The Linux operating system is like an onion. If you peel away the outer layers (the KDE desktop, the bash shell, and so on), you find a layer of operating system libraries. The libraries provide commonly used functions that enable applications to find things like the current date, the IP address of a given host, and so on.
Underneath the library layer is a set of system calls, which are functions that perform low-level operations like changing your user ID, allocating more memory, and opening a file. At the core of the onion, you find the kernel. The kernel uses device drivers to manage system hardware. The kernel also schedules disk I/O and CPU usage, responds to exter- nal signals, creates and tears down processes, and performs other low-level operations. But the kernel itself is layered, too. The Linux kernel has a portable layer that runs on any computer. At the very center of the onion is a hardware- dependent layer that is customized for each CPU (Intel x86, PowerPC, StrongARM, and so on).
Step 1: Making an Emergency Plan, or Boot Disk
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2.
When the YaST control center appears, click System (in the left-hand pane).3.
Click Create a Boot, Rescue, or Module Floppy.4.
Follow the on-screen instructions to create aboot floppy, rescue floppy, and module floppy. You may need to boot into your computer’s BIOS setup mode to change the boot sequence to test the floppy. How you enter setup varies with your machine, but instructions are typically displayed on-screen at boot time.
You can also use the first install disc of your distri- bution’s CD (or DVD) collection to boot into rescue mode.
To boot into rescue mode on a Mandrake system, fol- low these steps:
1.
Place the first install disc in the CD/DVD drive.2.
Power up your system.3.
When you see the Press <F1> for more optionsprompt, press F1.A screen full of help text appears, followed by the boot prompt (boot:).
4.
Type rescueand press Enter.Booting into rescue mode on a SuSE system is similar:
1.
Place the first install disc in the CD/DVD drive.2.
Power up your system.3.
When the boot menu appears, use the down arrow key to highlight Rescue System and press Enter.4.
When prompted, choose your preferred lan- guage from the menu and press Enter.To boot into rescue mode on a Fedora computer:
1.
Place the first install disc in the CD/DVD drive.2.
Power up your system.3.
When the boot:prompt appears, type in the following command and press Enter: boot: linux rescue(Don’t type the word boot:, that’s the boot prompt — just type in linux rescueand press Enter.)
4.
When prompted, choose your preferred lan- guage from the menu and press Enter.5.
Choose your keyboard type from the menu and press Enter.6.
At this point, Fedora asks if you want to start the network devices in your computer. Choose Yes or No (use the left- and right-arrow keys to select the option that you want) and press Enter to continue.Fedora will try to find the root file system on your hard drive and mount that file system so that you can carry out any repairs that you need to make. If you want to poke around a little without endan- gering anything on your hard drive, tell Fedora to mount the root file system in read-only mode.
7.
Choose the mount mode you prefer (choose Continue to mount your root file system in read/ write mode, Read-Only to safeguard your file systems, or Skip to tell Fedora not to mount your root file system).If Fedora locates and mounts your root file system, you can find it in the directory /mnt/ sysimage. If you look in that directory, you’ll see subdirectories such as /mnt/sysimage/bin, /mnt/ sysimage/boot, /mnt/sysimage/dev, and so on. Those subdirectories correspond to the /bin,
/boot, and /devdirectories on your computer’s root file system.
Regardless of which distribution you’re using, after you’ve booted into rescue mode, you eventually end up at a command line. From there, you can mount your root file system (and any other file systems that you may need), make any repairs that you need, and reboot.
Technique 24: Creating a Custom Kernel
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Step 3: Configuring
a New Kernel
After making a boot disk and installing your source code, it’s time to configure a new kernel.
To build a custom kernel, follow these steps:
1.
Open a terminal window and give yourselfsuperuser privileges with the sucommand.
2.
Type the following command and press Enter:# cd /usr/src/linux-2.4
If you’re using a kernel version newer than 2.4, cd
to that directory instead.
3.
Type the following command and press Enter: # make mrproperThis command cleans up any remnants of previ- ous builds that might confuse your new build.
4.
Identify the type of processor in your computer: # uname -pThe command displays the processor type that you’re currently using (we assume i686 in the examples that follow).
5.
Copy the configuration file that matches your processor type into your current directory: # cp configs/kernel-2.4.22-i686.config.config
By using a predefined configuration file, your new kernel starts out in a well-defined and func- tional state.
6.
Type the following command and press Enter: # make oldconfigThis step runs for a while and displays a ton of messages. Just ignore the messages and grab some caffeine.
Step 2: Finding the Source Code
To rebuild the kernel, you first need to be sure that the source code for the kernel is on your system. Fedora distributes the kernel source in the form of an RPM package. You could use the Red Hat Package Manager to install the kernel source package by hand, but you’d also need to install a number of dependencies. Here’s an easier way:Reusing the kernel that is included with the latest distribution saves time. It’s a lot faster to alter a kernel you already have handy than to go through the work of downloading, cus- tomizing, and building a whole new kernel from scratch.
1.
