Contents

Topics

• Reconfiguration
• Recompilation
• Installation
• Modules
• Patching the Kernel
• Bootable Floppy

Assignment

Reconfiguration

• cd to /usr/src/linux
• You should find several files, one of which should be a soft link to the current kernel version you are using (e.g. linux-2.4 -> linux-2.4.18-14)
• cd linux-2.4 (or whatever you are using)
• There is a great README file - read it.
• Run make menuconfig or make xconfig or make config. config is text based and hard to use. The other two are menu-based but xconfig requires that you be running the X server. You can also manually edit the config files but this is strongly discouraged if you aren't an expert at kernel configurations.
• Select the services that you want to delete or include.
  • Main Menu
  • Network Menu
• Exit and save your configuration. This creates or updates the .config file in your current directory.

• Don't put in stuff you don't need - e.g. SCSI devices when you don't have any. On the other hand, there may be interactions that could mean that things are needed that you don't really know about. If you add or remove things, its best to do it incrementally.
• However, it can be difficult to get everything right. The install kernels typically have everything in them, so you could remove some things and end up with a kernel that doesn't work on your hardware. Remember, the default configuration that you get when you recompile does not match your installed kernel.
• A HINT: Red Hat puts a file in /boot named config-version (e.g. config-2.4.18-14). If you copy this file into your source directory as .config you will have the configuration Red Hat used to create the kernel version that was installed. At least that's the theory.
• Before configuring your system, you can type make mrproper to insure all new object files and dependencies. However, it also deletes your configuration file (.config) so you should make a backup. If you are going to copy a configuration file from somewhere else, do the make mrproper first.

Recompilation

• Type make dep to set up the dependency links.
• If you have compiled before and did not do a make mrproper, do a make clean to get rid of any old .o files.
• From /usr/src/linux, enter make bzImage. You can also type make zImage, but bzImage is a compressed kernel that takes less disk space and is preferred.
• Go to lunch or at least coffee.

• If things go wrong, this could be a long process. Most likely problems are not having the proper gcc compilers installed, out-of-date include files or libaries.
• Type make install although you need look at the following section before you do this.
• If you reconfigured modules, you need to do the following:
  • make modules
  • make modules_install
• To make a bootable floppy version, insert a floppy and use "make bzdisk". This bootable version can be used to test a kernel for correctness or as a rescue floppy.
• If you have compiled before, use "make clean" to delete all the .o, .a and .s files before doing the "make bzImage" or "make bzdisk".

Installation

• If you are building a kernel that is a new version than the one you are currently using, you don't need to make a copy of anything.
• cd /boot and copy your current kernel to a backup file. If you aren't sure, look in /etc/lilo.conf. For example,

     cp vmlinuz-2.4.2-2 vmlinuz.save
          

• Enter make install or cd to /usr/src/linux/arch/i386/boot and

     cp bzImage /boot/vmlinuz-X.X.XX-X
          

  where the X's represent the current kernel version.
• cd /boot
• ln -s vmlinuz vmlinuz-X.X.XX-X to get a link for a general purpose kernel name if you want to.
• Edit your boot loader configuration file. It's a good idea to put in two linux boots - linux and linux-old for example, where linux-old points to your old kernel.

  default=0
  timeout=10
  splashimage=(hd0,0)/grub/splash.xpm.gz
  title Red Hat Linux (2.4.20-8)
          root (hd0,0)
          kernel /vmlinuz-2.4.20-8 ro root=/dev/md1 hdb=ide-scsi
          initrd /initrd-2.4.20-8.img
  title Red Hat Linux (2.4.22-2)
          root (hd0,0)
          kernel /vmlinuz-2.4.22-2 ro root=/dev/md1 hdb=ide-scsi
          initrd /initrd-2.4.22-2.img
  
          

