catch catch before program starts

catch catch before program starts

[Aleksandar Ristovski]

[-- Attachment #1: Type: text/plain, Size: 829 bytes --]

Hello,

When we first load symbols, we can not set "catch catch" or "catch throw". The error message will say "Function __cxa_... not defined".

However, we can set a breakpoint in .plt section. The attached patch illustrates this (it is not finished as there will be some error messages even though the catch will be set).

The idea is to try __cxa_begin_catch and then, failing that, to try with __cxa_begin_catch@plt. 

Now:
(gdb) catch catch
Function "__cxa_begin_catch" not defined.
(gdb)


After patch:
(gdb) file /tmp/sig/main
Reading symbols from /tmp/sig/main...done.
(gdb) catch catch
Oops...Function "__cxa_begin_catch" not defined.
Catchpoint 1 (catch)
(gdb) catch throw
Oops...Function "__cxa_throw" not defined.
Catchpoint 2 (throw)

(Ignore the "Oops" part).


Comments?

Aleksandar Ristovski
QNX Software Systems

[-- Attachment #2: breakpoint.c.catchatplt.diff --]
[-- Type: text/plain, Size: 2104 bytes --]

Index: gdb/breakpoint.c
===================================================================
RCS file: /cvs/src/src/gdb/breakpoint.c,v
retrieving revision 1.316
diff -u -p -r1.316 breakpoint.c
--- gdb/breakpoint.c	26 Apr 2008 05:43:45 -0000	1.316
+++ gdb/breakpoint.c	28 Apr 2008 20:43:44 -0000
@@ -6561,6 +6566,19 @@ static struct breakpoint_ops gnu_v3_exce
   print_mention_exception_catchpoint
 };
 
+struct decode_line_args
+{
+  char **argptr;
+  struct symtabs_and_lines *sals;
+};
+
+static int decode_line_no_exceptions (void *arg)
+{
+  struct decode_line_args *dlargs = arg;
+  *dlargs->sals = decode_line_1 (dlargs->argptr, 1, NULL, 0, NULL, NULL);
+  return 0;
+}
+
 static int
 handle_gnu_v3_exceptions (int tempflag, char *cond_string,
 			  enum exception_event_kind ex_event, int from_tty)
@@ -6568,18 +6586,39 @@ handle_gnu_v3_exceptions (int tempflag, 
   char *trigger_func_name, *nameptr;
   struct symtabs_and_lines sals;
   struct breakpoint *b;
+  struct decode_line_args dummy;
+
+  sals.nelts = 0;
 
   if (ex_event == EX_EVENT_CATCH)
     trigger_func_name = xstrdup ("__cxa_begin_catch");
-  else
+  else if (ex_event == EX_EVENT_THROW)
     trigger_func_name = xstrdup ("__cxa_throw");
+  else
+    trigger_func_name = NULL;
 
   nameptr = trigger_func_name;
-  sals = decode_line_1 (&nameptr, 1, NULL, 0, NULL, NULL);
+  dummy.argptr = &nameptr;
+  dummy.sals = &sals;
+  catch_errors (decode_line_no_exceptions, &dummy, "Oops...", RETURN_MASK_ERROR);  
   if (sals.nelts == 0)
     {
+      /* Try setting the breakpoint in trampoline.  */
       xfree (trigger_func_name);
-      return 0;
+
+      if (ex_event == EX_EVENT_CATCH)
+	trigger_func_name = xstrdup ("\"'__cxa_begin_catch@plt'\"");
+      else if (ex_event == EX_EVENT_THROW)
+	trigger_func_name = xstrdup ("\"'__cxa_throw@plt'\"");
+
+      nameptr = trigger_func_name;
+      catch_errors (decode_line_no_exceptions, 
+		    &dummy, "No go.", RETURN_MASK_ERROR);
+      if (sals.nelts == 0)
+	{
+	  xfree (trigger_func_name);
+	  return 0;
+	}
     }
 
   b = set_raw_breakpoint (sals.sals[0], bp_breakpoint);

Re: catch catch before program starts

[Daniel Jacobowitz]

On Mon, Apr 28, 2008 at 04:51:17PM -0400, Aleksandar Ristovski wrote:
> Hello,
>
> When we first load symbols, we can not set "catch catch" or "catch throw". The error message will say "Function __cxa_... not defined".
>
> However, we can set a breakpoint in .plt section. The attached patch illustrates this (it is not finished as there will be some error messages even though the catch will be set).

I suggest this be handled in the generic breakpoint-setting code, not
specific to "catch".  It's not a complete solution, by the way: it
will only help if the main executable uses catch / throw.  If only
shared libraries do, then there won't be a PLT entry in the
executable.  So maybe we should allow these breakpoints to be created
"pending"?

-- 
Daniel Jacobowitz
CodeSourcery