Stack Walk on linux using ptrace

Question

Following is my requirement.

while process A is running.

1. attach Process A from B with PTRACE_ATTACH.
2. Start a Loop
3. Stop Process A
4. read registers
5. Resume Process A
6. sleep(1)
7. end loop
8. detach A

i am facing issue with Start and Resume Process A from the loop. i tried combination of kill(pid,SIGSTOP), kill(pid,SIGCONT), PTRACE_CONT. but didnt work.

any other solutions please?

Thanks in advance. Sandeep

Answer 1

Following code is working for me and seems to fulfill your requirements -

A.c

#include<stdio.h>
int main()
{
   int i=0;
   printf("My PID is - %ld\n",getpid());
   while(i>=0)
   {
   }
   return 0;
}

B.c - Tracing process

int main()
{
   int pid;
   int status;
   struct user_regs_struct regs;
   unsigned int eip;

   printf("Enter pid to trace : \n");
   scanf("%d",&pid);
   printf("PID to be traced - %ld\n",pid);

   ptrace(PTRACE_ATTACH,pid,0,0);
   if(errno)
   {
        perror("attach");
        return -1;
   }

   waitpid(pid,&status,WUNTRACED);

   printf("Process Stopped\n");
   while(1)
   {
      ptrace(PTRACE_GETREGS,pid,0,&regs);
      eip=ptrace(PTRACE_PEEKTEXT,pid,regs.eip,0);

      printf("EIP - 0x%08x, instruction executed - 0x%08x\n",regs.eip,eip);

      ptrace(PTRACE_CONT,pid,0,0);
      waitpid(pid,&status,WUNTRACED);
   }

   return 0;

}

Signal passed -

kill -STOP 17779 kill -STOP 17779

Output of A -

xxxxx!xxxxx:~/myPer/stack_overflow [135]$ ./A
My PID is - 17779

Output of B -

XXXXX!xxxxx:~/myPer/stack_overflow [121]$ ./B
Enter pid to trace :
17779
PID to be traced - 17779
Process Stopped
EIP - 0x080483e1, instruction executed - 0x00f87d83
EIP - 0x080483e5, instruction executed - 0x00b8fa79
EIP - 0x080483e5, instruction executed - 0x00b8fa79

We see that B displays EIP value for each signal delivered to client. Basically signal is not getting delivered to A instead B wakes up and examines EIP and then continues in the loop. You can modify the code to deliver the signal if you want.

This is what i understood from your question. If i understood something else please let me know and i'll update answer accordingly

Answer 2

Sounds like a very challenging project to undertake from scratch. Have you considered leveraging the GNU debugger in any way? In particular there is an long running sub-project called libgdb2 which may suit your purposes even though it is far from finished or stable at this time.

Answer 3

You could try scripting/interfacing with gdb in the same way that a lot of IDE's do. See also http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/gdb/gdb-mi.html