Introduc)on to the Linux Kernel · – Device drivers – File system drivers • Implementaon of a speciﬁc ﬁle system – System calls – Network stack • Interprets a network

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

Shell

Kernel

Hardware


Privileged mode

Requestflow

Applications

Shell

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

VFS

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.

Documents

Introduc)on to the Linux Kernel · – Device drivers – File system drivers • Implementaon of a speciﬁc ﬁle system – System calls – Network stack • Interprets a network