______ Compiling the Linux Kernel (CS 273 (OS), Fall 2020)
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Compiling the Linux Kernel

CS 273 (OS), Fall 2020

Compiling the kernel

  1. Insure you have enough hard disk

    Your virtual disk will require more than 5-8 GB in order to recompile the kernel successfully. 16-20GB should be sufficient. You can enlarge a VDI virtual disk using the command-line program vboxmanage as described below

  2. Configure the kernel

    These instructions assume that you have performed all of the steps in the VirtualBox lab.
    Start up your virtual system, then enter the following commands:

      $ cd /usr/src/linux-5.0.21 
  $ sudo apt install flex bison make
  $ sudo make localmodconfig
  $ sudo make menuconfig

    • Configuration is selecting options for your kernel. There are dozens of configuration options to choose, and some may make the difference whether or not your kernel will work on your computer. In the great majority of cases, the default choice in these configuration steps is a good choice.

    • The config target for make (which we aren't using) presents each option by name with a selection of actions. For example: 

        WM8350 GPIOs (GPIO_WM8350) [N/m/y/?]
      prompts for one of three choices n, m, and y for a configuration option named GPIO_WM8350.

      • n means to decline that option

      • m means to make that option loadable while the kernel is running, as a module

      • y means to include that option in the kernel

      • ? prints some information about that option, then reprints the prompt

      The default choice (Enter key) is capitalized (N in this case). Note: Some items may additionally be labelled (NEW), (EXPERIMENTAL), etc. For our purposes, we will generally decline these choices unless we find we need them for some reason.

      • A prompt is provided for most or all NEW items, and the default response (capitalized) is ordinarily N, so we recommend accepting all defaults.

      • One new setting is Default security module, which requires a numerical response. RAB selected 1, for the well-known SELinux.

      We are avoiding the config target because it tediously presents hundreds of these prompts that each require a response, with no way to back up (other than re-entering everything again).

    • The localmodconfig target automatically selects only options that appear on the (virtual) system you are using. This will save on compilation time and kernel size, although it makes the resulting kernel less portable.

      Note: the localmodconfig target will prompt you whether to accept any new features, marked (NEW). You can safely respond Enter to accept all of those features (the default).

    • Use of menuconfig: We want to insure that the ext4 and iso file system supports are included, so check that these are included during the interactive menuconfig target. In April 2018, ihe iso file system was found under a submenu DVD/CD-ROM file systems. of the File systems menu, and needed to be selected (Y).

      • If you get a message saying that your terminal window is too small, enter CTRL/- in your terminal window to correct your terminal window's settings

      Enter TAB character to exit from a submenu, menu, or ultimately the menuconfig program

      Exit and save changes.

      • Choose the default filename .config , since that configuration file is used to make the kernel.

      • Also make yourself a backup copy, e.g., 

        $ sudo cp .config first.config
        This will allow you to restore these settings in case something happens to your .config file. (make menuconfig automatically creates a backup .config.bak, but that backup may change if you make the menuconfig target again.)

    • RECOMPILING: After your first successful compile (below), to recompile with your previous configuration, enter 

        $ sudo make oldconfig
      instead of either the localmodconfig or menuconfig targets. This saves you the unnecessary hassle of reconfiguring each time you compile the kernel.

  3. Compile the kernel

    First, an additional package needs to be installed. This installation only needs to be performed once per Ubuntu installation. 

    $ sudo apt install libssl-dev

    To compile the kernel: In /usr/src/linux-5.0.21, 

      $ sudo make

    • Expect perhaps 30-40 min for a complete compile.

    4. The instructions below are not yet checked/updated for Spring 2018

  4. Install the kernel

      $ sudo make modules_install install

    • This installs the new kernel in /boot

      • System.map-5.0.21

      • vmlinuz-5.0.21

      • initrd.img-5.0.21

        Initial RAMdisk. If missing, you can try to create or update one.

      • config-5.0.21

    • grub -- GNU GRand Unified Bootloader, allows you to choose which kernel to boot when rebooting your virtual OS within your virtual machine.

      During boot of your operating system (on your virtual machine), grub presents a menu for you to select which kernel to boot. The behavior of grub______

      ______

      The installation should update the file /boot/grub/grub.cfg by adding three sections for your new 5.0.21 kernel. If it doesn't, you must add those sections to the grub.cfg file, then update grub using 

        $ sudo update-grub
      Note: Do not perform update grub if sections were already added for your 5.0.21 kernel

      Note: The recovery entry in grub.cfg enables you to make repairs in a broken (virtual) OS setup in case of emergency.

    • RECOMPILING: to save time on a recompile, omit installing the modules on future recompiles

  5. Test the new kernel

    First, enter the command 

      $ uname -r
    which indicates the Linux version number for the kernel that came with your distribution of Ubuntu. Make a note of the output.

    Now, shut down and restart your virtual system.

