Рассмотрим введение какой-либо функции отладки в процесс ptraced и вызов ее через ptrace_setregs. Что-то вроде того, как GDB может запускать любую функцию процесса ptraced.
Также вы можете попытаться внедрить некоторый код в процесс через LD_PRELOAD. Вы даже можете попытаться сделать в работе без ptrace, используя сигналы.
upd1:
Внедрение GDB или «вызов функции низкого уровня» довольно сложно. Смотрите функцию call_function_by_hand в файле gdb-6.6.50.20070809 ›gdb› infcall.c здесь: http://sources.debian.net/src/gdb/7.6.2-1/gdb/infcall.c?hl=462#L462
/* All this stuff with a dummy frame may seem unnecessarily complicated
(why not just save registers in GDB?). The purpose of pushing a dummy
frame which looks just like a real frame is so that if you call a
function and then hit a breakpoint (get a signal, etc), "backtrace"
will look right. Whether the backtrace needs to actually show the
stack at the time the inferior function was called is debatable, but
it certainly needs to not display garbage. So if you are contemplating
making dummy frames be different from normal frames, consider that. */
/* Perform a function call in the inferior.
ARGS is a vector of values of arguments (NARGS of them).
FUNCTION is a value, the function to be called.
Returns a value representing what the function returned.
May fail to return, if a breakpoint or signal is hit
during the execution of the function.
ARGS is modified to contain coerced values. */
struct value *
call_function_by_hand (struct value *function, int nargs, struct value **args)
{
...
frame = get_current_frame ();
gdbarch = get_frame_arch (frame);
if (!gdbarch_push_dummy_call_p (gdbarch))
error (_("This target does not support function calls."));
/* A cleanup for the inferior status.
This is only needed while we're preparing the inferior function call. */
inf_status = save_infcall_control_state ();
inf_status_cleanup
= make_cleanup_restore_infcall_control_state (inf_status);
/* Save the caller's registers and other state associated with the
inferior itself so that they can be restored once the
callee returns. To allow nested calls the registers are (further
down) pushed onto a dummy frame stack. Include a cleanup (which
is tossed once the regcache has been pushed). */
caller_state = save_infcall_suspend_state ();
make_cleanup_restore_infcall_suspend_state (caller_state);
...
sp = push_dummy_code (gdbarch, sp, funaddr, args, nargs,
target_values_type, &real_pc, &bp_addr,
get_current_regcache ());
... pass args ...
/* Create the dummy stack frame. Pass in the call dummy address as,
presumably, the ABI code knows where, in the call dummy, the
return address should be pointed. */
sp = gdbarch_push_dummy_call (gdbarch, function, get_current_regcache (),
bp_addr, nargs, args,
sp, struct_return, struct_addr);
...
/* Everything's ready, push all the info needed to restore the
caller (and identify the dummy-frame) onto the dummy-frame
stack. */
dummy_frame_push (caller_state, &dummy_id);
...
/* Run the inferior until it stops. */
e = run_inferior_call (tp, real_pc);
}