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24Creating a CustomKernelFedora is a collection of applications, daemons, and drivers with aLinux kernel at the core. The kernel deals with hardware and pro- vides basic functions such as creating processes, managing privi- leges, and managing file systems. In most cases, you don t need to build your own kernel plenty of well- configured kernels are available for use. You may need features thataren t currently supported by the kernel included in the most recentFedora release. You may want to Add support for unusual hardware USB scanners, cameras, joy- sticks, and sound cards. Use additional encryption features that aren t part of the standardFedora kernel. Explore alternative file systems. Omit drivers for devices you may never have. Omit amateur radio support unless you re really into amateur radio. Many kernel features are included in the form of modules chunks ofcode that are not loaded until you actually use the features. Modules don ttake up a lot of space if you don t use them, but removing the unused mod- ules will save you time when you re rebuilding the kernel. You can also gainsome security by omitting modules that you don t need. In this technique, we show you how to build a kernel customized for yourneeds, based on a safe and sound prototype, and with all the drivers thatyou need to get your work done quickly. TechniqueSave Time By Building a custom kernelthat suits your needs Adding the kernel sourcecode and dependenciesin one easy step with thePackage Manager Adding device drivers ornew file systems to yourkernel30_
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1 (continued) To Do ThisMake This Change to the Timesaving Bonus InfoDefault Linux Command LineBoot into emergency repair Add the word emergencyto the Boot into emergency mode if you discover a problem mode.end of the command line.that prevents you from booting into single-user (or multi- For example:user) mode. When you boot into emergency mode, Linux grub append> ro root=does not run any of the normal startup scripts you re LABEL=/ emergencydumped at a command line, and you re ready to fix what- ever it is that s gone wrong.
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Do ThisMake This Change to the Timesaving Bonus InfoDefault Linux Command LineView detailed boot Remove splash=silentfrom Enabling detailed boot messages can help you pinpoint messages (SuSE or the command line.the cause of a boot problem. Mandrake). Make a menu of screen size Add the vga=askoption to the Choose a larger screen resolution to fit more text on the options appear when you end of the command line.screen. If you like a certain text size, you can make the boot.For example: change permanent by changing the menu.lstfile in the grub append> ro root=/boot/grubdirectory. To do so, open the file with your LABEL=/ rhgb vga=askfavorite text editor and append your changes to the linestarting with the word kernel. Save the file when you refinished, and next time you boot, the changes take effect. Be careful when you change this file because it s easy tomake your system hard to boot. Include more details in the Add the word debugto the If you re having problems booting, this is a quick way boot process as the system command line.to find the problem. boots. For example: grub append> ro root= LABEL=/ rhgb debugIf you don t use any USB Add the no usbcommand.You may gain a bit of CPU performance. Warning:Makedevices, you can turn off For example:sure you don t depend on USB devices (like mice) the USB device modules.grub append> ro root=before you disable this module. LABEL=/ rhgb no usbTurn off power-Add the acpi=offcommand You may want to do this if your laptop powers off management to the GRUB command line.intermittently or has battery problems (even when it s control features.For example:plugged in). grub append> ro root= LABEL=/ rhgb acpi=offBoot into a runlevel other Add the runlevel (1 5) to the Server machines typically don t need a graphical environ- than the default.end of the command line.ment most of the time (why run X Windows on a mail or For example: Web server?). Specify the runlevel on the command grub append> ro root=line if you need to do system maintenance work and want LABEL=/ 5friendly desktop environment. See Technique 22 for moreinformation about runlevels. Boot into single-user mode Add an Sto the end of the This is incredibly handy if you forget the rootpassword. without a password.command line. When you re at the command line, you can change the For example:rootpassword with the command sh-2.05b# passwd. grub append> ro root=Enter a new password and confirm the new password when LABEL=/ rhgb S prompted. When you re finished, type the command sh-2.05b# rebootto reboot the system so you can log inwith your newly assigned password. Warning:Anyone canuse the Soption. Be sure you know who has physicalaccess to your computer because with this tidbit of knowl- edge, anyone can bypass the rootpassword. (continued)
