How to Compile & Boot a Linux Kernel

Compile & Boot Linux from source code

It becomes absolutely necessary for a Kernel Hacker (Linux) to compile various kernel sources and boot them for either taking traces or testing a modification for a kernel module. Although there are lot of blogs talk about this compile and booting, I found the following resource useful (http://www.wikihow.com/Compile-the-Linux-Kernel). I will just explain the overview for the whole process, for detailed steps one can follow the above link.

Assumption: Logged-in user has root privileges
Steps:
1. Get the kernel image you want to work with (http://www.kernel.org)
2. Extract to /usr/src directory and compile (make) from there
3. Configure kernel per your need. (from the menu options). This is done with command (make menuconfig)
4. Compile Kernel and install it (make && make modules_install && make install)
5. Change the boot loader to point the new compiled version

Important Kernel routines

For most of the kernel hackers or device driver programmers => let us try to list some of the mostly used kernel routines and when to use them.

printk():

Used mainly for debugging, error tracking purpose. You may get them in syslogs/console depending on configuration.

Usage:
printk(KERN_INFO “Hello value = %d\n”, i);

we have other flags from highest to lowest:
KERN_EMERG high priority low number
KERN_ALERT
KERN_CRIT
KERN_ERR
KERN_WARNING
KERN_NOTICE
KERN_INFO
KERN_DEBUG low priority hi number

get_user() / put_user():

Used to get and put basic primitive values (such as an int, char, or long) from and to userspace. A pointer into userspace should never be simply dereferenced: data should be copied using these routines. Both return -EFAULT or 0.
These functions may sleep on a PAGE_FAULT and hence must always be called inside user context and with no interrupts disabled, or holding a spinlock.

Usage:
get_user( x, ptr );
put_user( x, ptr );

copy_to_user() / copy_from_user():

They are used to copy an aritrary amount of data to and from userspace. Very similar to their primitive counterpart these functions may sleep on a PAGE_FAULT and hence must always be called inside user context and with no interrupts disabled, or holding a spinlock.

Usage:
copy_to_user( to, from, n );
copy_from_user( to, from, n );

kmalloc() / kfree():

Very similar to malloc() and free() routines in user program (for allocation in userspace), we have these routines to dynamically request pointer-aligned chunks of memory. kmalloc() takes an extra flag giving more details about the memory chunk to be allocated.

GFP_KERNEL: Most reliable way to allocate memory. The request may sleep and swap to free memory hence it is only allowed in user context (not in interrupt context).
GFP_ATOMIC: This call will never sleep, hence can be called in interrupt context. Less reliable than GFP_KERNEL. Out-of-memory error handling must be done to avoid any kernel panics.

Reference

https://www.wikihow.com/Compile-the-Linux-Kernel

https://developer.ibm.com/technologies/linux/

http://linuxforthenew.blogspot.com/2013/01/why-doshould-we-use-copyfromuser-or.html

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