x86 privilege introduce

最新推荐文章于 2025-12-27 16:32:14 发布

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#CPL & RPL & DPL

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本文探讨了处理器中实现的特权概念，通过为关键对象分配0到3的值来定义特权等级。详细介绍了描述符、选择器和当前特权级的作用，并解释了不同特权级别如何在数据访问控制中发挥作用，确保程序只能访问相同或更低特权级别的数据。

1.Concept of privilege
     The concept of privilege is implemented by assigning a value from zero to three to key objects recognized by the processor.This value is called the privilege level.
    ● Descriptors contain a field called the descriptor privilege level(DPL).
     ● Selectors contain a field called the requestor's privilege level(RPL). The RPL is intended to represent the privilege level ofthe procedure that originates a selector.
     ● An internal processor register records the current privilege level(CPL).Normally the CPL is equal to the DPL of the segment thatthe processor is currently executing.CPL changes as control is transferred to segments with differing DPLs.

2.Restricting access to Data
     Three different privilege levels enter into this type of privilege check:
     1)The CPL (current privilege level).
     2)The RPL (requestor's privilege level) of the selector used to specify the target segment.
     3)The DPL of the descriptor of the target segment.
     Instructions may load a data-segment register only if the DPL of the target segment is numerically greaterthan or equal to the maximum of the CPL and the selector's RPL.
     In other words, a procedure can only access data that is at the same or lessprivileged level.

Level value: 0(kernel) & 3(application) And Privilege priority : kernel > application