write memory of the parent process via /proc/$PPID/mem

gistfile1.txt

$ gcc write_ppid_memory.c -o write_ppid_memory

$ sudo chown root write_ppid_memory

$ sudo chmod u+s write_ppid_memory

$ ls -al
-rwsr-xr-x 1 root user 8984 Jan  4 14:35 write_ppid_memory*
-rw-r--r-- 1 user user  475 Jan  4 14:28 write_ppid_memory.c

$ ./write_ppid_memory
Trace/breakpoint trap (core dumped)

write_ppid_memory.c

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

int main()
{
    int pid = fork();
    if (pid == 0) {
        /* child */
        char fpath[256];
        snprintf(fpath, sizeof(fpath), "/proc/%d/mem", getppid());
        int fd = open(fpath, 2);
        lseek(fd, 0x400801, 0);
        write(fd, "\xcc", 1);
    } else {
        /* parent */
        waitpid(pid, NULL, 0);
    }
    return 0;
}