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arguments (note, you have only a few sec- onds before the boot loader starts your kernel). Ifyou re running Mandrake or SuSE Linux, press theEsc key (while the boot selection screen is dis- played) to reach the Linux boot prompt. If you have multiple kernels or multiple oper- ating systems installed on your system, they redisplayed on the boot loader selection screen. The Linux kernel supports a huge variety of optionsand parameters, but you only need to know about afew of them. To find a complete (though slightly unattrac- tive) list of boot time parameters, install thekernel-docRPM package, which is includedwith most Linux distributions. You find the listin /usr/src/linux-$(uname -r)/ Documentation/kernel-parameters.txt. Just open the file with your favorite editor. The default command line for Fedora is as follows: grub append> ro root=LABEL=/ rhgbIf you re running Mandrake or SuSE Linux, thedefault command line will differ slightly. Table 23-1 lists various parameters that you maywant to use to alter the default command line. Youcan change the kernel command line to modify theway your Linux kernel boots, or to change the waythe Linux kernel runs after it s done booting. Removing modules with rmmodThe rmmodcommand removes a loadable modulefrom the kernel. You can t remove a module if it s inuse (lsmodwill tell you whether or not the module isbeing used). To remove a module, use the followingcommand: # rmmod modulenameWhen you delete a module, it s still on your com- puter; it s just not loaded into the kernel. Manipulating Boot TimeParametersThe Linux kernel is a program just like any otherprogram (well, maybe that s a bit of oversimplifica- tion): It has a command line, and you can specifyoptions and parameters to the kernel when you bootyour system. Each time you boot your system, the boot loaderpauses for a moment at the boot selection screen you usually hit Enter (or just wait for the built-intimer to expire) when you see the selection screen, and your computer happily boots the Linux kernelusing the default boot parameters. Linux can do a lotmore. If you re running Fedora Linux, press the A key whilethe boot selection screen is displayed to modify theTABLE23-1: FEDORABOOT-LINEPARAMETERSTo Do ThisMake This Change to the Timesaving Bonus InfoDefault Linux Command LineView detailed boot Remove the rhgb(Red Hat Removing rhgbsaves a little bit of boot time, and you messages (Fedora only).Graphical Boot) from the have a better starting point if you have to investigate a end of the command line.boot problem.
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the interdependencies among kernel modules and writes the results to /lib/ modules/$(uname -r)/modules.dep. The modprobecommand reads dependency information from thatfile. depmodruns each time you boot your system sothe dependency database is always up-to-date, unless you build or install a new module. To rebuildthe dependency database, use the following command: # depmod -aAs we mention earlier, modprobeuses the informationgathered by depmodto automatically load any mod- ules dependent on the one you want to use. The syn- tax for modprobeis essentially the same as the syntaxfor insmod: # modprobe modulenameIf any of the dependencies can tbe resolved, modprobegives up and undoes any of the work it salready done. Loading a module for a slightly different kernel with insmod and modprobeBecause modprobeautomatically resolves anydependencies for you, why would you ever want touse insmod? Every kernel module is compiled for aspecific kernel version. Occasionally, you run into amodule that s compiled for the wrong kernel, butyou re pretty sure it will work with your version. (For example, you may find a great sound carddriver compiled for kernel version 2.4.22-1.2115. nptl, and you re running 2.4.22-1.2116.nptl.) If youtry to modprobea module with the wrong version, modprobewon t do it. You can force insmodto loadthe module (even though it s been built for thewrong version) by including the –forceoption onthe command line: # insmod –force iforceBut, insmodwill still complain about unresolveddependencies. Don t give up you can still use modprobeto do the hard work for you. Just use the -nand -voptions: # modprobe -n -v iforceinsmod input.oinsmod usbcore.oinsmod serio.oinsmod iforce.oWhen you use the -nand -voptions, modprobeshowsyou the insmodcommands that you need in order toload dependent modules in the correct order (but itdoesn t actually execute those commands). Now youcan execute those commands yourself; just be sureto include the –forceoption when you load the mis- matched module: # modprobe -n -v iforceinsmod input.oinsmod usbcore.oinsmod serio.oinsmod iforce.o# insmod input.o# insmod usbcore.o# insmod serio.o# insmod –force iforce.oThe lsmodcommand displays a list of the modulesthat are currently loaded on your system (see Figure 23-1). Figure 23-1:Currently loaded kernel modules.
