How to boot linux kernel

Manually booting the Linux kernel from GRUB

It’s never a comforting scenario when you go to boot up your system and you get the following screen:

 GNU GRUB version 1.99 Minimal BASH-like line editing is supported. For the first word, TAB lists possible command completions. Anywhere else TAB lists possible 
device or file completions. grub> _ 

«Where’s the OS?» one might demand. But don’t fret, this may not be the end. There are a number of reasons you could be getting this prompt, but it usually all boils down to the fact that GRUB is unable to see any information on how it should load your kernel and operating system.

What is booting?

In computing, booting (also known as a boot-up) is the initial set of operations that a computer system performs after being turned on. When the machine finishes it’s Power-On Self-Test (POST), it will look for instructions on how to actually load your Operating System. In the case of a hard disk (which is most common), it will load the code found in the Master Boot Record (MBR), which will generally locate and load the operating system’s «Boot Loader» into memory. In the case here, the boot loader is GRUB. The boot loader is then responsible for preping and starting the Operating System.

How does it work?

The GRand Unified Bootloader (GRUB) was initially developed as a boot loader for the GNU/Hurd project. There are two versions of GRUB in common use, though GRUB version 2 is now used by most distributions (and will be the focus here). GRUB will check it’s configs for the location of the requested kernel and attempt to load (or strap) that image into memory. Once loaded, GRUB will pass parameters (if any) and transfer control to the kernel. The kernel will then load both the default configuration file and any other modules needed.

So how do we fix it when things go wrong?

Manually booting your operating system from GRUB is actually pretty easy once you know what you need to do. Before trying to actually do anything with GRUB, you should examine what GRUB can actually see in your system. For starters, if you can see the GRUB prompt you know that the MBR is intact, and that GRUB has been properly loaded into memory. Great! Now let’s poke around and see which disks may be visible to GRUB. You can start by using the ls command:
grub> ls
(hd0) (hd0,msdos2) (hd0,msdos1)

All of our partitions are showing up here (yours may look slightly different, depending on how things are partitioned). Since it can see our boot volume, let’s actually tell it to use that:
grub> root=(hd0,msdos1)

Now we can tell it to load or kernel image. (Just note that you must put in the full filename of the image. You can use TAB completion here to help you, especially if you don’t remember the name of the file.)
grub> linux /vmlinuz root=/dev/sda2

Next, we need to tell the kernel where it can find it’s initialization RAM disk (initrd). Remember to use TAB completion to help you:
grub> initrd /initrd.img

Finally, you can go ahead and boot your system!
grub> boot

For the most part, you should be done. That wasn’t so bad, right? LVM gets a little trickier though, as you will need to activate volume groups and specify logical volumes for root partitions (I may cover this in an update to this article).

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Linux kernel release 6.x ¶

These are the release notes for Linux version 6. Read them carefully, as they tell you what this is all about, explain how to install the kernel, and what to do if something goes wrong.

What is Linux?¶

Linux is a clone of the operating system Unix, written from scratch by Linus Torvalds with assistance from a loosely-knit team of hackers across the Net. It aims towards POSIX and Single UNIX Specification compliance.

It has all the features you would expect in a modern fully-fledged Unix, including true multitasking, virtual memory, shared libraries, demand loading, shared copy-on-write executables, proper memory management, and multistack networking including IPv4 and IPv6.

It is distributed under the GNU General Public License v2 — see the accompanying COPYING file for more details.

On what hardware does it run?¶

Although originally developed first for 32-bit x86-based PCs (386 or higher), today Linux also runs on (at least) the Compaq Alpha AXP, Sun SPARC and UltraSPARC, Motorola 68000, PowerPC, PowerPC64, ARM, Hitachi SuperH, Cell, IBM S/390, MIPS, HP PA-RISC, Intel IA-64, DEC VAX, AMD x86-64 Xtensa, and ARC architectures.

Linux is easily portable to most general-purpose 32- or 64-bit architectures as long as they have a paged memory management unit (PMMU) and a port of the GNU C compiler (gcc) (part of The GNU Compiler Collection, GCC). Linux has also been ported to a number of architectures without a PMMU, although functionality is then obviously somewhat limited. Linux has also been ported to itself. You can now run the kernel as a userspace application — this is called UserMode Linux (UML).

Documentation¶

• There is a lot of documentation available both in electronic form on the Internet and in books, both Linux-specific and pertaining to general UNIX questions. I’d recommend looking into the documentation subdirectories on any Linux FTP site for the LDP (Linux Documentation Project) books. This README is not meant to be documentation on the system: there are much better sources available.
• There are various README files in the Documentation/ subdirectory: these typically contain kernel-specific installation notes for some drivers for example. Please read the Documentation/process/changes.rst file, as it contains information about the problems, which may result by upgrading your kernel.

