Ptrace segfault

[Garrett Cooper]

On Thu, Apr 29, 2010 at 12:06 PM, Gunnar Hinriksson <tomtinn at gmail.com> wrote:
> Hello
>
> Im having a little problem using ptrace on my system.
> If I use ptrace to attach to another process the child process
> segfaults once I detach.
> For example using this simple program.
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/types.h>
> #include <sys/ptrace.h>
> #include <sys/wait.h>
>
> int main(int argc, char *argv[])
> {
>        int pid = atoi(argv[1]);
>        ptrace(PT_ATTACH, pid, 0, 0);
>        wait(NULL);
>        ptrace(PT_DETACH, pid, 0, 0);
>        return 0;
> }
>
> Am I using ptrace incorrectly or is there perhaps a bug in ptrace that
> causes the child to always segfault ?

    Nope -- it's a bug in your code. From ptrace(2):

     PT_CONTINUE   The traced process continues execution.  The addr argument
                   is an address specifying the place where execution is to be
                   resumed (a new value for the program counter), or
                   (caddr_t)1 to indicate that execution is to pick up where
                   it left off.  The data argument provides a signal number to
                   be delivered to the traced process as it resumes execution,
                   or 0 if no signal is to be sent.

[...]

     PT_DETACH     This request is like PT_CONTINUE, except that it does not
                   allow specifying an alternate place to continue execution,
                   and after it succeeds, the traced process is no longer
                   traced and continues execution normally.

    Note very carefully the fact that PT_DETACH is like PT_CONTINUE,
and that PT_CONTINUE says that addr references the memory where the
execution is going to be resumed.
HTH,
-Garrett