Tuesday, July 12, 2011

How to recompile Kernel?

Kernel Recompilation
Compiling custom kernel has its own advantages and disadvantages.  It helps to optimize the kernel to your environment (hardware and usage patterns).  I shall try to guide you through Kernel recompilation process.
Step 1:
Download the kernel source
cd /usr/local/src
wget  http://www.kernel.org/pub/linux/kernel/v2.6/linux-x.y.z.tar.bz2
Note: Replace x.y.z with actual version number.
Step 2:
Extract the  source file
tar -xjvf linux-x.y.z.tar.bz2
Step 3:
Patching the Kernel
If you are requested to apply any patches , follow these steps
a) Move the downloaded kernel patch to the /usr/local/src directory.
b)  Extract the patch file
c)  Patch the kernel  source using the extracted  patch file
cd /usr/local/src/linux-x.y.z
patch -p1 < patchfile-2.2.x
Now the  Kernel Source is patched against known  vulnerabilities.
Step 4:
If you are trying to upgrade the Kernel of already running server , it is always better use the existing configuration. To do this follow these steps
#uname -a
Linux  Server1  2.6.18-164.el5 #1 SMP Thu Sep 3 03:28:30 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux
# cd /boot
There you can see different configuration files as given below
config-2.6.18-128.el5    initrd-2.6.18-128.el5.img  message
config-2.6.18-164.el5     initrd-2.6.18-164.el5.img  quota.user*
identify the configuration file corresponding to the version of OS installed , In our case it is config-2.6.18-164.el5 . We are  copying this file to the downloaded kernel source  to use it during configuration.
#cp -p config-2.6.18-164.el5 /usr/local/src/linux-x.y.z/.config
# make clean
# make mrproper
# make menuconfig
You have to select different options as per your need.   If you  intended to use the existing configuration ,specify the path to the file  ( .config in this case) by selecting the option
“Load an Alternative configuration file”
Step 5: Compilation
Compile the Kernel using the following commands
Compile to create a compressed kernel image
# make
Compile kernel modules:
# make modules
Install kernel modules
# make modules_install
Step 6: Install Kernel
If the above steps completed without any errors , now its the time to Install the new Kernel
# make install
It will install three files into /boot directory as well as modification to your kernel grub configuration file:
Step 7:  Create the Initrd image
Type the following command :
# cd /boot
mkinitrd -o initrd.img-x.y.x  x.y.z
initrd images contains device driver which needed to load rest of the operating system later on. Not all computer requires it, but it is  better  to create one
Step 8: Boot Loader Modification
Mofdify the boot loader to boot the new OS as default . Check the documentaions corresponding to your boot loader
Step 9: The last step
execute the following command
Wait a few minutes and once it is up , you can see that the new Kernel is loaded :-)