ptrace and ORIG_EAX on ppc

ashwin tanugula ashwin.tanugula at
Thu Mar 31 05:15:17 EST 2005

On Tue, 29 Mar 2005 23:25:58 -0800, Brad Boyer <flar at> wrote:
> On Tue, Mar 29, 2005 at 08:01:06PM -0500, ashwin tanugula wrote:
> > Hi!
> > Can somebody tell me how to set references to ORIG_EAX in ppc kernel.
> > We know ORIG_EAX is defined in unistd.h of i386. How do i make the
> > following program print two same pids in ppc?
> The ORIG_EAX is a reference to a register as saved in the stack. The
> equivalent on ppc would seem to be PT_ORIG_R3. You're digging around
> in an area that is extremely non-portable. This program appears to
> be messing around with the syscall number before dispatch in the
> syscall table. The implementation of ptrace on ppc and ppc64
> explicitly does not allow changing this value, so I think you'll need
> to find a completely different way to achieve your desired result.
> Just to lessen the confusion, what are you trying to accomplish? Not
> in the low-level detail sense, but what is the big picture goal?
>         Brad Boyer
>         flar at
Thanks to Brad for his help.
ORIG_EAX has to be changed to PT_R0 not to PT_ORIG_R3.
The output i got after changing ORIG_EAX to PT_R0 is
root at slemieux:/home/ashwin # gcc ptrace_test.c
root at slemieux:/home/ashwin # ./a.out
Parent pid = 16682
getpid() returned 16682
getpid() returned 16682


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