Vmlinux
Encyclopedia
On Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
systems, vmlinux is a statically linked
Static library
In computer science, a static library or statically-linked library is a set of routines, external functions and variables which are resolved in a caller at compile-time and copied into a target application by a compiler, linker, or binder, producing an object file and a stand-alone executable...
executable
Executable
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...
file that contains the Linux kernel
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
in one of the object file
Object file
An object file is a file containing relocatable format machine code that is usually not directly executable. Object files are produced by an assembler, compiler, or other language translator, and used as input to the linker....
formats supported by Linux, including ELF
Executable and Linkable Format
In computing, the Executable and Linkable Format is a common standard file format for executables, object code, shared libraries, and core dumps. First published in the System V Application Binary Interface specification, and later in the Tool Interface Standard, it was quickly accepted among...
, COFF
COFF
The Common Object File Format is a specification of a format for executable, object code, and shared library computer files used on Unix systems...
and a.out
A.out (file format)
a.out is a file format used in older versions of Unix-like computer operating systems for executables, object code, and, in later systems, shared libraries...
. The vmlinux file might be required for kernel debugging
Debugging
Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware, thus making it behave as expected. Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge...
, symbol table
Symbol table
In computer science, a symbol table is a data structure used by a language translator such as a compiler or interpreter, where each identifier in a program's source code is associated with information relating to its declaration or appearance in the source, such as its type, scope level and...
generation or other operations, but must be made bootable before being used as an operating system kernel by adding a multiboot
Multiboot Specification
The Multiboot Specification is an open standard originally created in 1995 and developed by the Free Software Foundation. The specification describes a method of loading various multiboot kernels using a single compliant boot loader. GNU Hurd, VMware ESXi, Xen, and L4 microkernels all need to be...
header, bootsector and setup routines.
Etymology
Traditionally, UNIX platforms called the kernel image/unix. With the development of virtual memoryVirtual memory
In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
, kernels that supported this feature were given the vm- prefix to differentiate them. The name vmlinux is a mutation of vmunix, while in vmlinuz the letter z at the end denotes that it is compressed (zipped).
Location
Traditionally, the kernel was located in the root directoryRoot directory
In computer file systems, the root directory is the first or top-most directory in a hierarchy. It can be likened to the root of a tree — the starting point where all branches originate.-Metaphor:...
of the filesystem hierarchy; however, as the bootloader must use BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
drivers to access the hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
, limitations on some i386 systems meant only the first 1024 cylinders
Cylinder 1024
Cylinder 1024 is the first cylinder of a hard disk that was inaccessible in the original IBM PC compatible hardware specification, interrupt 13h, which uses cylinder-head-sector addressing...
of the hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
were addressable.
To overcome this, Linux distributors encouraged users to create a partition
Disk partitioning
Disk partitioning is the act of dividing a hard disk drive into multiple logical storage units referred to as partitions, to treat one physical disk drive as if it were multiple disks. Partitions are also termed "slices" for operating systems based on BSD, Solaris or GNU Hurd...
at the beginning of their drives specifically for storing bootloader and kernel related files. GRUB, LILO
LILO (boot loader)
LILO is a generic boot loader for Linux.-Overview:LILO does not depend on a specific file system, and can boot an operating system from floppy disks and hard disks. One of up to sixteen different images can be selected at boot time. Various parameters, such as the root device, can be set...
and SYSLINUX
SYSLINUX
The SYSLINUX Project is a suite of lightweight IBM PC MBR bootloaders for starting up computers with the Linux kernel. It is the work of H. Peter Anvin, and consists of several separate systems, the best-known of which is ISOLINUX.-List:...
are common bootloaders.
By convention, this partition is mount
Mount (computing)
Mounting takes place before a computer can use any kind of storage device . The user or their operating system must make it accessible through the computer's file system. A user can access only files on mounted media.- Mount point :A mount point is a physical location in the partition used as a...
ed on the filesystem hierarchy as
/boot. This was later standardised by the Filesystem Hierarchy StandardFilesystem Hierarchy Standard
The Filesystem Hierarchy Standard defines the main directories and their contents in Linux operating systems. For the most part, it is a formalization and extension of the traditional BSD filesystem hierarchy....
