就是模拟setuid seteuid setreuid setresuid，感觉代码比书上大段的文字好记，就写成代码形式了。
// 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;
}
real = (ruid != -1) ? ruid : real;
effective = (euid != -1) ? euid : effective;
if (ruid != -1 || effective != 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);
}
main函数是TLPI第9章习题第1道的运行结果，然后模拟了一遍功能，后面几道也很简单就能做出来了。以后忘记的话看遍代码就能很快记起来了。