Linux Kernel Architecture

Mauerer runapp02.tex V2 - 09/04/2008 6:09pm Page 1143

Appendix B: Working with the Source Code

❑ crypto/contains the files of the crypto layer (which is not discussed in this book). It includes

implementations of various ciphers that are needed primarily to support IPSec (encrypted IP

connection).

❑ Thesecurity/directory is used for security frameworks and key management for cryptogra-

phy. For kernel 2. 6 .24, it contains only the SELinux security framework,^1 but kernel 2. 6 .25, which

was still under development at the time of this writing, will also contain the SMACK framework.

❑ Documentation/contains numerous text files to document various aspects of the kernel. How-

ever, some of this information is very old (documenting software is not the favorite pastime of

kernel developers).

❑ arch/holds all architecture-specific files, both include files and C and Assembler sources.

There is a separate subdirectory for each processor architecture supported by the kernel. The

architecture-specific directories differ only slightly and are similar to the top-level directory of

the kernel in that they include subdirectories such asarch/mm/,arch/kernel,andsoon.

❑ scripts/contains all scripts and utilities needed to compile the kernel or to perform other useful

tasks.

The source size distribution among the various kernel components is illustrated in Figure B-1.

Initialization

IPC

Block Layer

Security Framework

Library Functions

Cryptographic Routines

Memory Management

Scripts

Core Kernel

Documentation

Sound Drivers

Networking Namespaces

Filesystems

Header Files

Architecture-Specific Code

Device Drivers

0 20 40 60 80 100 120 140

Size [MiB]

Figure B-1: Distribution of code sizes among components found in the top-level directory of kernel

2.6.24.

(^1) SELinux extends the classic DAC (discretionary access control) rights model of the kernel to include role-based access control options,
MAC (mandatory access control) and MLS (multilevel security). These special topics aren’t discussed here because their implementa-
tion is lengthy, the underlying concept is complex, and the options are available in only a small number of Linux distributions.