[RFC] Implementation of DW_OP_deref and DW_OP_deref_size operators

Mark Kettenis
2002-07-04T22:02:50Z

After staring for a while at some compiler warnings resulting from an
attempt to make an i386 x x86-64 cross-debugger, and failing to
understand the code that implements the DW_OP_deref and
DW_OP_deref_size operators I think I've found out what's wrong.

These expressions are supposed to dereference pointers. In GDB this
means that they should look at the address indicated by the pointer
they're dereferencing *in the target*. The current code is trying to
dereference a pointer in GDB. If we look at the history of this code
it is perfectly understandable. It is largely copied from the
unwinder that comes with GCC. This unwinder is supposed to be linked
into the program that contains the DWARF2 code, so in that context it
makes sense to lookup pointers in the program this code is running in.

Does this sound reasonable?

To fix this, I propose the following patch.

Ok, to check this in?

Mark

Index: ChangeLog
from Mark Kettenis <kettenis@gnu.org>

* dwarf2cfi.c (execute_stack_op): Fix implementation of the
DW_OP_deref and DW_OP_deref_size operators by letting do their
lookup in the target.

Index: dwarf2cfi.c
===================================================================
RCS file: /cvs/src/src/gdb/dwarf2cfi.c,v
retrieving revision 1.12
diff -u -p -r1.12 dwarf2cfi.c
--- dwarf2cfi.c 4 Jul 2002 14:43:51 -0000 1.12
+++ dwarf2cfi.c 4 Jul 2002 21:50:39 -0000
@@ -21,6 +21,7 @@
Boston, MA 02111-1307, USA. */

#include "defs.h"
+#include "gdbcore.h"
#include "symtab.h"
#include "symfile.h"
#include "objfiles.h"
@@ -1119,32 +1120,21 @@ execute_stack_op (struct objfile *objfil
{
case DW_OP_deref:
{
- char *ptr = (char *) result;
- result = read_pointer (objfile->obfd, &ptr);
+ int len = TARGET_ADDR_BIT / TARGET_CHAR_BIT;
+ if (len != 4 && len != 8)
+ internal_error (__FILE__, __LINE__,
+ "execute_stack_op error");
+ result = read_memory_unsigned_integer (result, len);
}
break;

case DW_OP_deref_size:
{
- char *ptr = (char *) result;
- switch (*op_ptr++)
- {
- case 1:
- result = read_1u (objfile->obfd, &ptr);
- break;
- case 2:
- result = read_2u (objfile->obfd, &ptr);
- break;
- case 4:
- result = read_4u (objfile->obfd, &ptr);
- break;
- case 8:
- result = read_8u (objfile->obfd, &ptr);
- break;
- default:
- internal_error (__FILE__, __LINE__,
- "execute_stack_op error");
- }
+ int len = *op_ptr++;
+ if (len != 1 && len != 2 && len != 4 && len !=8)
+ internal_error (__FILE__, __LINE__,
+ "execute_stack_op error");
+ result = read_memory_unsigned_integer (result, len);
}
break;