模拟Linux修改实际、有效和保存设置标识

// setuid.cc: 模拟<unistd.h>中的设置用户ID的方法的作用
#include <stdio.h>
#include <errno.h>

int real = 0;          // 实际用户ID
int effective = 0;     // 有效用户ID
int saved = 0;         // 保存的设置用户ID

void showid();  // 打印所有ID

inline bool hasPrivilege() { return effective == 0; }
#define EINVAL_RETURN { errno = EINVAL; return -1; }
#define EPERM_RETURN { errno = EPERM; return -1; }

int setuid(int uid)  // 修改所有用户ID
{
    if (uid < 0)
        EINVAL_RETURN;
    if (!hasPrivilege() && uid != real && uid != saved)
        EPERM_RETURN;
    if (hasPrivilege())
        real = effective = saved = uid;
    else
        effective = uid;
    return 0;
}

int seteuid(int euid)  // 修改有效用户ID
{
    if (euid < 0)
        EINVAL_RETURN;
    if (!hasPrivilege() && euid != real && euid != saved)
        EPERM_RETURN;
    effective = euid;
    return 0;
}

int setreuid(int ruid, int euid)  // 修改实际/有效用户ID
{
    if (ruid < -1 || euid < -1)
        EINVAL_RETURN;
    if (!hasPrivilege())
    {
        if (ruid != -1 && ruid != real && ruid != effective)
            EPERM_RETURN;
        if (euid != -1 && euid != real && euid != effective && euid != saved)
            EPERM_RETURN;
    }
    int old_real = real;
    real = (ruid != -1) ? ruid : real;
    effective = (euid != -1) ? euid : effective;
    if (effective != old_real)
        saved = effective;
    return 0;
}

// 非SUSv3规范, 其他UNIX实现对其也鲜有支持
int setresuid(int ruid, int euid, int suid)  // 修改实际/有效/保存用户ID
{
    if (ruid < -1 || euid < -1 || suid < -1)
        EINVAL_RETURN;
    if (!hasPrivilege())
    {
        if (ruid != -1 && ruid != real && ruid != effective && ruid != saved)
            EPERM_RETURN;
        if (euid != -1 && euid != real && euid != effective && euid != saved)
            EPERM_RETURN;
        if (suid != -1 && suid != real && suid != effective && suid != saved)
            EPERM_RETURN;
    }
    real = (ruid != -1) ? ruid : real;
    effective = (euid != -1) ? euid : effective;
    saved = (suid != -1) ? suid : saved;
    return 0;
}

int main()
{
    real = 1000;
    // 下面4句只能执行其中1句
//    setuid(2000);
    setreuid(-1, 2000);
//    seteuid(2000);
//    setresuid(-1, 2000, 3000);

    showid();
    return 0;
}

void showid()
{
    printf("实际用户ID:       %4d\n", real);
    printf("有效用户ID:       %4d\n", effective);
    printf("保存的设置用户ID: %4d\n", saved);
}