Introduc)on to the Linux Kernel · – Device drivers – File system drivers • Implementaon of a...

transcript

Introduc)ontotheLinuxKernel

Prak)kumKernelProgrammingUniversityofHamburgScien)ficCompu)ng

Wintersemester2015/2016

Konstan)nosChasapisKonstan)nos.chasapis@informa)k.uni-hamburg.de

Outline

•  Introduc)on(story,licence,versioning)•  Mainparts•  LoadableKernelModules•  SystemCalls•  Security

4/11/15 Linux Kernel Intro. 1

Introduc)on•  DevelopedbyLinusTorvalds(1991)

–  JustforFun:TheStoryofanAccidentalRevolu)onarybyLinusTorvalds

•  BasedonUnix•  1stversionsupportedIntel80386•  Currentlyvariouspla\ormsaresupported•  ImplementedinGNUC•  SeveralDistribu)ons(distro)

–  RedHat,CentOS,Ubuntu,SUSE,Debian,Arch–  Differentpackagesystem,configura)onetc.–  Applydifferentpatches

Introduc)on(cont.)•  X-ServerisnotimplementedwithintheKernel•  Everythingrunin“Kernelmode”

–  Privilegedaccesstohardware•  Monolithicbutboastsmodulardesign

–  Kernelpreemp)on(undercertaincondi)ons)•  Theschedulerispermieedtoforciblyperforma

contextswitch–  Supportskernelthreads–  Dynamicloadandunloadbinaries(kernelmodules)–  Reentrant,severalprocessescanbeinkernelmode

simultaneously

Introduc)on(cont.)•  LicenseTerms

–  islicensedundertheVersion2oftheGNUGeneralPublicLicense(GPL)

–  AllowsanybodytoredistributeandevensellaproductcoveredbyGPLaslongastherecipienthasaccesstothesourceandisabletoexercisethesamerights

–  AnysogwarederivedbyaproductcoveredbyGPLmustbereleasedundertheGPL

•  Democra)ze,everyonecancontribute–  Ifyouwantyourcodetogointothemainlineoryouhave

modifiedthekernelthenyouhavetouseGPL-compa)blelicense

Introduc)on(cont.)•  UseofbinaryBlobs(Modules,firmware)

–  Thesourceisnotgiven–  Maycontainpartofthedriverfromanotherfilesystem–  Ifthecodehasbeenportedfromanotheropera)ngsystem

islegal–  Ifacompanywantstokeepthesourceprivate–  Usingsuchsogwareisdiscourage

•  Versioning–  $uname–a

3 . 17 . 1

major . minor . revision 4/11/15 Linux Kernel Intro. 5

Outline

Αpplications

Linuxsystemoverview

Kernel

Hardware

Privileged mode

Requestflow

Applications

Kernel Hardware

Mainparts

Process Management (PM)

Memory Management (MM)

Network Stack

Virtual File System (VFS)

Device Drivers (DD) Arch

System Call Interface

Mainparts(cont.)•  Systemcallinterface(SCI)

–  Athinlayerthatprovidesamethodtointeractfromuserspacetokernelspace

•  ProcessManagement(PM)–  Create,destroyprocesses–  Communica)onbetweendifferentprocesses(kernelthreads)–  CPUscheduling

•  MemoryManagement(MM)–  Physicaltovirtualmemorymanagement–  Memoryalloca)on–  Swapping,frommemorytoharddisk

Mainparts--I/OPath•  VirtualFileSystem(VFS)

–  Eportsthecommonfileinterface–  Abstractfilesystemfunc)onalityfrom

implementa)on

•  FileSystems–  Implementa)onofFSfunc)onality

•  BufferCache–  Asetoffunc)onstomanipulatemain

memorydesignedforFS•  DeviceDriver•  PhysicalDevice

–  Wheredatalive

ext4 /proc XFS

Buffer Cache

Device Drivers

Physical Devices 4/11/15 Linux Kernel Intro. 11

Mainparts(cont.)•  NetworkStack

–  Implementthenetworkprotocols–  Deliverpacketsacrossprogramsandnetworkinterfaces

•  DeviceDrivers(DD)–  Interactwiththehardware–  Extractanabstrac)onofthedevicefunc)onali)es

•  Arch–  Architecturedependentcode

Outline

LKMs•  LKMs(LoadableKernelModules)•  Pre-compiledbinarypieces•  Eachpieceiscalled“module”•  Canbeloadedatrun)me•  Extendthefunc)onalityofthesystem•  Enforcemodularity

–  Easytodevelop,debugandmaintain–  Noneedtorebuildthekernel

•  Cansavememory(loadonlythenecessary)

WhatareLKMsusedfor•  Everythingthatisnotrequiredinthecore•  6maincategories

–  Devicedrivers–  Filesystemdrivers

•  Implementa)onofaspecificfilesystem–  Systemcalls–  Networkstack

•  Interpretsanetworkprotocol–  TTYlinedisciplines–  Executableinterpretersforthesupportedformats

CharacterDeviceDriver

•  ReadorWriteabyteata)me•  Accessedbyastreamofbytes•  Usuallypermitonlysequen)alaccess•  Implement:open,close,read,write•  Similartoregularfiles

– /dev/console– /dev/eyS0

BlockDeviceDriver

•  ReadorWriteblock-sizemul)ples•  Permitrandomaccess•  Accessedinthe/dev/•  Filesystemscanbemountontop•  HandleI/Oopera)ons•  Differwiththecharmoduleinthewaythemanagedatainsidethekernel

•  Differentinterfacetothekernelthancharmodules

NetworkDrivers•  Handleanynetworktransac)onmade•  Transferpacketsofdata•  Independentofaspecificprotocol•  Recep)onandTransmissioninsteadofRead/Write•  Usuallytheinterfaceisahardwaredevicebutitcanalsobe

sogwareliketheloopback–  loopbackisusedtocommunicatewiththeserversthat

runinthesamenode,debuggingetc.•  Theyarenotmappedtothefilesystem;theyareiden)fied

byaname

Outline

Systemcalls•  Asyscallcausesaprogrammedexcep)on(trap)ontheCPU

–  syscall(number,arguments)•  Withinthekernelyoucannotaccessuserspacebuffers

directly

Syscall Table

User Space

Kernel Space

write(fd, ptr, sz)

syscall(WRITE, fd, ptr,sz)

sys_write(f, up, sz)

vfs_write(f, p, sz) etx4_write(f, p, sz)

copy_from/to_user (to,from,sz)

Outline

Securityconsidera)ons•  Securitycheckisenforcedbythekernel•  IftheKernelhas„holes“Systemhasholes•  Avoidintroducingtypicalprogrammingbugs

– Moduleparameters–  Bufferoverrun–  Memorycorrup)on

•  Zeroorini)alizememorygiventouser•  Runprecompiledkernelsfoundinyourdistro•  Inofficialdistrosonlythesuperusercanloadandunload

modules

Kernel programming is vital for as long as new hardware is being

designed and produced or old-obsolete hardware is maintained.

Introduc)on to the Linux Kernel · – Device drivers – File system drivers • Implementaon of a...

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