Installing the kernel source¶

• If you install the full sources, put the kernel tarball in a directory where you have permissions (e.g. your home directory) and unpack it:

xz -cd linux-6.x.tar.xz | tar xvf -

xz -cd ../patch-6.x.xz | patch -p1

Replace «x» for all versions bigger than the version «x» of your current source tree, in_order, and you should be ok. You may want to remove the backup files (some-file-name~ or some-file-name.orig), and make sure that there are no failed patches (some-file-name# or some-file-name.rej). If there are, either you or I have made a mistake. Unlike patches for the 6.x kernels, patches for the 6.x.y kernels (also known as the -stable kernels) are not incremental but instead apply directly to the base 6.x kernel. For example, if your base kernel is 6.0 and you want to apply the 6.0.3 patch, you must not first apply the 6.0.1 and 6.0.2 patches. Similarly, if you are running kernel version 6.0.2 and want to jump to 6.0.3, you must first reverse the 6.0.2 patch (that is, patch -R) before applying the 6.0.3 patch. You can read more on this in Documentation/process/applying-patches.rst . Alternatively, the script patch-kernel can be used to automate this process. It determines the current kernel version and applies any patches found:

linux/scripts/patch-kernel linux

Software requirements¶

Compiling and running the 6.x kernels requires up-to-date versions of various software packages. Consult Documentation/process/changes.rst for the minimum version numbers required and how to get updates for these packages. Beware that using excessively old versions of these packages can cause indirect errors that are very difficult to track down, so don’t assume that you can just update packages when obvious problems arise during build or operation.

Build directory for the kernel¶

When compiling the kernel, all output files will per default be stored together with the kernel source code. Using the option make O=output/dir allows you to specify an alternate place for the output files (including .config). Example:

kernel source code: /usr/src/linux-6.x build directory: /home/name/build/kernel

To configure and build the kernel, use:

cd /usr/src/linux-6.x make O=/home/name/build/kernel menuconfig make O=/home/name/build/kernel sudo make O=/home/name/build/kernel modules_install install

Please note: If the O=output/dir option is used, then it must be used for all invocations of make.

Configuring the kernel¶

Do not skip this step even if you are only upgrading one minor version. New configuration options are added in each release, and odd problems will turn up if the configuration files are not set up as expected. If you want to carry your existing configuration to a new version with minimal work, use make oldconfig , which will only ask you for the answers to new questions.

"make config" Plain text interface. "make menuconfig" Text based color menus, radiolists & dialogs. "make nconfig" Enhanced text based color menus. "make xconfig" Qt based configuration tool. "make gconfig" GTK+ based configuration tool. "make oldconfig" Default all questions based on the contents of your existing ./.config file and asking about new config symbols. "make olddefconfig" Like above, but sets new symbols to their default values without prompting. "make defconfig" Create a ./.config file by using the default symbol values from either arch/$ARCH/defconfig or arch/$ARCH/configs/$_defconfig, depending on the architecture. "make $_defconfig" Create a ./.config file by using the default symbol values from arch/$ARCH/configs/$_defconfig. Use "make help" to get a list of all available platforms of your architecture. "make allyesconfig" Create a ./.config file by setting symbol values to 'y' as much as possible. "make allmodconfig" Create a ./.config file by setting symbol values to 'm' as much as possible. "make allnoconfig" Create a ./.config file by setting symbol values to 'n' as much as possible. "make randconfig" Create a ./.config file by setting symbol values to random values. "make localmodconfig" Create a config based on current config and loaded modules (lsmod). Disables any module option that is not needed for the loaded modules. To create a localmodconfig for another machine, store the lsmod of that machine into a file and pass it in as a LSMOD parameter. Also, you can preserve modules in certain folders or kconfig files by specifying their paths in parameter LMC_KEEP. target$ lsmod > /tmp/mylsmod target$ scp /tmp/mylsmod host:/tmp host$ make LSMOD=/tmp/mylsmod \ LMC_KEEP="drivers/usb:drivers/gpu:fs" \ localmodconfig The above also works when cross compiling. "make localyesconfig" Similar to localmodconfig, except it will convert all module options to built in (=y) options. You can also preserve modules by LMC_KEEP. "make kvm_guest.config" Enable additional options for kvm guest kernel support. "make xen.config" Enable additional options for xen dom0 guest kernel support. "make tinyconfig" Configure the tiniest possible kernel.

• Having unnecessary drivers will make the kernel bigger, and can under some circumstances lead to problems: probing for a nonexistent controller card may confuse your other controllers.
• 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. The kernel will be slightly larger, but will work on different machines regardless of whether they have a math coprocessor or not.
• The «kernel hacking» configuration details usually result in a bigger or slower kernel (or both), and can even make the kernel less stable by configuring some routines to actively try to break bad code to find kernel problems ( kmalloc() ). Thus you should probably answer ‘n’ to the questions for «development», «experimental», or «debugging» features.

Compiling the kernel¶

• Make sure you have at least gcc 5.1 available. For more information, refer to Documentation/process/changes.rst .
• Do a make to create a compressed kernel image. It is also possible to do make install if you have lilo installed to suit the kernel makefiles, but you may want to check your particular lilo setup first. To do the actual install, you have to be root, but none of the normal build should require that. Don’t take the name of root in vain.
• If you configured any of the parts of the kernel as modules , you will also have to do make modules_install .
• Verbose kernel compile/build output: Normally, the kernel build system runs in a fairly quiet mode (but not totally silent). However, sometimes you or other kernel developers need to see compile, link, or other commands exactly as they are executed. For this, use «verbose» build mode. This is done by passing V=1 to the make command, e.g.:

If something goes wrong¶

If you have problems that seem to be due to kernel bugs, please follow the instructions at ‘ Reporting issues ‘.

Hints on understanding kernel bug reports are in ‘ Bug hunting ‘. More on debugging the kernel with gdb is in ‘ Debugging kernel and modules via gdb ‘ and ‘ Using kgdb, kdb and the kernel debugger internals ‘.

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