Open the Main Menu and choose System Settings➪Add/Remove Applications.2.
Enter the rootpassword when prompted.The Package Manager checks the system for installed packages and opens the Add or Remove Packages window, showing both the installed and available packages.
3.
Scroll down the list and check the box next to Kernel Development.4.
Click the Update button.The System Preparation dialog opens.
5.
Click the Continue button.If prompted, insert the required disc and click OK. When the installation is complete, a confirmation window is displayed.
The Add/Remove Applications tool may get confused if you’re installing software from a DVD instead of a CD. If the disc you’re using gives you trouble, just insert it and let the autorun procedure start. Then follow the setup wizard to install additional packages.
Step 4: Customizing the Kernel
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Step 4: Customizing the Kernel
When you’re done with the preceding steps, you’re ready for the fun part: customization. Here’s how it works:
1.
Enter the following command: # make menuconfigThe Linux Kernel Configuration window opens, as shown in Figure 24-1.
• Figure 24-1: The Linux Kernel Configuration window. If you run into some documentation that suggests make xconfigrather than make menuconfig, ignore it. xconfighas a nice user interface, but in the 2.4 kernel series, it has a serious flaw that will cause you all sorts of grief.
2.
The menuconfigwindow (refer to Figure 24-1)displays a list of feature groups. Use the arrow keys to move up and down through the list and press Enter to select the highlighted group. The left- and right-arrow keys move you through the <Select>, <Exit>, and <Help>choices at the bottom of the window. To return to the previous screen, choose <Exit>.
When you select a feature group and press Enter, you see the list of features within that group (see Figure 24-2). To the left of each feature, you see an indicator that shows the state of the feature (see Table 24-1).
• Figure 24-2: The File Systems submenu.
TABLE24-1: MENUCONFIGINDICATORS
Indicator Description
[ ] The feature is not selected and won’t be included in the new kernel. You can build the feature as a loadable module.
[*] The feature will be included (and can only be compiled) in the new kernel.
< > The feature is notselected and won’t be compiled as a loadable module.
<M> The feature will become a loadable kernel module.
Indented lines show separate components of a feature. You can’t enable a component without the parent feature.
The <Help>button displays a help screen that describes the highlighted feature. Most features are well documented and include links to more information, as shown in Figure 24-3.
Finding features in the Kernel Configuration menu is like digging around in an old attic. You’ll find features for technologies that are long since retired or so obscure that the aver- age human will likely never know anyone who needs them. Still, if you do need them, they’re here — just look around.
Technique 24: Creating a Custom Kernel
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2.
Enter the following command and press Enter: # make cleanThis command cleans up old unwanted files to prepare for the build.
3.
Choose a name for your new kernel (typically, just add the current date to the end of the name). Use your favorite editor to change the EXTRAVERSIONvariable in the Makefilefile. To use kate(a KDE-friendly editor), type in the fol- lowing command and press Enter:# kate Makefile
4.
Add the date to the end of the EXTRAVERSION variable (after the word custom).EXTRAVERSION=-1.2115.nptlcustom012504
Save the file and close the editor.
5.
At the command line, type the following com- mand and press Enter:# make bzImage
This step takes quite a while, possibly a few hours, because you’re compiling most of the ker- nel source code here.
6.
Type the following command and press Enter: # make modulesThis compiles the source code for the kernel modules that you selected. This step also takes some time.
You did make a boot disk and test it, right? If not, stop what you’re doing, go back to “Step 1: Making an Emergency Plan, or Boot Disk,” earlier in this technique, and follow the steps there.
7.
Type the following command and press Enter: # make modules_installThis step installs the new kernel modules. Don’t worry — you’re not replacing your current ker- nel; you’re just adding another choice.
• Figure 24-3: The Configuration help screen.
3.
Make any changes that you want.A couple of useful features to point out are alter- nate file systems and USB support for otherwise unsupported devices (such as cameras or USB scanners).
If you have a single CPU in your machine, (which is likely), you should choose to disable SMP support in your new kernel. This feature is included on the Processor Type and Features menu and is labeled Symmetric Multiprocess- ing Support. Click the button next to the n before continuing.
4.
When you’ve chosen the options you want to include in your new kernel, choose <Exit>and save your work.Step 5: Building the Kernel
Now it’s time to start building the kernel. (Note:Some of these steps will run for quite some time, so you better grab a donut.) Follow these steps:
1.
At the command line, enter the following com- mand and press Enter:# make dep
The make depcommand computes dependencies for the features that have changed.
Step 5: Building the Kernel
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8.
Type the following command and press Enter: # make installThis step copies the new kernel into place and adds the kernel to your boot menu.
9.
To use the new kernel, reboot and highlight the new kernel in the list of choices. Then press Enter.Your system boots with the newly built kernel.
Building kernels can be a tricky business. If you experience trouble at boot time (kernel panic!), reboot into your old kernel, and try building the new kernel again.
Write down error messages! You can Google for them later to see if other Linux users have had similar problems and published work- arounds on the Web.
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