c - setuid 和 seteuid 函数的区别

Question

程序 1:使用 setuid() 的示例

    #include<stdio.h>
    #include<sys/types.h>
    #include<unistd.h>
    void main()
    {
        printf("Real user id = %d, Effective User id = %d\n",getuid(),geteuid());
        setuid(1000);
        printf("Real user id = %d, Effective User id = %d\n",getuid(),geteuid());
        setuid(1014);
        printf("Real user id = %d, Effective User id = %d\n",getuid(),geteuid());
    }

输出:

    guest $ ./a.out 
    Real user id = 1000, Effective User id = 1014
    Real user id = 1000, Effective User id = 1000
    Real user id = 1000, Effective User id = 1014
    guest $

程序 2:seteuid() 示例

    #include<stdio.h>
    #include<sys/types.h>
    #include<unistd.h>
    void main()
    {
        printf("Real user id = %d, Effective User id = %d\n",getuid(),geteuid());
        seteuid(1000);
        printf("Real user id = %d, Effective User id = %d\n",getuid(),geteuid());
        seteuid(1014);
        printf("Real user id = %d, Effective User id = %d\n",getuid(),geteuid());
    }

输出:

    guest $ ./a.out 
    Real user id = 1000, Effective User id = 1014
    Real user id = 1000, Effective User id = 1000
    Real user id = 1000, Effective User id = 1014
    guest $

两个程序给出相同的输出。那么，这两个函数有什么区别呢？根据引用资料(手册页)，这两个函数都用于设置进程的有效用户 ID。这两个程序的功能有何不同？

Answer 1

The documentation很清楚区别:

If the user is root or the program is set-user-ID-root, special care must be taken. The setuid() function checks the effective user ID of the caller and if it is the superuser, all process-related user ID's are set to uid. After this has occurred, it is impossible for the program to regain root privileges.

Thus, a set-user-ID-root program wishing to temporarily drop root privileges, assume the identity of an unprivileged user, and then regain root privileges afterward cannot use setuid(). You can accomplish this with seteuid.