SystemTap probe point if it exists.  */
-      if (!bp_objfile_data->exception_searched)
+      if (!ret.empty ())
 	{
-	  std::vector<probe *> ret
-	    = find_probes_in_objfile (objfile, "libgcc", "unwind");
+	  /* We are only interested in checking one element.  */
+	  probe *p = ret[0];
 
-	  if (!ret.empty ())
+	  if (!p->can_evaluate_arguments ())
 	    {
-	      /* We are only interested in checking one element.  */
-	      probe *p = ret[0];
-
-	      if (!p->can_evaluate_arguments ())
-		{
-		  /* We cannot use the probe interface here, because it does
-		     not know how to evaluate arguments.  */
-		  ret.clear ();
-		}
+	      /* We cannot use the probe interface here, because it does
+		 not know how to evaluate arguments.  */
+	      ret.clear ();
 	    }
-	  bp_objfile_data->exception_probes = ret;
-	  bp_objfile_data->exception_searched = 1;
 	}
+      bp_objfile_data->exception_probes = ret;
+      bp_objfile_data->exception_searched = 1;
+    }
 
-      if (!bp_objfile_data->exception_probes.empty ())
-	{
-	  gdbarch = objfile->arch ();
+  if (bp_objfile_data->exception_probes.empty ())
+    return false;
 
-	  for (probe *p : bp_objfile_data->exception_probes)
-	    {
-	      b = create_internal_breakpoint (gdbarch,
-					      p->get_relocated_address (objfile),
-					      bp_exception_master,
-					      &internal_breakpoint_ops);
-	      b->location = new_probe_location ("-probe-stap libgcc:unwind");
-	      b->enable_state = bp_disabled;
-	    }
+  gdbarch = objfile->arch ();
 
-	  continue;
-	}
+  for (probe *p : bp_objfile_data->exception_probes)
+    {
+      b = create_internal_breakpoint (gdbarch,
+				      p->get_relocated_address (objfile),
+				      bp_exception_master,
+				      &internal_breakpoint_ops);
+      b->location = new_probe_location ("-probe-stap libgcc:unwind");
+      b->enable_state = bp_disabled;
+    }
 
-      /* Otherwise, try the hook function.  */
+  return true;
+}
 
-      if (msym_not_found_p (bp_objfile_data->exception_msym.minsym))
-	continue;
+/* Install a master breakpoint on the unwinder's debug hook for OBJFILE using
+   _Unwind_DebugHook.  Return true if a breakpoint was installed.  */
+
+static bool
+create_exception_master_breakpoint_hook (objfile *objfile)
+{
+  const char *const func_name = "_Unwind_DebugHook";
+  struct breakpoint *b;
+  struct gdbarch *gdbarch;
+  struct breakpoint_objfile_data *bp_objfile_data;
+  CORE_ADDR addr;
+  struct explicit_location explicit_loc;
 
-      gdbarch = objfile->arch ();
+  bp_objfile_data = get_breakpoint_objfile_data (objfile);
 
-      if (bp_objfile_data->exception_msym.minsym == NULL)
-	{
-	  struct bound_minimal_symbol debug_hook;
+  if (msym_not_found_p (bp_objfile_data->exception_msym.minsym))
+    return false;
 
-	  debug_hook = lookup_minimal_symbol (func_name, NULL, objfile);
-	  if (debug_hook.minsym == NULL)
-	    {
-	      bp_objfile_data->exception_msym.minsym = &msym_not_found;
-	      continue;
-	    }
+  gdbarch = objfile->arch ();
+
+  if (bp_objfile_data->exception_msym.minsym == NULL)
+    {
+      struct bound_minimal_symbol debug_hook;
 
-	  bp_objfile_data->exception_msym = debug_hook;
+      debug_hook = lookup_minimal_symbol (func_name, NULL, objfile);
+      if (debug_hook.minsym == NULL)
+	{
+	  bp_objfile_data->exception_msym.minsym = &msym_not_found;
+	  return false;
 	}
 
-      addr = BMSYMBOL_VALUE_ADDRESS (bp_objfile_data->exception_msym);
-      addr = gdbarch_convert_from_func_ptr_addr (gdbarch, addr,
-						 current_top_target ());
-      b = create_internal_breakpoint (gdbarch, addr, bp_exception_master,
-				      &internal_breakpoint_ops);
-      initialize_explicit_location (&explicit_loc);
-      explicit_loc.function_name = ASTRDUP (func_name);
-      b->location = new_explicit_location (&explicit_loc);
-      b->enable_state = bp_disabled;
+      bp_objfile_data->exception_msym = debug_hook;
+    }
+
+  addr = BMSYMBOL_VALUE_ADDRESS (bp_objfile_data->exception_msym);
+  addr = gdbarch_convert_from_func_ptr_addr (gdbarch, addr,
+					     current_top_target ());
+  b = create_internal_breakpoint (gdbarch, addr, bp_exception_master,
+				  &internal_breakpoint_ops);
+  initialize_explicit_location (&explicit_loc);
+  explicit_loc.function_name = ASTRDUP (func_name);
+  b->location = new_explicit_location (&explicit_loc);
+  b->enable_state = bp_disabled;
+
+  return true;
+}
+
+/* Install a master breakpoint on the unwinder's debug hook.  */
+
+static void
+create_exception_master_breakpoint (void)
+{
+  for (objfile *obj : current_program_space->objfiles ())
+    {
+      /* Skip separate debug object.  */
+      if (obj->separate_debug_objfile_backlink)
+	continue;
+
+      /* Try a probe kind breakpoint.  */
+      if (create_exception_master_breakpoint_probe (obj))
+	continue;
+
+      /* Iterate over separate debug objects and try an _Unwind_DebugHook
+	 kind breakpoint.  */
+      for (objfile *sepdebug = obj->separate_debug_objfile;
+	   sepdebug != nullptr; sepdebug = sepdebug->separate_debug_objfile)
+	if (create_exception_master_breakpoint_hook (sepdebug))
+	  break;
     }
 }