Re: How to recgnize the local varialble

Re: How to recgnize the local varialble

[Jan Kratochvil]

On Wed, 29 Apr 2015 10:42:07 +0200, 高国胜 wrote:
> Accord to the red part,

Please do not depend on the HTML part of mail, it is difficult to display.

> we can get the memory address of the variable i, but how can the injected
> code identify the symbol i ?

So it looks like:
------------------------------------------------------------------------------
void _gdb_expr (struct __gdb_regs *__regs) {
  __attribute__ ((unused)) __gdb_uintptr __i_ptr; {
[...]
    __i_ptr = __gdb_stack[__gdb_tos];
  }
#pragma GCC user_expression
{
#line 1 "gdb command line"
i=20
; } }
------------------------------------------------------------------------------

Starting at that line
	#pragma GCC user_expression
GCC starts to ask GDB about any unknown symbols.  With
	(gdb) set debug compile 1
you can see it as line:
	gcc_convert_symbol "i": local symbol
It is the GDB line:
          decl = C_CTX (context)->c_ops->build_decl (C_CTX (context),
                                                     SYMBOL_NATURAL_NAME (sym),
                                                     kind,
                                                     sym_type,
                                                     symbol_name, addr,
                                                     filename, line);
and 'symbol_name' is "__i_ptr" in this case passed as parameter
             const char *,            /* Argument SUBSTITUTION_NAME.  */

How GCC binds the created symbol "i"'s address to the value of the "__i_ptr"
variable is GCC's magic beyond my knowledge.


Jan