level19.c

#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>

int main(int argc, char **argv, char **envp)
{
	pid_t pid;
	char buf[256];
	struct stat statbuf;

	/* Get the parent's /proc entry, so we can verify its user id */

	snprintf(buf, sizeof(buf)-1, "/proc/%d", getppid());

	/* stat() it */

	if(stat(buf, &statbuf) == -1) {
		printf("Unable to check parent process\n");
		exit(EXIT_FAILURE);
	}

	/* check the owner id */

	if(statbuf.st_uid == 0) {
		/* If root started us, it is ok to start the shell */

		execve("/bin/sh", argv, envp);
		err(1, "Unable to execve");
	}

	printf("You are unauthorized to run this program\n");
}