How to modify EIP's tracee forked procee?

Question

I\'m working on a Linux application incorporating ptrace to observe another process which had been created by fork() system call.

Strictly speaking: I want to implement

Answer 1

You're not modifying the EIP, you're adding something to the value of the instruction at EIP, and probably resulting in a bad address reference. To change EIP, use PTRACE_SETREGS

      wait(NULL);
      ptrace(PTRACE_GETREGS, child,NULL, ®s);
      printf("\n EIP @  0x %#lx\n",regs.eip);
      regs.eip += ???;
      ptrace(PTRACE_SETREGS, child, NULL, ®s);
      ptrace(PTRACE_CONT, child, NULL, NULL);