回想一下内核模式表示代码执行的特权模式，而用户模式则表示非特权模式（用于那些运行在内核之外的程序）。

Recall that kernel mode represents the privileged mode for code execution, while user mode represents the non-privileged mode (for programs running outside the kernel).

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这使 set-user-ID（非根）程序能够放弃它的所有用户特权，执行一些非特权操作，然后恢复原来的有效用户 ID。

This allows a set-user-ID (other than root) program to drop all of its user privileges, do some un-privileged work, and then re-engage the original effective user ID in a secure manner.

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