ptrace attach to vsftpd hangs - linux

Question

I am trying to ptrace a vsftpd server process on linux to be able to get control whenever vsftpd process makes a system call. I start the vsftpd process and pass this process id as command line to the following program which traces vsftpd.
however, when I run the following program it just hangs and does not print anything.Can anyone point out what could be wrong? Thanks a lot for your help!!
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <linux/user.h>
#include <sys/syscall.h> /* For SYS_write etc */
#include<sys/reg.h>
int main(int argc,char* argv[])
{ pid_t child;
long orig_eax, eax;
long params[3];
int status;
int insyscall = 0;
child = atoi(argv[1]);
ptrace(PTRACE_ATTACH,child,NULL,NULL);
while(1) {
wait(&status);
if(WIFEXITED(status))
break;
orig_eax = ptrace(PTRACE_PEEKUSER,
child, 4 * ORIG_EAX, NULL);
if(orig_eax == __NR_clone || orig_eax == __NR_open || orig_eax == __NR_write)
{
if(insyscall == 0) {
/* Syscall entry */
insyscall = 1;
params[0] = ptrace(PTRACE_PEEKUSER,
child, 4 * EBX,
NULL);
params[1] = ptrace(PTRACE_PEEKUSER,
child, 4 * ECX,
NULL);
params[2] = ptrace(PTRACE_PEEKUSER,
child, 4 * EDX,
NULL);
if(orig_eax == __NR_clone)
{
printf("\nClone");
}
else if(orig_eax == __NR_open)
printf("\nOpen");
else if(orig_eax == __NR_write)
printf("\nWrite");
printf(" called with "
"%ld, %ld, %ld\n",
params[0], params[1],
params[2]);
}
else { /* Syscall exit */
eax = ptrace(PTRACE_PEEKUSER,
child, 4 * EAX, NULL);
printf("Returned "
"with %ld\n", eax);
insyscall = 0;
}
}
ptrace(PTRACE_SYSCALL,
child, NULL, NULL);
}
return 0;
}

Answer 1

You need to have the privilege to trace VSFTPD. Run this as root. To test, put the result of ptrace(PTRACE_ATTACH,child,NULL,NULL); into a variable and print it, ie.
long result = ptrace(PTRACE_ATTACH,child,NULL,NULL);
printf("%ld",result);
On my system if result == -1, I do not have permission. If result == 0, I do.