setuid函数的学习笔记 转自：http://blog.chinaunix.net/space.php?uid=1877180&do=blog&cuid=1132108

转自：http://blog.chinaunix.net/space.php?uid=1877180&do=blog&cuid=1132108

在linux中每个进程有三个[实际上有第4个]用户标识符.

real uid : 真实用户ID.
saved uid : 已保存用户ID
effective uid : 有效用户ID
真实用户ID(real uid)是login时的用户.而在运行过程中,
用于所有的安全检查的是有效用户ID(effective uid).
一般情况下:
real uid = saved uid = effective uid

在某些场合下,使用用setuid,setruid函数可以改变effective uid,从而
使得程序运行时具有特殊的权限.常见的例子是linux系统中的passwd命令,
由于所有的用户信息包括用户密码都保存在/etc/passwd文件中,而/etc/passwd
文件只有root权限可以读写,若想让每个用户都只可以修改自己的密码,就必须
让普通用户暂时获得有限的读写/etc/passwd的权限.用setuid就可以解决这个
问题.

Linux setuid(uid)函数:
(1)如果由普通用户调用,将当前进程的有效ID设置为uid.
(2)如果由有效用户ID符为0的进程调用,则将真实,有效和已保存用户ID都设
置为uid.
Linux的setuid函数和Unix中的setuid函数的行为是不同的.
Unix中.setuid(uid)函数的行为:
(1)如果进程没有超级用户特权,且uid等于实际用户ID或已保存用户ID,则只
将有效的用户ID设置为uid.否则返回错误.
(2)如果进程是有超级用户特权,则将真实,有效和
已保存用户表示符都设置为uid.

这里主要的区别在于普通用户调用时的行为.产生这个问题的原因是POSIX和
BSD的实现差异,而linux却同时支持这两者.BSD中使用
setreuid(uid_t ruid, uid_t euid)
来设定真实用户ID(real uid)和有效用户ID(effective uid).这个函数在由有效
用户ID符为0的进程调用时,不会改变已保存用户ID.函数seteuid(uid_t uid)等价
于setreuid(-1,uid),只改变有效用户ID(effective uid).


例子:
使用setuid或是setruid,让非root用户也可以读取只有root用户有读写权限的
文件.

#假设此程序名为:setuid_ex
#要读取的文件为:root_only.txt
$ users
dd admin
$ ls -l root_only.txt
-rw------- 1 root root 33 Jan 11 17:07 root_only.txt
$ vi setuid_ex.cpp
1 /**
2 * Linux setuid函数的例子.
3 * */
4 #include <unistd.h> //setuid() and getuid()
5 #include <iostream>
6 #include <fstream>
7
8 using namespace std;
9 void test_read_file(const char *name)
10 {
11 ifstream f(name);
12 if(f.fail())
13 cout<<"=[ERROR]: read failed."<<endl;
14 else
15 cout<<"=[OK]: read successful."<<endl;
16 }
17 //打印uid和euid.
18 inline void p_states(void)
19 {
20 int uid,euid;
21 cout<<"-----Current states--------------------------"<<endl;
22 cout<<"real uid\t"<<getuid()<<endl;
23 cout<<"effective uid\t"<<geteuid()<<endl;
24 cout<<"---------------------------------------------"<<endl;
25 }
26 //调用setuid.
27 inline void run_setuid_fun(int uid)
28 {
29 if(setuid(uid) == -1)
30 cout<<"=[ERROR]: setuid("<<uid<<") error"<<endl;
31 p_states();
32 }
33 //调用seteuid.
34 inline void run_seteuid_fun(int uid)
35 {
36 if(seteuid(uid) == -1)
37 cout<<"=[ERROR]: seteuid("<<uid<<") error"<<endl;
38 p_states();
39 }
40
41 int main()
42 {
43 int t_re=0;
44 const char * file = "root_only.txt";
45
46 cout<<endl<<"TEST 1: "<<endl;
47 p_states();
48 //[1]此时 real uid= login user id
49 // effective uid = root
50 // saved uid = root
51 test_read_file(file);
52
53 cout<<endl<<"TEST 2: seteuid(getuid())"<<endl;
54 run_seteuid_fun(getuid());
55 //[2]此时 real uid= login user id
56 // effective uid = login user id
57 // saved uid = root
58 test_read_file(file);
59
60 cout<<endl<<"TEST 3: seteuid(0)"<<endl;
61 run_seteuid_fun(0);
62 //[3]此时 real uid= login user id
63 // effective uid = root
64 // saved uid = root
65 test_read_file(file);
66
67 cout<<endl<<"TEST 4: setuid(0)"<<endl;
68 run_setuid_fun(0);
69 //[4]此时 real uid= root
70 // effective uid = root
71 // saved uid = root
72 test_read_file(file);
73
74 cout<<endl<<"TEST 5: setuid(503)"<<endl;
75 run_setuid_fun(503);
76 //[5]此时 real uid= login user id
77 // effective uid = login user id
78 // saved uid = login user id
79 test_read_file(file);
80
81 cout<<endl<<"TEST 6: setuid(0)"<<endl;
82 //[6]此时 real uid= login user id
83 // effective uid = login user id
84 // saved uid = login user id
85 // 运行setuid(0)是将返回错误值-1.
86 run_setuid_fun(0);
87 test_read_file(file);
88 return 0;
89 }
90
91
$ vi Makefile
1 src=setuid_ex.cpp
2 exe=setuid_ex
3 cc=g++
4 flags=-g
5
6 all:${src}
7 ${cc} ${flags} -o ${exe}
8 chown root:root ${exe}
9 ### 实际上chmod 4111改变了effective id 和saved uid的值.
10 ### 这也是setuid setruid函数在不同权限间正常切换的前提.
11 chmod 4111 ${exe}

$ sudo make #使用root进行Make.
$ ls -l setuid_ex
---s--x--x 1 root root 58579 Jan 15 11:27 setuid_ex
$ ./setuid_ex
TEST 1:
-----Current states--------------------------
real uid 503
effective uid 0
---------------------------------------------
=[OK]: read successful.

TEST 2: seteuid(getuid())
-----Current states--------------------------
real uid 503
effective uid 503
---------------------------------------------
=[ERROR]: read failed.

TEST 3: seteuid(0)
-----Current states--------------------------
real uid 503
effective uid 0
---------------------------------------------
=[OK]: read successful.

TEST 4: setuid(0)
-----Current states--------------------------
real uid 0
effective uid 0
---------------------------------------------
=[OK]: read successful.

TEST 5: setuid(503)
-----Current states--------------------------
real uid 503
effective uid 503
---------------------------------------------
=[ERROR]: read failed.

TEST 6: setuid(0)
=[ERROR]: setuid(0) error
-----Current states--------------------------
real uid 503
effective uid 503
---------------------------------------------
=[ERROR]: read failed.