How to play with ptrace on x86-64?

Question

I'm following the tutorial here, and modified a little for x86-64(basically replace eax to rax,etc) so that it compiles:

#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <sys/user.h>
#include <sys/reg.h>
#include <unistd.h>


int main()
{   pid_t child;
    long orig_eax;
    child = fork();
    if(child == 0) {
        ptrace(PTRACE_TRACEME, 0, NULL, NULL);
        execl("/bin/ls", "ls", NULL);
    }
    else {
        wait(NULL);
        orig_eax = ptrace(PTRACE_PEEKUSER,
                          child, 4 * ORIG_RAX,
                          NULL);
        printf("The child made a "
               "system call %ld\n", orig_eax);
        ptrace(PTRACE_CONT, child, NULL, NULL);
    }
    return 0;
}

But it doesn't actually work as expected, it always says:

The child made a system call -1

What's wrong in the code?

Answer 1

ptrace returns -1 with errno EIO because what you're trying to read is not correctly aligned. Taken from ptrace manpage:

   PTRACE_PEEKUSER
          Reads a word at offset addr in  the  child's  USER  area,  which
          holds the registers and other information about the process (see
          <sys/user.h>).  The word  is  returned  as  the  result  of  the
          ptrace()  call.   Typically  the  offset  must  be word-aligned,
          though this might vary by architecture.  See  NOTES.   (data  is
          ignored.)

In my 64-bits system, 4 * ORIG_RAX is not 8-byte-aligned. Try with values such 0 or 8 and it should work.

Answer 2

In 64 bit = 8 * ORIG_RAX

8 = sizeof(long)