lecture_1_introduction_linux_1234567.ppt

Introduction to the Kernel
and Device Drivers
Mark Stanovich
COP 5641 / CIS 4930

Introduction
 Device drivers
 Software interface to hardware device
 Use standardized calls
 Independent of the specific driver
 Main role
 Map standard calls to device-specific operations
 Can be developed separately from the rest of the
kernel
 Plugged in at runtime when needed

Role of the Device Driver
 Implements the mechanisms to access the
hardware
 E.g., Hard disk
 Read/write blocks of data
 Access as a continuous array
 Does not force particular policies on the user
 Examples
 Who many access the drive
 Whether the drive is accessed via a file system
 Whether users may mount file systems on the drive

Policy-Free Implementation
 Simplifies the design
 Separation of concerns
 Capabilities provided
 Use of Capabilities
 Reuse
 Different policies do not require changes to the
kernel

Splitting the Kernel
 Kernel handles resource requests
 Process management
 Creates, destroys processes
 Supports communication among processes
 Signals, pipes, etc.
 Schedules how processes share the CPU/cores
 Memory management
 Virtual addressing

Splitting the Kernel
 File systems
 Everything in UNIX can be treated as a file
 Linux supports multiple file systems
 Device control
 System operation maps to a physical device
 Networking
 Handles packets
 Routing and network address resolution

(Dynamically) Loadable Modules
 The ability to add and remove kernel features
at runtime
 Each unit of extension is called a module
 Use insmod and modprobe program to
add a kernel module
 Use rmmod program to remove a kernel
module

Classes of Devices and Modules
 Character devices
 Block devices
 Network devices
 Others

Character Devices
 Abstraction: a stream of bytes
 Examples
 Text console (/dev/console)
 Serial ports (/dev/ttyS0)
 Usually supports open, close, read, write
 Accessed sequentially (in most cases)
 Might not support file seeks
 Accessed through a filesystem node

Block Devices
 Abstraction: continuous array of storage
blocks (e.g. sectors)
 Applications can access a block device in bytes
 Accessed through a file system node
 A block device can host a file system

Network Devices
 Abstraction: data packets
 Send and receive packets
 Do not know about individual connections
 Have unique names (e.g., eth0)
 Not in the file system
 Support protocols and streams related to packet
transmission (i.e., no read and write)

Other Classes of Devices
 Examples that do not fit to previous
categories:
 USB
 SCSI
 FireWire
 MTD

File System Modules
 Software drivers, not device drivers
 Serve as a layer between user API and block
devices
 Intended to be device-independent

Security Issues
 Deliberate vs. incidental damage
 Kernel modules present possibilities for both
 System does only rudimentary checks at
module load time
 Relies on limiting privilege to load modules
 And trusts the driver writers
 Driver writer must be on guard for security
problems

Security Issues
 Do not define security policies
 Provide mechanisms to enforce policies
 Be aware of operations that affect global
resources
 Setting up an interrupt line
 Could cause another device to malfunction
 Setting up a default block size
 Could affect other users

Security Issues
 Buffer overrun
 Overwriting unrelated data
 Treat input/parameters with utmost suspicion
 Uninitialized memory
 Kernel memory should be zeroed before being
made available to a user
 Otherwise, information leakage could result
 Passwords

Security Issues
 Avoid running kernels/device drivers
compiled by an untrusted friend
 Modified kernel could allow anyone to load a
module

Version Numbering
 Every software package used in Linux has a
release number
 Version dependency
 You need a particular version of one package to run a
particular version of another package
 Prepackaged distribution contains compatible
versions of various packages

Version Numbering
 Different throughout the years
 1.0 … 2.4 … 2.6… 3.0
 After version 1.0 but before 3.0
 <major>.<minor>.<release>.<bugfix>
 Time based releases (after two to three months)
 3.x
 Moved to 3.0 to better reflect newer development
process
 <version>.<release>.<bugfix>
 https://lkml.org/lkml/2011/5/29/204

License Terms
 GNU General Public License (GPL2)
 GPL allows anybody to redistribute and sell a
product covered by GPL
 As long as the recipient has access to the source
 And is able to exercise the same rights
 Any software product derived from a product
covered by the GPL must be released under GPL

License Terms
 If you want your code to go into the mainline
kernel
 Must use a GPL-compatible license

Joining the Kernel Development
Community
 The central gathering point
 Linux-kernel mailing list
 http://www.tux.org/lkml
 https://lkml.org/
 Chapter 20 of LKD Book further discusses
the community and accepted coding style

lecture_1_introduction_linux_1234567.ppt

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