User and group permissions

u+s How is it realized ？

When a user changes his password , Need modification shadow file , This file is inaccessible to ordinary users ; Why ordinary users can change their passwords ？

This is because passwd Document has u+s jurisdiction ; When executed passwd Command will be switched to root identity ;

shell Implementation process of permission ;

 sudo The implementation of the program ：mysu.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

int main(int argc, char **argv)
{
    pid_t pid;
    
    if(argc < 3)
    {
        fprintf(stderr, "Usage: %s <command> <file>\n", argv[0]);
        exit(1);
    }

    pid = fork();
    if(pid < 0)
    {
        perror("fork()");
        exit(1);
    }
    if(pid == 0)
    {
        setuid(atoi(argv[1]));
        execvp(argv[2], argv+2);
        perror("execvp()");
        exit(1);
    }
    wait(NULL);

    exit(0);
}

Execution requires ：

chown root mysu

chmod u+s mysu

./mysu 0 cat /etc/shadow        // With 0 Execute as user No