Content
Each UNIX proces has 3 UIDs associated to it. Superuser privilege is UID=0.
每一个Unix进程拥有3个用户ID与之关联.超级权限的用户ID为0.
Real UID
真实用户ID
This is the UID of the user/process that created THIS process. It can be changed only if the running process has EUID=0.
这是创建这个进程的用户/进程的用户ID.只有有效用户ID是0才能修改这项信息.
Effective UID
有效用户ID
This UID is used to evaluate privileges of the process to perform a particular action. EUID can be change either to RUID, or SUID if EUID!=0. If EUID=0, it can be changed to anything.
这项用户ID是用来扩充进程的权力以用来开展一项特定的操作.有效用户ID在有效用户ID不为0时,能被切换为真实用户ID或者保存用户ID.如果有效用户ID为0,它能被切换成任何用户ID.
Saved UID
保存用户ID
If the binary image file, that was launched has a Set-UID bit on, SUID will be the UID of the owner of the file. Otherwise, SUID will be the RUID.
如果一项二进制图像文件,含有Set-UID的二进制标记,那么保存用户ID将会成为这个文件拥有者的用户ID.否则,保存用户ID将会变成真实用户ID.
What is the idea behind this?
这之后的意义是什么?
Normal programs, like “ls”, “cat”, “echo” will be run by a normal user, under that users UID. Special programs that allow user to have controlled access to protected data, can have Set-UID bit to allow the program to be run under privileged UID.
常见的程序,像"ls",“cat”,"echo"一般用户所运行时,是基于这个一般用户的用户ID进行的.特定的程序当用户在访问限制数据的时候会受到限制,能通过设置Set-UID位来允许该程序通过特权用户ID来运行该程序.
An example of such program is “passwd”. If you list it in full, you will see that it has Set-UID bit and the owner is “root”. When a normal user, say “ananta”, runs “passwd”, passwd starts with:
一个这样的例子就是"passwd".如果你采取命令显示文件的所有信息,就能看到它标记了set-uid位并且该文件的拥有者是超级权限"root".当一个一般用户"ananta",运行"passwd"程序,passwd的用户ID设置如下:
Real-UID = ananta
Effective-UID = ananta
Saved-UID = root
The the program calls a system call “seteuid( 0 )” and since SUID=0, the call will succede and the UIDs will be:
这个程序会执行一个系统调用"seteiud(0)"而且,如果SUID=0,这次调用将会成功并且用户ID配置将会变成:
Real-UID = ananta
Effective-UID = root
Saved-UID = root
After that, “passwd” process will be able to access /etc/passwd and change password for user “ananta”. Note that user “ananta” cannot write to /etc/passwd on it’s own. Note one other thing, setting a Set-UID on a executable file is not enough to make it run as privileged process. The program itself must make a system call.
在此之后,"passwd"进程就将能够使用/etc/passwd 然后为"ananta"修改密码.注意,ananta用户本身并不能对/etc/passwd文件进行写入操作.另外,还需要注意,为一个可执行文件设置set-uid位并不足以让该程序在运行的时候获得相应的特殊权限,该程序本身必须要执行一个系统调用才能达成相应的目的
Conclusion
总而言之,按照这里的描述,
RealUID是根据当前的操作用户/进程而定的.
SavedUID则是充当一个暂存的容器,如果设置了set-uid位,则其设置为和当前文件拥有者相同,不然,则设置为和RealUID相同.
EffectiveUID的作用是作为实际起作用的,发挥效果的UID信息,这个UID是随着运行状态不断变化的,当前的EUID的实际设置将直接决定这个EUID的可变化状态.
