Linux Kernel Architecture

Mauerer runapp02.tex V2 - 09/04/2008 6:09pm Page 1173

Appendix B: Working with the Source Code

short stub code in the kernel to provide an interface to a debugger running on a second system. Because

GDBsupports remote debugging, the kernel can make use of this form of debugging, which features

breakpoints, single-step processing, and the like.

KGDB is not included in kernel 2.6.24, but after long years of struggle, it will have been included in kernel

2.6.26 by the time you are reading this. Patches are also available for older kernel versions in case you

need KGDB support for them.

Once you have obtained a kernel with KGDB support, the configuration includes the new menu item

Kernel hacking->KGDB: kernel debugging with remote gdbthat must be enabled. If the serial interface

is used for data transfer, the correct settings must be made for the particular hardware. Debug symbols

should, of course, also be included in the kernel binary.

Because KGDB is evolving rather dynamically at the time of this writing, you should refer to

Documentation/DocBook/kgdb.html(which can be generated withmake htmldocs) for information on

how to connectgdbto a running kernel.

B.6 User-Mode Linux

UML (User-Mode Linux) is a port of Linux to Linux itself. The kernel runs on a Linux box as a userspace

process — a setup that is not without a certain beauty.

This facilitates many applications that would be difficult or impossible to implement with a classic Linux

kernel on genuine hardware, above all the ability to successively test new kernel features without the

need for dozens of time-consuming restarts.

UML also supports the use of debuggers, either of thebuilt-in, console-based type or as external pro-

grams. This section briefly describes howDDDcan be used in conjunction with UML to provide versatile

options for analyzing the kernel and its data structures. As with KGDB, breakpoints can be set and

variables can be changed in memory; however, only a single system is needed.

ARCH=ummust be specified in the command line to indicate that the kernel is to be generated for UML and

not for the local processor (CROSS_COMPILEneed not be set as would otherwise be necessary for genuine

cross-compilation.) For example:

wolfgang@meitner>make menuconfig ARCH=um

wolfgang@meitner>make linux ARCH=um

To compile UML on an AMD64 architecture, you also need to addSUBARCH=i386. The default configura-

tion of UML is a reasonable setup for most purposes and need not be modified.

The compilation result is an executable file calledlinuxthat is located in the main directory of the kernel

sources. This file contains Linux as a user process.

You can debug UML in the same way as a normal Linux process. As Figure B-9 shows, it is also possible

to set breakpoints.

UML offers many other options; for example, shared use of a filesystem with the host for ease of data

exchange, or a setup of network connections between the host and UML (and even between several UML