• Reboot and choose the new kernel.
• /var/log/dmesg has all of the boot up messages if you need to look at them.
• If you get a message of the form:

  
          unable to handle kernel paging request at address C0000010
          Oops: 0002
          EIP:   0010:LLLLLLLL
          eax: xxxxxxxx   ebx: xxxxxxxx   ecx: xxxxxxxx   edx: xxxxxxxx
          esi: xxxxxxxx   edi: xxxxxxxx   ebp: xxxxxxxx
          ds: xxxx  es: xxxx  fs: xxxx  gs: xxxx
          Pid: xx, process nr: xx
          xx xx xx xx xx xx xx xx xx xx
  
     

  the LLLLLLLL values tells you where in the kernel it had a problem.
• Run namelist against the kernel to find out where it is:

     nm vmlinuz | sort | less
     

• Always be prepared to go back, so keep track of the changes that you made.

Example

You decide that you need to upgrade your kernel to the latest version. So you go to the kernel mirror list, and pick a good place to download the kernel. You go to the kernel directory and see a list of kernel versions. 2.4 is the largest number so you check there. The README file makes it clear that this version is not ready for prime time, so you try 2.3. The latest version is 2.3.99-pre9:

• Download the gzipped tar file to /usr/src.
• tar --gzip linux-X.X.XX-X.tar.gz
• ln -s linux-X.X.XX-X linux
• cd linux
• make mrproper
• make menuconfig
• make dep
• make bzImage
• make install
• make modules
• make modules_install
• cd /boot
• Assuming that the current version is X.X.XX-X, edit /etc/grub.conf and add a boot option for the new version, but leave the old one there and rename the label.
• If you prefer to have a generic link, such as vmlinuz, that points to the latest version, that is a common alternative.

Modules

• Modules reside in /lib/modules/version. For example,
  /lib/modules/2.2.12-20
• The typical directory contents are:

     
  .rhkmvtag  cdrom/  ipv4/  modules.dep  pcmcia/  video/
  block/     fs/     misc/  net/         scsi/                            
     
     

• Each directory contains a set of modules. For example, cdrom might contain,

     
  aztcd.o   cm206.o  isp16.o  mcdx.o   sbpcd.o  sonycd535.o
  cdu31a.o  gscd.o   mcd.o    optcd.o  sjcd.o
     
     

  The module configuration is found in /etc/modules.conf. Older Linux versions use conf.modules and it will still be used if modules.conf is not present.

  A typical module configuration file might be:

     
  alias eth0 eepro100         
  alias usb-controller usb-uhci
  alias eth0 tulip
     
     

  When the system boots, it loads the modules specified by modules.conf. Modules can also be managed manually.

  lsmod lists all currently loaded modules.

  insmod will install a loadable kernel module.

  insmod [options] [-o module name] object_file [arguments]

  insmod /lib/modules/2.2.12-20/scsi/aha152x.o

  insmod -f -o aha1527 /lib/modules/2.2.12-20/scsi/aha152x.o

  -f ignores kernel version mismatches

  modinfo will output information about a module file

  • -a = author
  • -p = parameters
  • -d = description

  depmod outputs a set of dependencies based on the set of modules mentioned on the command line or in a default place (/lib/modules/) and places the dependency file in a known place (/lib/modules/version/modules.dep).

  modprobe takes the dependency list created by depmod and loads the appropriate modules. For example, this is done in /etc/rc.d/rc.sysinit

  lsmod lists all currently loaded modules.

Example

You decide to go for a nice SCSI tape drive on your system. Unfortunately, the manufacturer doesn't provide Linux drivers. So you look on the web and find a site like HREF="http://lhd.zdnet.com/lhd_press_drivers.html">zdnet that has quite a few drives and lo, there it is. They have a loadable kernel module for your controller. Download the file (probably a tar file), untar it and put the output (.c and .h files) in /usr/src/linux-X.X/drivers/scsi (where your current linux source tree resides). A module might also come as a patch as discussed later.

Since its a module, check to make sure that you have enabled loadable modules in your kernel configuration. If not, rebuild it. If you install the modules, it should be in /lib/modules/your-version-number in the the scsi subdirectory.