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1: VIEWINGMODULEINFORMATIONWITHMODINFO# modinfo iforcefilename: /lib/modules/ 2.4.22-1.2115.nptl/kernel/ drivers/char/joystick/iforce.odescription: USB/RS232 I-Force joysticks and wheels driver author: Vojtech Pavlik, Johann Deneux license: GPL Installing a module with insmodThe insmodcommand installs a module into a run- ning kernel (or at least tries to). The typical com- mand syntax is# insmod modulenameTaking care of dependencies automaticallywith modprobe and depmodWe said that insmodtries to install a module, but it snot always successful. Some modules depend onother modules. For example, if you want to load theIforce joystick driver, you must first load the genericserial joystick driver and USB driver. If you try toload a module with unsatisfied symbols, you seeerror messages like this: unresolved symbol serio_close_R393d70c3unresolved symbol serio_open_R17abfb2funresolved symbol serio_unregister_deviceunresolved symbol serio_register_deviceDeciphering these messages can be nearly impossible. Fortunately, modprobesolves dependency problemsfor you. The modprobecommand works with theinformation computed by depmodto automaticallyload any dependent modules. Adding and Removing KernelModulesLinux contains some handy commands you can useto manipulate your kernel while it s running. The ker- nel module tools make it easy to experiment withkernel changes without making any lasting or perma- nent changes. These commands don t make permanentchanges to your kernel; the changes last onlyuntil you reboot. To automatically implement achange at boot time, you need to add thechange to a startup script. For the quickestway to make a change to your system startupscript, check out Technique 26, where weshow you a couple of easy-to-follow examplesin the section about closing down securitygaps with /proc. To use the kernel tools, you need to open a terminalwindow and give yourself superuser privileges withthe su -command. Be sure to include the hyphen, which ensures that the module tools are on your$PATHsearch path. Learning about modulesYou can find out what kernel modules are on yoursystem by entering the following command: # ls -R /lib/modules/$(uname -r) If you don t have the kernel module that youneed to control a device or configure a pieceof hardware, do a Google search for the kernelmodule you need. Lots of modules are avail- able on the Web. The modinfocommand displays all sorts of informa- tion about a kernel module, as shown in Listing 23-1.29_
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23TechniqueTaking Good Care of Your KernelThe kernel is the software core of your computer. Kernel modulesmake up the software interface between your system hardware andthe system software. On a fresh Linux installation, over 900 kernelmodules are ready to load into your kernel. The actual number may varydepending on the specific hardware that Linux finds when it sets uphousekeeping, but on our Fedora system, 968 modules exist. You can also download and install new kernel modules. You can findmany open-source modules bouncing around the Web that do everythingfrom silencing annoying beeps to enabling wireless network cards. Thebeauty of the system is the flexibility that kernel modules provide thecomputer industry. Every time a new piece of hardware is invented, youdon t have to modify and rebuild the Linux kernel you just add a newmodule. Manipulating kernel modules is quick and simple, thanks to a series ofcommands that work at the command line to update your kernel withouta complete rebuild. You can also find-tune your kernel by using the boottime parameters. Boot time parameters are the kernel options that areenforced every time you boot your system. Not all features are modules, and you may want your kernel to be config- ured so that some features can t be changed. If that is the case, a com- plete kernel rebuild is called for (see Technique 24 for information aboutbuilding a kernel from scratch). If you can get by with a simple patch tothe kernel, this is the technique for you. In this technique, we introduceyou to the commands that let you customize your kernel quickly andeasily. Save Time By Manipulating your kernelon the fly Using your boot timeparameters to custom- tailor your kernel29_
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Part IVTweaking the Kernel onYour Linux System28_
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Click Kill to confirm your choice. The process is terminated. Figure 22-9:The KDE System Guard window. Figure 22-10:Confirm the process termination. With a little vigilance, you ll avoid the piles of filesthat ex-users can amass on your system and savethe resources for the users that need them.
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might be prudent to consult with the othermembers of that group before deleting the file. 9.If the list includes files that you can t recognizeby filename or location, you can Open the file and manually inspect the con- tents. Depending on how prolific the ex-userwas, this might be the best option to startwith. However, if the user left you with hun- dreds of files, you ll probably want to use theother option. Narrow the inspection a bit by deleting thethings that aren t important. Right-click thefilename and choose Open from the pop-upmenu. You re treated to a view of the file, inthe default viewer for that type of file. If you see a listing with /proc/processIDinthe In Subdirectory column of the result table, it means that the ex-user still has a processrunning somewhere on your system. Make anote of the process ID so you can use KDESystem Guard to kill off the process. (SeeTechnique 41 for complete details; see the fol- lowing steps for the short version.) To kill off abandoned processes, follow these steps: 1.Open a terminal window and gain superuserprivileges with the sucommand. 2.Type the following command and press Enter: # ksysguardThe KDE System Guard window opens, as shownin Figure 22-9.3.Click the Process Table tab to move to a list ofcurrently running processes, and look for theprocess ID in the PID column. 4.When you ve found the process ID, highlightthe entry for that process and click the Killbutton. A dialog opens asking if you really want to killthe selected process (see Figure 22-10). You can delete old system files that containconfiguration information and preferences foryour ex-user without much concern for inter- fering with other users, but most other filesshould be investigated more closely. 6.To delete an old file, choose Delete from thepop-up menu. You re asked to confirm the deletion.Click Yes todelete the file. You can select multiple files by holding downthe Shift key and highlighting the files with amouse click (or by using the arrow keys toselect multiple files). Then right-click the filegroup and choose Delete from the pop-upmenu. You re asked to confirm the deletion. Click Yes, and the files disappear quickly! 7.If you see files in the list that may contain workor data important to others, right-click andchoose Properties from the pop-up menu toopen the Properties dialog (see Figure 22-8). Figure 22-8:The Properties dialog. 8.Click the Permissions tab to view the file own- ership information, and note the name of thegroup that owns the file.
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