有两种情况，

1. 您想在执行setuid程序时暂时删除root权限
2. 您想在执行setuid程序时永久删除root权限...

• 您可以通过将euid设置为真实用户id，然后将uid更改为您想要的任何内容来临时执行此操作.稍后，当您需要恢复root权限时，可以将uid设置为root，有效的userid将更改为root.这是因为保存的用户id没有更改.
• 通过直接将uid更改为权限较低的用户id，可以永久删除权限.之后，无论发生什么，都无法恢复根权限.

case 1:

在setuid程序开始执行后

1.seteuid(600);
2.setuid(1000);
3.setuid(0);

在这种情况下，根权限可以再次获得.

              +----+------+------------+
              | uid|euid  |saved-uid   |
              |----|------|------------|
            1.|1000| 0    | 0          |
            2.|1000| 600  | 0          |
            3.|1000| 1000 | 0          |
            4.|1000|  0   | 0          |
              |    |      |            |
              +------------------------+

case 2:

在setuid程序开始执行后，

1.setuid(1000);
2.setuid(0);



               +----+------+------------+
               | uid|euid  |saved-uid   |
               |----|------|------------|
             1.|1000|0     | 0          |
             2.|1000|1000  | 1000       |
               |    |      |            |
               +------------------------+

在这种情况下，您无法获得root权限.

cat/proc/PROCID/task/PROCID/status | less

Uid:    1000    0       0       0
Gid:    1000    0       0       0

这个命令将显示Uid和Gid，它将有4个字段(前三个字段是我们关注的字段).类似上面的东西

The three fields represent uid,euid and saved-user-id. You can introduce a pause (an input from user) in your setuid program and check for each step the cat/proc/PROCID/task/PROCID/status | less command. During each step you can check the saved uid getting changed as mentioned.

If you're euid is root and you change the uid, the privileges gets dropped permanently.If effective user id is not root then saved user id is never touched and you can regain the root privilege back anytime you want in your program.