[PATCH][gdb/breakpoints] Fix longjmp master breakpoint with separate debug info

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

Hi,

When running test-case gdb.base/longjmp.exp with target board unix/-m32, we
run into:
...
(gdb) next^M
Warning:^M
Cannot insert breakpoint 0.^M
Cannot access memory at address 0x7dbf7353^M
^M
__libc_siglongjmp (env=0x804a040 <env>, val=1) at longjmp.c:28^M
28        longjmps++;^M
(gdb) FAIL: gdb.base/longjmp.exp: next over longjmp(1)
...

The failure to access memory happens in i386_get_longjmp_target and is due to
glibc having pointer encryption (aka "pointer mangling" or "pointer guard") of
the long jump buffer.  This is a known problem.

In create_longjmp_master_breakpoint (which attempts to install a master
longjmp breakpoint) a preference scheme is present, which installs a
probe breakpoint if a libc:longjmp probe is present, and otherwise falls back
to setting breakpoints at the names in the longjmp_names array.

But in fact, both the probe breakpoint and the longjmp_names breakpoints are
set.  The latter ones are set when processing libc.so.debug, and the former
one when processing libc.so.  In other words, this is the longjmp variant of
PR26881, which describes the same problem for master exception breakpoints.

This problem only triggers when the glibc debug info package is installed,
which is not due to the debug info itself in libc.so.debug, but due to the
minimal symbols (because create_longjmp_master_breakpoint uses minimal symbols
to translate the longjmp_names to addresses).

The problem doesn't trigger for -m64, because there tdep->jb_pc_offset is not
set.

Fix this similar to commit 1940319c0ef (the fix for PR26881): only install
longjmp_names breakpoints in libc.so/libc.so.debug if installing the
libc:longjmp probe in libc.so failed.

Tested on x86_64-linux.

Any comments?

Thanks,
- Tom

[gdb/breakpoints] Fix longjmp master breakpoint with separate debug info

gdb/ChangeLog:

2021-01-27  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/27205
	* breakpoint.c (create_longjmp_master_breakpoint_probe)
	(create_longjmp_master_breakpoint_names): New function, factored out
	of ...
	(create_longjmp_master_breakpoint): ... here.  Only try to install
	longjmp_names breakpoints in libc.so/libc.so.debug if installing probe
	breakpoint in libc.so failed.

---
 gdb/breakpoint.c | 187 +++++++++++++++++++++++++++++++++----------------------
 1 file changed, 113 insertions(+), 74 deletions(-)

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 70a4d0e1690..cdf647b44b2 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -3327,100 +3327,139 @@ create_overlay_event_breakpoint (void)
     }
 }
 
-static void
-create_longjmp_master_breakpoint (void)
+/* Install a master longjmp breakpoint for OBJFILE using a probe.  Return
+   true if a breakpoint was installed.  */
+
+static bool
+create_longjmp_master_breakpoint_probe (objfile *objfile)
 {
-  scoped_restore_current_program_space restore_pspace;
+  struct gdbarch *gdbarch = objfile->arch ();
+  struct breakpoint_objfile_data *bp_objfile_data
+    = get_breakpoint_objfile_data (objfile);
 
-  for (struct program_space *pspace : program_spaces)
+  if (!bp_objfile_data->longjmp_searched)
     {
-      set_current_program_space (pspace);
+      std::vector<probe *> ret
+	= find_probes_in_objfile (objfile, "libc", "longjmp");
 
-      for (objfile *objfile : current_program_space->objfiles ())
+      if (!ret.empty ())
 	{
-	  int i;
-	  struct gdbarch *gdbarch;
-	  struct breakpoint_objfile_data *bp_objfile_data;
+	  /* We are only interested in checking one element.  */
+	  probe *p = ret[0];
 
-	  gdbarch = objfile->arch ();
+	  if (!p->can_evaluate_arguments ())
+	    {
+	      /* We cannot use the probe interface here,
+		 because it does not know how to evaluate
+		 arguments.  */
+	      ret.clear ();
+	    }
+	}
+      bp_objfile_data->longjmp_probes = ret;
+      bp_objfile_data->longjmp_searched = 1;
+    }
 
-	  bp_objfile_data = get_breakpoint_objfile_data (objfile);
+  if (bp_objfile_data->longjmp_probes.empty ())
+    return false;
 
-	  if (!bp_objfile_data->longjmp_searched)
-	    {
-	      std::vector<probe *> ret
-		= find_probes_in_objfile (objfile, "libc", "longjmp");
+  for (probe *p : bp_objfile_data->longjmp_probes)
+    {
+      struct breakpoint *b;
 
-	      if (!ret.empty ())
-		{
-		  /* We are only interested in checking one element.  */
-		  probe *p = ret[0];
+      b = create_internal_breakpoint (gdbarch,
+				      p->get_relocated_address (objfile),
+				      bp_longjmp_master,
+				      &internal_breakpoint_ops);
+      b->location = new_probe_location ("-probe-stap libc:longjmp");
+      b->enable_state = bp_disabled;
+    }
 
