gdb: fix crash when getting the value of a label symbol

When the source program contains a goto label, it turns out it's
actually pretty hard for a user to find out more about that label.
For example:

  (gdb) p some_label
  No symbol "some_label" in current context.
  (gdb) disassemble some_label
  No symbol "some_label" in current context.
  (gdb) x/10i some_label
  No symbol "some_label" in current context.
  (gdb) break some_label
  Breakpoint 2 at 0x401135: file /tmp/py-label-symbol-value.c, line 35.

In all cases, some_label is a goto label within the current frame.
Only placing a breakpoint on the label worked.

This all seems a little strange to me, it feels like asking about a
goto label would not be an unreasonable thing for a user to do.

This commit doesn't fix any of the above issues, I mention them just
to provide a little context for why the following issue has probably
not been seen before.

It turns out there is one way a user can access the symbol for a goto
label, through the Python API:

  python frame = gdb.selected_frame()
  python frame_pc = frame.pc()
  python block = gdb.current_progspace().block_for_pc(frame_pc)
  python symbol,_ = gdb.lookup_symbol('some_label', block, gdb.SYMBOL_LABEL_DOMAIN)
  python print(str(symbol.value()))
  ../../src/gdb/findvar.c:204: internal-error: store_typed_address: Assertion `type->is_pointer_or_reference ()' failed.

The problem is that label symbols are created using the
builtin_core_addr type, which is a pure integer type.

When GDB tries to fetch the value of a label symbol then we end up in
findvar.c, in the function language_defn::read_var_value, in the
LOC_LABEL case.  From here store_typed_address is called to store the
address of the label into a value object with builtin_core_addr type.

The problem is that store_typed_address requires that the destination
type be a pointer or reference, which the builtin_core_addr type is
not.

Now it's not clear what type a goto label address should have, but
GCC has an extension that allows users to take the address of a goto
label (using &&), in that case the result is of type 'void *'.

I propose that when we convert the CORE_ADDR value to a GDB value
object, we use builtin_func_ptr type instead of builtin_core_addr,
this means the result will be of type 'void (*) ()'.  The benefit of
this approach is that when gdbarch_address_to_pointer is called the
target type will be correctly identified as a pointer to code, which
should mean any architecture specific adjustments are done correctly.

We can then cast the new value to 'void *' type with a call to
value_cast_pointer, this should not change the values bit
representation, but will just update the type.

After this asking for the value of a label symbol works just fine:

  (gdb) python print(str(symbol.value()))
  0x401135 <main+35>

And the type is maybe what we'd expect:

  (gdb) python print(str(symbol.value().type))
  void *

gdb/findvar.c

@@ -593,20 +593,32 @@ language_defn::read_var_value (struct symbol *var,
       return v;
 
     case LOC_LABEL:
-      /* Put the constant back in target format.  */
-      v = allocate_value (type);
-      if (overlay_debugging)
-	{
-	  struct objfile *var_objfile = var->objfile ();
-	  addr = symbol_overlayed_address (var->value_address (),
-					   var->obj_section (var_objfile));
-	  store_typed_address (value_contents_raw (v).data (), type, addr);
-	}
-      else
-	store_typed_address (value_contents_raw (v).data (), type,
-			      var->value_address ());
-      VALUE_LVAL (v) = not_lval;
-      return v;
+      {
+	/* Put the constant back in target format.  */
+	if (overlay_debugging)
+	  {
+	    struct objfile *var_objfile = var->objfile ();
+	    addr = symbol_overlayed_address (var->value_address (),
+					     var->obj_section (var_objfile));
+	  }
+	else
+	  addr = var->value_address ();
+
+	/* First convert the CORE_ADDR to a function pointer type, this
+	   ensures the gdbarch knows what type of pointer we are
+	   manipulating when value_from_pointer is called.  */
+	type = builtin_type (var->arch ())->builtin_func_ptr;
+	v = value_from_pointer (type, addr);
+
+	/* But we want to present the value as 'void *', so cast it to the
+	   required type now, this will not change the values bit
+	   representation.  */
+	struct type *void_ptr_type
+	  = builtin_type (var->arch ())->builtin_data_ptr;
+	v = value_cast_pointers (void_ptr_type, v, 0);
+	VALUE_LVAL (v) = not_lval;
+	return v;
+      }
 
     case LOC_CONST_BYTES:
       if (is_dynamic_type (type))

gdb/testsuite/gdb.python/py-label-symbol-value.c

@@ -0,0 +1,38 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see  <http://www.gnu.org/licenses/>.  */
+
+volatile int global_var = 1;
+
+int
+get_value ()
+{
+  return global_var;
+}
+
+int
+main (void)
+{
+  int value = get_value ();
+  if (value > 0)
+    goto some_label;
+
+  return 1;
+
+ some_label:
+
+  return 0;
+}

gdb/testsuite/gdb.python/py-label-symbol-value.exp

@@ -0,0 +1,39 @@
+# Copyright 2022 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check that GDB handles the user asking for the value of a label
+# symbol (i.e. a symbol for a goto label).
+
+load_lib gdb-python.exp
+standard_testfile
+
+if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
+    return -1
+}
+
+# Skip all tests if Python scripting is not enabled.
+if { [skip_python_tests] } { continue }
+
+if ![runto_main] {
+   return -1
+}
+
+# Use Python to print the value of the 'some_label' symbol.
+gdb_test "python frame = gdb.selected_frame()"
+gdb_test "python frame_pc = frame.pc()"
+gdb_test "python block = gdb.current_progspace().block_for_pc(frame_pc)"
+gdb_test "python symbol,_ = gdb.lookup_symbol('some_label', block, gdb.SYMBOL_LABEL_DOMAIN)"
+gdb_test "python print(str(symbol.value()))" "$hex <main\\+$decimal>"
+gdb_test "python print(str(symbol.value().type))" "void \\*"