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This restriction prevents application programs or

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Unformatted text preview: fer to Handler Stack Usage with Privilege-Level Change Interrupted Procedure’s Stack ESP Before Transfer to Handler Handler’s Stack ESP After Transfer to Handler SS ESP EFLAGS CS EIP Error Code Figure 5-4. Stack Usage on Transfers to Interrupt and Exception-Handling Routines 5.10.1.1. PROTECTION OF EXCEPTION- AND INTERRUPT-HANDLER PROCEDURES The privilege-level protection for exception- and interrupt-handler procedures is similar to that used for ordinary procedure calls when called through a call gate (refer to Section 4.8.4., “Accessing a Code Segment Through a Call Gate” in Chapter 4, Protection). The processor does not permit transfer of execution to an exception- or interrupt-handler procedure in a less privileged code segment (numerically greater privilege level) than the CPL. An attempt to violate this rule results in a general-protection exception (#GP). The protection mechanism for exceptionand interrupt-handler procedures is different in the following ways: • • Because interrupt and exception vectors have no RPL, the RPL is not checked on implicit calls to exception and interrupt handlers. The processor checks the DPL of the interrupt or trap gate only if an exception or interrupt is generated with an INT n, INT 3, or INTO instruction. Here, the CPL must be less than or equal to the DPL of the gate. This restriction prevents application programs or procedures running at privilege level 3 from using a software interrupt to access critical exception 5-17 INTERRUPT AND EXCEPTION HANDLING handlers, such as the page-fault handler, providing that those handlers are placed in more privileged code segments (numerically lower privilege level). For hardware-generated interrupts and processor-detected exceptions, the processor ignores the DPL of interrupt and trap gates. Because exceptions and interrupts generally do not occur at predictable times, these privilege rules effectively impose restrictions on the privilege levels at which exception and interrupthandling procedures can run. Either of the following techniques can be used to avoid privilegelevel violations. • The exception or interrupt handler can be placed in a conforming code segment. This technique can be used for handlers that only need to access data available on the stack (for example, divide error exceptions). If the handler needs data from a data segment, the data segment needs to be accessible from privilege level 3, which would make it unprotected. The handler can be placed in a nonconforming code segment with privilege level 0. This handler would always run, regardless of the CPL that the interrupted program or task is running at. FLAG USAGE BY EXCEPTION- OR INTERRUPT-HANDLER PROCEDURE • 5.10.1.2. When accessing an exception or interrupt handler through either an interrupt gate or a trap gate, the processor clears the TF flag in the EFLAGS register after it saves the contents of the EFLAGS register on the stack. (On calls to exception and interrupt handlers, the processor also clears the VM, RF, and NT flags in the EFLAGS register, after they are saved on the stack.) Clearing the TF flag prevents instruction tracing from affecting interrupt response. A subsequent IRET instruction restores the TF (and VM, RF, and NT) flags to the values in the saved contents of the EFLAGS register on the stack. The only difference between an interrupt gate and a trap gate is the way the processor handles the IF flag in the EFLAGS register. When accessing an exception- or interrupt-handling procedure through an interrupt gate, the processor clears the IF flag to prevent other interrupts from interfering with the current interrupt handler. A subsequent IRET instruction restores the IF flag to its value in the saved contents of the EFLAGS register on the stack. Accessing a handler procedure through a trap gate does not affect the IF flag. 5.10.2. Interrupt Tasks When an exception or interrupt handler is accessed through a task gate in the IDT, a task switch results. Handling an exception or interrupt with a separate task offers several advantages: • • The entire context of the interrupted program or task is saved automatically. A new TSS permits the handler to use a new privilege level 0 stack when handling the exception or interrupt. If an exception or interrupt occurs when the current privilege level 0 stack is corrupted, accessing the handler through a task gate can prevent a system crash by providing the handler with a new privilege level 0 stack. 5-18 INTERRUPT AND EXCEPTION HANDLING • The handler can be further isolated from other tasks by giving it a separate address space. This is done by giving it a separate LDT. The disadvantage of handling an interrupt with a separate task is that the amount of machine state that must be saved on a task switch makes it slower than using an interrupt gate, resulting in increased interrupt latency. A task gate in the IDT references a TSS descriptor in the GDT (refer to Figure 5-5). A switch to the handler task is handled in the same manner as an ordinary task switch (refer to Section 6.3., “Task...
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