Now, you can enter:

alias sza aha9999

initrd

The initrd files that you commonly see in the /boot directory and referenced in the grub.conf file are RAM disk files. That is, they are images that can be loaded directly into memory to complement the kernel that is to be booted. They contain things (usually modules) that need to be loaded before the kernel can run. For example, if you have a SCSI disk and your kernel doesn't have the SCSI functionality compiled in, you can force the necessary driver modules to be loaded into memory and be available to load the kernel from the disk.

mkinird is the command used to create initrd files. It will automatically load anything mentioned in /etc/modules.conf as well as the default SCSI modules. There are two options for adding another module; with for adding modules to be loaded after the SCSI modules, and preload for modules to be loaded before the SCSI modules. For example,

mkinitrd --preload=hpt366 --with=mymodule initrd-test.img 2.4.20

which creates an initrd image named initrd-test.img, based on the 2.4.20 kernel (which has to be present) and in addition to the default contents, also preloads the hpt366 driver and postloads mymodule.

Interestingly, an initrd image is simply a compressed file image, so you can add things to the file system directly. For example,

gzip -dc /boot/initrd-2.4.18 /tmp/initrd
mount -o loop /tmp/initrd /mnt/floppy
cd /mnt/floppy

You will find a number of things, including the modules you want loaded under the lib directory. Once mounted like this, you can add modules and other features if you so desire. Then,

umount /mnt/floppy
gzip -c /tmp/initrd > /boot/initrd-2.4.18-new.img

Patching the Kernel

Kernel upgrades are often done with patches. A patch is a set of diff files. diff is a program that will compare two files and output the differences on a line-by-line basis. If you have an old file and a set of differences between the old file , you can upgrade the old file to be a duplicate of the new file. Kernel patches have the diff files between different versions or mods and when applied, can upgrade the kernel.

Finding a Patch

Download a Patch

Extract the Patch

Install the Patch

patch [options] originalfile [patchfile]

but the usual form is:

patch -pnum < patchfile

For example, in the situation above, we have /usr/src/linux-2.4 contains our current version, and we want to patch it with the data we put in directory patch-2.4.26-rc2. So we would do the following:

cd /usr/src/linux-2.4
patch -p1 < ../patch-2.4.26-rc2

The -p1 option tells patch how to process filenames. The diff files directory structure will mimic the directory structure of /usr/src/linux-2.4 and patch will find files with the matching pathname to make changes. The filenames will be ../patch-2.4.26-rc2/..., but we need the files to match at the level of the patch-2.4.26-rc2. That is, if /usr/src/linux-2.4 has a file named config, then you would expect to find the same name in the patch directory at the same level. -p1 tells patch to ignore the first element in the path, so it becomes patch-2.4.26-rc2. If this directory contains a config, then we have what we need. You can think about it this way, each n in -pn removes one slash from a pathname and everything in front of it.

After executing the patch command, the kernel is ready to be rebuilt. Remember to perform an mrproper.

Bootable Floppy

• To boot given a particular hard drive configuration using the rescue option at boot time.
• To create a standalone bootable set, it is easiest to get a set from someplace, like
  ftp://metalab.unc.edu/pub/linux/recovery www://www.toms.net/rb www.knoppix.com to get a bootable CD.
• uname -r - will give you the version of your OS.
• mkbootdisk version
• Also, can use the the zdisk option when making the kernel.

     
     uname -r 
     
     2.2.13-7mdk
  
     
     mkbootdisk 2.2.13-7mdk
     
     Insert a disk in /dev/fd0. Any information on the disk will be lost.
     Press  to continue or ^C to abort.
  
     
     mount -text2 /dev/fd0 /mnt/floppy
     ls
     
     ./ ../ boot/ dev/ etc/ initrd.img lost+found/ vmlinux-2.2.13-7mdk
     

Assignment

1. Make some inconsequential change in your kernel config, recomplile and install. Wheee!! Some good choice are things like SCSI support, removing unused network cards or sound card stuff.
2. Make a bootable floppy and try it out.