Download Win32 Programming

Secure Operating Systems
Lesson 3: OS Structures
What Services Does an OS provide?
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An environment for the execution of
programs… which means what?
Right: a set of APIs that allow a programmer
to leverage services controlled by the OS
User-facing
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The GUI
Program Execution
I/O Operations
File-system manipulation
Comms
Error Detection
Under the Hood
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Resource Allocation
Accounting
Protection and Security
The OS Interface
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Of course, the GUI
But also the command interpreter and shell
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The Interface to programs is the API
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Compare and contrast Unix and Windows on this
This requires a change in domain
The API isn’t (usually) the same as the underlying
system call
Let’s walk through a simple fopen()
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Why is this an OS function?
How does it relate to the OS? Why?
Example: CreateFileA?W
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HANDLE WINAPI CreateFile(
_In_
LPCTSTR lpFileName,
_In_
DWORD dwDesiredAccess,
_In_
DWORD dwShareMode,
_In_opt_ LPSECURITY_ATTRIBUTES
lpSecurityAttributes,
_In_
DWORD dwCreationDisposition,
_In_
DWORD dwFlagsAndAttributes,
_In_opt_ HANDLE hTemplateFile
);
How the Call Works
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Three ways
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Often, via the stack
Or via registers
Or via a pointer to a block of memory
The Confused Deputy
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We’ll look at this in much more detail later,
but the OS can be thought of as being
deputized by an application to do work for
hire
This is a risky proposition
System Programs/Utilities
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May not need more privilege
Handle things like:
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File management
Status information
File modification
Programming language support
Program loading and execution
Comms – Message passing and Shared Memory
OS Design Goals
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Perhaps where we fail most is when we set
our design goals: poor goals lead to a bad
design, and no wonder it doesn’t work
Performance – especially worst case – is often
overlooked
We tend to “hand wave” over requirements in
the OS
Mechanisms and Policies
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Mechanisms are how something gets done
Policies define what to do…
Easily to confuse; when we mix them, we end
up with inflexible systems
OS Structure
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Simple layers, like MSDOS
Application
Programs
Resident
Programs
MS-DOS
Devices
ROM BIOS
Or Layers (like Unix)
The Users
Shells and Commands
System Call Interface
Signals
File System
CPU Scheduling
Kernel Interface to Hardware
Terminal Controllers
Device Controllers
Memory Controllers
Microkernels
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A large kernel is dangerous from a security
perspective (why?)
Possible solution is the microkernel
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Move everything you can from the lowest domain
The kernel focuses on message passing
Makes it easier to expand
But performance can be problematic
Modular Kernels
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A standard form for kernel loadable modules
Runtime loading allows you to add when you
need things…
Problem: if you load my code into your
kernel… yes… ahem…
OS Debugging
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A tricky business (think about it!)
VMs can help
Core Dump: dump as much state as able at a
critical error point
OS debugging has a number of challenges
related to complexity, concurrence and
attribution
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Typically, we do some type of remote debugging
Assignment
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Read Chapter 2 of OSC
Read “Protection” by Butler Lampson
A word to the wise – don’t behind on the
reading, because there’s a lot of it. I’ll set less
assignments based on that, but the material
will be on the final.
Questions & Comments
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What do you want to know?