Recompiling the Linux Kernel

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Recompiling the Linux Kernel

• by
• Mahendra M
• Mahendra_M_at_infosys.com
• http//www.infosys.com/

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Traditional Kernel Architectures

• Micro Kernels
• Macro Kernels
• Modular

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Micro Kernel Architecture

• A Core Kernel
• All functionalities in User Space
• e.g. File-system, Sound driver, video
• Communication Using Message Passing
• Fast in Theory Experimental Setups
• Practically Slow Less Efficient
• A crash in a subsystem doesnt bring down the
  whole system
• e.g. Minix, QNX, GNU/Hurd (??)

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Macro Kernel Architecture

• A single monolithic kernel
• Everything in Kernel Space
• Communication using function calls
• Practically fast
• Easy to implement
• Crash in any region can bring down the whole
  system
• e.g. DOS

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Linux ( The Kernel )

• Modular, Monolithic Kernel
• Modules can be compiled and dynamically linked
  into kernel address space.
• Useful for device drivers that need not always be
  resident until needed.
• Keeps core kernel "footprint" small.
• Can be used to "extend" functionality of kernel
  too!
• Modules are loaded into kernel address space and
  unresolved symbols in the module are linked to
  the symbol table of running kernel.

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Linux ( The Kernel ) contd...

• Advantages
• Footprint of running kernel is small
• Unused modules are flushed out. Increases
  efficiency
• Flexibility - Dynamically add new features
• Disadvantages
• A faulty module can bring down the system
• Slight overhead in loading the modules
• Advantages outweigh Disadvantages
• Murphys law If it aint broken, dont fix it

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Precautions

• Compulsory non-modular cases
• Hard Disk support
• will prevent loading of any module, since the
  hard-disk cannot be probed to access modules
• Filesystem of root partition
• Diskless clients ethernet drivers / NFS root
  filesystem
• will prevent booting up of the system, since the
  kernel image cannot be obtained over the network.
• Maybe even TCP/IP support.

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Module Handling Tools

• insmod - link a module into the running kernel by
  resolving all symbols from the kernels exported
  symbol table.
• -root option for enhanced security
• depmod - creates module dependency file -
  modules.dep
• modprobe - loads all relevant modules from above
• rmmod - removes unused modules
• kmod - kernel thread
• replacement for kerneld ( does not remove modules
  )
• kmod calls modprobe
• together they can loop (if modules.dep is
  absent). Handled by a compile time option.
• "kmod runaway modprobe loop assumed and stopped
• eg modprobe started with syslog option. (
  needs unix sockets )

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Steps in loading a module

• Kernel notices a feature request not resident in
  the kernel
• Sends a message to kmod with a symbolic
  description of the feature
• kmod asks modprobe to load a module
• modprobe looks into its internal alias
  translation table for a match
• insmod is asked to insert all modules needed
• configured accroding to the options in
  /etc/modules.conf
• kernel uses the freshly installed feature as if
  it was configured as a resident part

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Recompiling the kernel

• Why recompile ?
• Feel like owning a custom made BMW.
• Highly optimized than kernels in distros ( eg
  i386 compilation )
• Reduce kernel footprint.
• Increase efficiency. Higher performance
• Notice If you get it correct first time,
  contact Torvalds.
• Notes .

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Notes

• "Processor type" is set higher than 386 -
• Will not work on a 386
• A kernel with math-emulation compiled in will
  still use the coprocessor if one is present
• The math emulation will just never get used in
  that case.
• Having unnecessary drivers will make the kernel
  bigger
• Can lead to problems
• It is YOUR call.
• Enable Magic System Request Key Hacks
• Have FUN !!!

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The Five steps to Freedom

• make config ( or gconfig/menuconfig )
• select resident modular parts
• make dep
• make bzImage
• build the actual image
• make modules
• builds all modules
• make module_install
• installs the modules in /lib/modules/2.6./

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Compilation

• Dependence on gcc
• Work in progress
• -j option for make
• -jN ( N Num of Processors 1 )
• EXTRAVERSION
• To protect your working modules/kernel
• Dont change VERSION, PATCHLEVEL or SUBLEVEL
• Applications may refer to these

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Finally

• Extensive Documentation
• The Kernel Source Tree
• Demo . . .

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Links

• http//infylug/kernel
• http//kernel.org
• http//www.kernelnewbies.org
• http//www.sudolinux.com/kernel_links.htm
• Lots of Kernel related links
• http//www.digitalhermit.com/linux/kernel.html
• Excellent Document on Recompiling the kernel
• http//www.linux.it/kerneldocs/kconf/
• Reconfiguring the Linux Kernel

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Thankyou