-		  if (!p->can_evaluate_arguments ())
-		    {
-		      /* We cannot use the probe interface here,
-			 because it does not know how to evaluate
-			 arguments.  */
-		      ret.clear ();
-		    }
-		}
-	      bp_objfile_data->longjmp_probes = ret;
-	      bp_objfile_data->longjmp_searched = 1;
-	    }
+  return true;
+}
 
-	  if (!bp_objfile_data->longjmp_probes.empty ())
-	    {
-	      for (probe *p : bp_objfile_data->longjmp_probes)
-		{
-		  struct breakpoint *b;
-
-		  b = create_internal_breakpoint (gdbarch,
-						  p->get_relocated_address (objfile),
-						  bp_longjmp_master,
-						  &internal_breakpoint_ops);
-		  b->location = new_probe_location ("-probe-stap libc:longjmp");
-		  b->enable_state = bp_disabled;
-		}
+/* Install master longjmp breakpoints for OBJFILE using longjmp_names.
+   Return true if at least one breakpoint was installed.  */
+
+static bool
+create_longjmp_master_breakpoint_names (objfile *objfile)
+{
+  struct gdbarch *gdbarch = objfile->arch ();
+  if (!gdbarch_get_longjmp_target_p (gdbarch))
+    return false;
+
+  struct breakpoint_objfile_data *bp_objfile_data
+    = get_breakpoint_objfile_data (objfile);
+  unsigned int installed_bp = 0;
 
+  for (int i = 0; i < NUM_LONGJMP_NAMES; i++)
+    {
+      struct breakpoint *b;
+      const char *func_name;
+      CORE_ADDR addr;
+      struct explicit_location explicit_loc;
+
+      if (msym_not_found_p (bp_objfile_data->longjmp_msym[i].minsym))
+	continue;
+
+      func_name = longjmp_names[i];
+      if (bp_objfile_data->longjmp_msym[i].minsym == NULL)
+	{
+	  struct bound_minimal_symbol m;
+
+	  m = lookup_minimal_symbol_text (func_name, objfile);
+	  if (m.minsym == NULL)
+	    {
+	      /* Prevent future lookups in this objfile.  */
+	      bp_objfile_data->longjmp_msym[i].minsym = &msym_not_found;
 	      continue;
 	    }
+	  bp_objfile_data->longjmp_msym[i] = m;
+	}
 
-	  if (!gdbarch_get_longjmp_target_p (gdbarch))
-	    continue;
+      addr = BMSYMBOL_VALUE_ADDRESS (bp_objfile_data->longjmp_msym[i]);
+      b = create_internal_breakpoint (gdbarch, addr, bp_longjmp_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;
+      installed_bp++;
+    }
 
-	  for (i = 0; i < NUM_LONGJMP_NAMES; i++)
-	    {
-	      struct breakpoint *b;
-	      const char *func_name;
-	      CORE_ADDR addr;
-	      struct explicit_location explicit_loc;
+  return installed_bp > 0;
+}
 
-	      if (msym_not_found_p (bp_objfile_data->longjmp_msym[i].minsym))
-		continue;
+/* Create a master longjmp breakpoint.  */
 
-	      func_name = longjmp_names[i];
-	      if (bp_objfile_data->longjmp_msym[i].minsym == NULL)
-		{
-		  struct bound_minimal_symbol m;
+static void
+create_longjmp_master_breakpoint (void)
+{
+  scoped_restore_current_program_space restore_pspace;
 
-		  m = lookup_minimal_symbol_text (func_name, objfile);
-		  if (m.minsym == NULL)
-		    {
-		      /* Prevent future lookups in this objfile.  */
-		      bp_objfile_data->longjmp_msym[i].minsym = &msym_not_found;
-		      continue;
-		    }
-		  bp_objfile_data->longjmp_msym[i] = m;
-		}
+  for (struct program_space *pspace : program_spaces)
+    {
+      set_current_program_space (pspace);
 
-	      addr = BMSYMBOL_VALUE_ADDRESS (bp_objfile_data->longjmp_msym[i]);
-	      b = create_internal_breakpoint (gdbarch, addr, bp_longjmp_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;
-	    }
+      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_longjmp_master_breakpoint_probe (obj))
+	    continue;
+
+	  /* Try longjmp_names kind breakpoints.  */
+	  if (create_longjmp_master_breakpoint_names (obj))
+	    continue;
+
+	  /* Iterate over separate debug objects and try longjmp_names
+	     kind breakpoints.  */
+	  for (objfile *sepdebug = obj->separate_debug_objfile;
+	       sepdebug != nullptr; sepdebug = sepdebug->separate_debug_objfile)
+	    if (create_longjmp_master_breakpoint_names (sepdebug))
+	      break;
 	}
     }
 }