, or FHS, which now requires the Linux kernel image to be located in either
/ or /boot, although there is no technical restriction enforcing this. See section 3.5.2 of FHS 2.3Compression
Traditionally, when creating a bootable kernel image, the kernel is also compressedData compression
In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....
using the zlib
Zlib
zlib is a software library used for data compression. zlib was written by Jean-Loup Gailly and Mark Adler and is an abstraction of the DEFLATE compression algorithm used in their gzip file compression program. Zlib is also a crucial component of many software platforms including Linux, Mac OS X,...
algorithm, or since Linux 2.6.30, using LZMA or BZIP2
Bzip2
bzip2 is a free and open source implementation of the Burrows–Wheeler algorithm. It is developed and maintained by Julian Seward. Seward made the first public release of bzip2, version 0.15, in July 1996.-Compression efficiency:...
, which requires a very small decompression
Data compression
In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....
stub to be included in the resulting image. The stub decompresses the kernel code, on some systems printing dots to the console to indicate progress, and then continues the boot process.
The decompression routine is a negligible factor in boot time, and prior to the development of the bzImage, the size constraints of some architectures, notably i386, were extremely limiting, making compression a necessity.
On the SPARC
SPARC
SPARC is a RISC instruction set architecture developed by Sun Microsystems and introduced in mid-1987....
architecture, the vmlinux file is compressed using simple gzip
Gzip
Gzip is any of several software applications used for file compression and decompression. The term usually refers to the GNU Project's implementation, "gzip" standing for GNU zip. It is based on the DEFLATE algorithm, which is a combination of Lempel-Ziv and Huffman coding...
, because the SILO
SILO (boot loader)
The SPARC Improved bootLOader is the bootloader used by the SPARC port of the Linux operating system; it can also be used for Solaris as a replacement for the standard Solaris boot loader....
boot loader transparently decompresses gzipped images.
The filename of the bootable image is not important, but by convention it is called vmlinuz or zImage.
bzImage
The bzImage (big zImage) format was developed to overcome this limitation by cleverly splitting the kernel over discontiguous memory
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
regions.
The bzImage was compressed using the zlib algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
until Linux 2.6.30 which introduced more algorithms. Although there is the popular misconception that the bz prefix means that bzip2
Bzip2
bzip2 is a free and open source implementation of the Burrows–Wheeler algorithm. It is developed and maintained by Julian Seward. Seward made the first public release of bzip2, version 0.15, in July 1996.-Compression efficiency:...
compression is used (the bzip2 package is often distributed with tools prefixed with bz, such as bzless, bzcat, etc.), this is not the case.
The bzImage file is in a specific format: It contains concatenated bootsect.o + setup.o + misc.o + piggy.o.
piggy.o contains the gzipped vmlinux file in its data section (ELF) (see compressed/Makefile piggy.o). All source files mentioned are in arch/i386/boot/.
No specific tool exists to unpack the bzImage file, but there exists a script named extract-ikconfig, which decompresses the image and extracts build configuration directives from the image. One may modify it to get the decompressed image directly. Some distributions (e.g. Red Hat and clones) may come with a kernel-debuginfo RPM that contains the vmlinux file for the matching kernel RPM, it typically gets installed under /usr/lib/debug/lib/modules/`uname -r`/vmlinux
See also this LKML post.
Object format
This is the ELFExecutable and Linkable Format
In computing, the Executable and Linkable Format is a common standard file format for executables, object code, shared libraries, and core dumps. First published in the System V Application Binary Interface specification, and later in the Tool Interface Standard, it was quickly accepted among...
header from an x86-64 Gentoo 2.6.29 executable kernel image.
$ readelf -h vmlinux
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x1000000
Start of program headers: 64 (bytes into file)
Start of section headers: 13951312 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 5
Size of section headers: 64 (bytes)
Number of section headers: 45
Section header string table index: 42