Bài giảng An toàn hệ điều hành

Control Flow

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Concept of Control Flow

ꢀ The program counter (PC) assumes a sequence of

values:

a₁,a₂,a₃,…,a_n-1

ꢀ a_kis the address of some corresponding instruction I_k.

Each transition from a_kto a_k+1is called a control

transfer.

ꢀ A sequence of such control transfers is called the

ﬂow of control, or control ﬂow of the processor.

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Concept of Exceptional Control Flow

• The simplest kind of control ﬂow is a smooth sequence where each I_k

and I_k+1are adjacent in memory.

• I_k+1is not adjacent to I_k,such as jumps, calls, and return=> they are

necessary mechanisms that allow programs to react to changes in

internal program state represented by program variables.

• Other hand, changes in system state that are not captured by internal

program variables, such as timer, I/O… Modern systems react to

these changes by making abrupt changes in the control ﬂow. It is

refered as exceptional control ﬂow.

• It exists all levels (hardware, operating system, application)

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Exceptions

ꢀ A form of exceptional control ﬂow that are implemented

partly by the hardware and partly by the operating system.

ꢀ An exception is an abrupt change in the control ﬂow in

response to some change in the processor’s state

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Exception Handling

ꢀ Each type of possible exception in a system is assigned a

unique non-negative integer exception number by the

designers of the processor and designers of the operating

system kernel

ꢀ At system boot time the operating system allocates and

initializes a jump table called an exception table, so that

entry k contains the address of the handler for exception k.

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Classes of Exceptions

ꢀ Four classes: Interrupts, Traps, Faults, and Aborts

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Interrupts

ꢀ Interrupts occur asynchronously as a result of signals

from I/O devices

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Traps

ꢀ Traps are intentional exceptions that occur as a result of executing

an instruction

ꢀ The most important use of traps is to provide a procedure like

interface between user programs and the kernel known as a system

call

ꢀ Executing the syscall instruction causes a trap to an exception

handler that decodes the argument and calls the appropriate kernel

routine

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Faults

ꢀ When a fault occurs, the processor transfers control to the fault

handler.

ꢀ If the handler is able to correct the error condition, it returns

control to the faulting instruction, thereby reexecuting it.

ꢀ Otherwise, the handler returns to an abort routine in the kernel

that terminates the application program that caused the fault.

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Aborts

ꢀ Result from unrecoverable fatal errors

ꢀ Abort handlers never return control to the application

program.The handler returns control to an abort routine

that terminates the application program.

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Exceptions in Intel Processors

ꢀ Up to 256 different exception types

ꢀ Numbers in the range 0 to 31 correspond to exceptions

that are deﬁned by the Pentium architecture.

ꢀ Numbers in the range 32 to 255 correspond to interrupts

and traps that are deﬁned by the operating system.

ꢀ System calls are provided on IA32 systems via a trapping

instruction called INT n, where n can be the index of any

of the 256 entries in the exception table. Historically,

system calls are provided through exception 128 (0x80)

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Processes

ꢀ An instance of a program in execution.

ꢀ State includes the program’s code and data stored in

memory, its stack, the contents of its general-purpose

registers, its program counter, environment variables,

and the set of open ﬁle descriptors

ꢀ Each program in the system runs in the context of some

process. The context consists of the state that the

program needs to run correctly

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Key abstractions that a process

provides to the application

ꢀ An independent logical control ﬂow that provides the

illusion that our program has exclusive use of the

processor

ꢀ A private address space that provides the illusion that our

program has exclusive use of the memory system.

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Logical Control Flow

ꢀ A process provides each program with the illusion that it has exclusive

use (độc quyền sử dụng) of the processor

ꢀ Sequence of PC values is known as a logical control ﬂow (PC:

Program Counter)

ꢀ Concurrent process ; multitasking; time slice

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Private Address Space

ꢀ A process also provides each program with the illusion that

it has exclusive use of the system’s address space

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User and Kernel Modes

ꢀ In order for the kernel to provide an airtight process abstraction

ꢀ Use a mechanism that restricts the instructions that an application can execute, as

well as the portions of the address space that it can access.

ꢀ Controlled by using the mode bit in some control register

ꢀ Mode bit is set ꢀ process in kernel mode (supersisor mode)

ꢀ User mode: process is not allowed to execute privileged instructions that do things

such as halt the processor, change the mode bit, or initiate an I/O operation, and to

directly reference code or data in the kernel area of the address space

ꢀ User programs must access kernel code and data indirectly via the system call

interface

ꢀ The only way for the process to change from user mode to kernel mode is via an

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exception such as an Interrupt, a fault, or a trapping system call

Context Switches (1/2)

ꢀ Context consists of the values of objects such as the general-purpose registers,

the ﬂoating-point registers, the program counter, user’s stack, status registers,

kernel’s stack, and various kernel data structures such as a page table that

characterizes the address space, a process table that contains information about

the current process, and a ﬁle table that contains information about the ﬁles that

the process has opened.

ꢀ The kernel maintains a context for each process. The context is the state that the

kernel needs to restart a preempted process

ꢀ The kernel has scheduled a new process to run, it preempts the current process

and transfers control to the new process using a mechanism called a context

switch

ꢀ Context switch mechanism is built on top of the lower-level exception mechanism

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Context Switches (2/2)

ꢀ A context switch can occur while the kernel is executing a

system call on behalf of the user.

ꢀ A context switch can also occur as a result of an interrupt

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Bài giảng An toàn hệ điều hành - Control Flow