    When you restart your system, your new kernel should may not be selected by default (Fall 2020). To choose your new kernel manually, reboot as follows.

    • When restarting your VM, as soon as the VM's window appears, press and hold the right-shift key in that new VM window.

    • While you are holding the right-shift key, a menu such as the following will appear:

      • GRUB is the GNU boot loader, which allows you to choose which kernel to boot.

    • Use the down-arrow key and enter to select "Advanced options for Ubuntu". Then select your kernel labelled with , and press the enter key to boot from that kernel.

      • Don't choose the kernel labelled "(recovery mode)", which is for taking extraordinary recovery actions, for example, if the file system on your virtual disk becomes corrupted.

    The boot should now proceed as normal using your kernel. (If your new kernel doesn't successfully boot, you can power your VM off then use GRUB to restart choosing a different kernel.

    After you log in, the command 

      $ uname -r
    indicates the Linux version number for the currently running kernel (we expect 5.0.21 for your new kernel)

    • Note: In Spring 2018, at least one person (Mac system) found that after booting the new kernel, first login screen seemed to receive mouse events but not keyboard events (which prevented entering a password).

      A workaround was to select the Guest account for login (which has no password), then logging out from that Guest account. Future login screens accepted the keyboard events, even after rebooting the OS or powering down the VM.

How to enlarge a VDI hard disk using vboxmanage

Note: These instructions worked in Fall 2015, but are not yet tested in Spring 2018.

  • Be sure to shut down your virtual machine.

  • Find the file name of your virtual disk.

    You can enter the command 

      % vboxmanage list hdds
    on a terminal window on your host OS will list your hard disks. Note: On a Windows computer, you may need to precede the vboxmanage command with its full path.

    If it's not obvious which disk goes with your virtual machine, you can use the GUI-based Virtual Box manager to find the disk filename by: (1) selecting your virtual machine, then clicking Settings; (2) select storage, and click on the virtual disk on the left pane; (3) look for the location entry on the right; (4) copying that filename.

  • Now, resize the hard disk using 

      $ vboxmanage modifyhd "filename" --resize 20480
    Here, filename is the file name you found before, and the value 20480 (in megabytes) specifies exactly 20GB. Use 16384 (in megabytes) to specify 16GB.

  • Shutdown and restart Virtual Box, so that your image will recognize the new hard disk size

  • Note: Increasing the size of a disk image like this is easy, but decreasing one may require compacting the file system within (rearranging blocks to create a range of unused blocks at the end of that file system), and is not recommended even if vboxmanage supports it (which is doubtful).

  • You have now enlarged the virtual disk, but this does not enlarge the partition on that virtual disk that contains your file system. This will require editing the partition table on your virtual disk.

    First, back up your virtual disk so you can retrieve your data in case something goes wrong. (Making an error in the partition table can render your disk unusable, but you could start over again with the copy.) To make a backup copy,

    • Determine the location of your virtual disk, either within VirtualBox (click on Storage for your virtual machine and mouse-over the virtual disk's name) or using vboxmanage

      vboxmanage list hdds

    • Make sure your VM is shut down, then make a copy of that file, perhaps using the same file name plus an added .bu

    Note: This backs up your virtual disk, which your VM needs in order to connect to your files. It's also possible to back up your VM itself, i.e., the VirtualBox settings for your VM. VirtualBox uses an ID number system to avoid starting VMs with disks that don't belong to it.

  • Obtain a bootable copy of a disk partition editor. We will assume gparted

  • Boot from gparted

    • While your VM is still off, select that VM and click on Storage.

    • If an ISO image is already loaded on your IDE controller, virtually eject it ("remove the attachment from the storage tree") using the second of the four control buttons

    • Then, click on Controller: IDE and use the first (leftmost) of those buttons to virtually insert your gparted ISO image ("add a new attachment to the storage tree"). You will need to browse to that image.

    • Then restart your VM, use the F12 key and choose to boot from CDrom instead of your virtual disk

  • gparted boots as a "live CD" that will first ask some setup questions (default answers will probably work for you). Then, you will see a display such as the following:

    Rather than take the risk of moving any partitions, we will add a new ext4 partition to hold additional files.
    Note: gparted operates by preparing a "to-do" list of pending partitions before making any change in your disk. This gives you an opportunity to undo any or all of those pending operations before performing any actions that cannot be undone.

    • Select the Unallocated area if that isn't already selected.

    • Under Partition, select New, then select Add. This installs a fresh file system on a newly created partition.

    • Check that the pending operations correctly reflect the changes you want, then click Apply (green check-mark icon).

    • Note the path of the new file system (such as /dev/sda3)

  • Now, shut down your virtual machine and reboot into Ubuntu as usual. Within your VM, you should be able to mount your new file system on a directory of your choice such as the generic mount point /mnt, using a command such as this: 

    $  sudo mount /dev/sda3 /mnt
    Thereafter, you should be able to store and access files on /mnt.




rab@stolaf.edu, April 10, 2022