Topics - NIX Kernel
Linux: the choice of a GNU generation -email@example.com (more like) “Compiling the colonel sanders” - my SO, wisdom
- The material below was cobbled together for personal use, from attributed sources, and endured some mild look/feel massage.
- Document Purpose: Conveniently scoped refresher on the listed Linux material.
- LPIC2 Prep - Linux Academy
- Photo by Pierre Châtel-Innocenti on Unsplash
The Linux Kernel
- Utilize kernel components that are necessary to specific hardware, hardware drivers, system resources and requirements.
- Implement different types of kernel images, identifying stable and development kernels and patches, as well as using kernel modules.
- Focus on standard way of doing things.
- Centos - 2.x.x kernel tree
- Debian - 3.x kernel tree
201.1 Kernel Components
Kernel Source and Documentation
- Weighting of 2 on LPIC2, so only expect a couple questions.
- Kernel source is typically not installed by default
- Challenge - each distro has special additional steps that differs from others. Centos
uname -ato see current kernel
yum install kernel-devel. When this completes, a folder for current kernel in this dir.
- LPIC2 Exam - requires general steps for building a kernel
- 2.x and older, docs separate. 3.x+ docs come with.
usr/src/linux/documentation, or possibly (centos)
- Often online doc will point you to a file in here. Debian
apt install source linux-image-$(uname -r)
- Boot directory
- vmlinuz is the actual kernel itself
- Z-image (compressed image of your kernel)
- Low memory sized image to support older systems with 512kb limit
- On modern distros (2.0+)
- “BZ image type”, aka “Big Z Image”. High-memory image.
- Since 2.6, uses better compressing bzip2
- 4.0+ xz compression. XZ is better compression ratio, but slower. Uses more resources.
201.2 Compiling Kernel
Prep - Dependencies
- Group Install
yum groupinstall "Development Tools
- ncurses-devel, qt-devel, etc
Your going to need a bigger boat for all the devel libraries needed (99-ish)
apt install make gcc lib-ncurses5-dev
apt install build-dep linux-image-$(uname-r)
- make distclean, make mrproper, make clean
- General information for gcc on how to build software (not specific to a kernel bld)
- Contains two kinds of information, information for entire env (flags and env vars), and make with targets
- ie make clean
- targets determine behavior of make action (based on passed in arg, ie make clean)
make cleanFirst thing we need to do is create a configuration file There are a dizzying list of options
make menuconfigis a nicer (ncurses) way of answering those option questions.
- Reads a number of different scripts, and current config for kernel
- When we installed kernel, we got a config file
- We will pick the things we want installed and not installed
- Go into sections..
- ..anything with:
- star - built in
- blank - excluded, not present loadable or otherwise. Just means it isn’t built into K.
- Doesn’t mean we can’t use it later. LKM
" is a module,"< >" Module capable
- There are other options, but menuconfig is the most reasonable.
(2) Walkthrough, editing .config via make menuconfig
- You could drop floppy driver support
- Block devices
- “Normal floppy disk support” - change from “M” to excluded.
- You can go through and make a large number of changes. For a general desktop/kernel, you’ll never have a reason.
- Write file and exit.
- Takes hours
- Make sure you have a couple gig of disk space available
- Start by making large image.
make bzImage. Mainline kernel based on chosen options.
- Runs a while (say 10s of minutes), builds kernel and kernel modules that we need.
- First kernel image completes. Then you build k modules.
make modules, from in src dir. Runs a while (say 1s of hours).
(4) make modules_install
- Completes, see output lines prefaced by “INSTALL”
(5) Relocate completed binary
- Move, use standard naming convention
mv bZimage /boot/vmlinuz-3.xx.x-xx.x86_64
- Need to create initramfs for boot
- dracut utility
mkinitrd /boot/initrd-3.xx.x-xx.img 3.xx.x-xx(example you’d do in normal compile, and is what LPIC2 looks for)
- Debian special -
mkinitramfs -o /boot/initrd-3.xx.x-xx.img 3.xx.x-xx
- Last, we would deal with GRUB
201.3 Kernel Runtime Mgmt and Troubleshooting
Weighted 2x as heavily as kernel building Unlike modules built-in during compile, there are loaded in as needed to supplement the base kernel.
- They are not loaded in and taking up memory when the K starts.
- Only when there is a device or a reason for them to exist there.
- Called LKM, loadable kernel modules
- Review how to load, configure, add, remove
- Manage and/or query a 2.6.x or 3.x kernel and its loadable modules.
- Identify and correct common boot and run time issues.
- Understand device detection and management using udev, including troubleshooting udev rules.
Popular command, can give info about the kernel
rmay@dev-vm:~/Projects/dojo$ uname -a Linux dev-vm 4.15.0-43-generic #46-Ubuntu SMP Thu Dec 6 14:45:28 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
- SMP - kernel designed for multi-core procs
- kernel name, uname -r
- machine name, hardware type, platform (m,p,i)
- .ko, kernel objects
kernel/, broken out by type
- Different levels in each dir
- Higher level, more general driver
- These are all drivers we can load via LKM.
depmod and modules.dep
- You might need to copy an already compiled kernel module to your system.
- Those typically have an installation program that moves associated files.
- There might a possibility that you just have object itself.
- If all dependency libraries are present, you just need to drop the file.
- In order to use it, you need to be aware of a command and couple of files.
- Those tell the system what other things it might depend on, and map out how that KO fits in to device driver hierarchy.
- Utility is called depmod, also handles map files. Resulting output is called modules.dep.
- Run anywhere is smart enough to find needed things.
- See modules map files:
rmay@dev-vm:~/Projects/dojo$ ls /lib/modules/4.15.0-43-generic/ build initrd kernel modules.alias modules.alias.bin modules.builtin modules.builtin.bin ...
- If you copy a module into here, you’ll need to regenerate the maps
depmodLooks for KOs and sees where they fit.
cat modules.depHas a bunch of info. Listing of all detected modules on systems
- Delimted by a colon, two fields.
- First col - K module
- Second col -dependencies, separated.
- If blank, no deps.
- Map files are further definitions, of what K modules and devices are mapped to what types of information.
- Example, .usbmap has references to interfaces, drivers
- TLDR: A way for the K to know, if something is plugged in, how to locate & load the appropriate device driver.
Listing, Adding, Removing Loadable Kernel Modules (KM)
Review - Loadable modules extend the kernel as needed
- When something is there, it should be loaded, when something is not there, unloaded.
- There are instances where you might need:
- ..to manually load a KM for testing.
- .. you installed it and want to make sure it is installed correctly (missing dependencies errors)
- .. remove it b/c it is installed, but you want to free up the memory
LS Mod command
- Module, Memory footprint in bytes, Used by
- Good for determining how safe it would be to remove a module.
lsmod | grep lp
rmmod- see man. Also could use
modprobe -r. Never recommend using force flag. Boot into safe mode and disable.
- Two ways to re-add: Old way(hard), New way(easy).
- Hard way - insmod
- Hard b/c we need to know exact module and path. Use modinfo.
- Modinfo. Defaults to running kernel, no need for a path.
- Use output for insmod. Ex:
modinfo lptells us ‘filename’ path for lp mod.
- Use that in insmod.
- Easy way - modprobe
modprobe lpIf it succeeded, completes silently.
- modinfo tells us params, like with lp you can
modprobe lp reset=1
Other Module Changes of Interest Alias Force install/load (like if a dep could not be detected) Remove even if it has a device that would normally cause it to load Blacklist, if wanted to override its use on system at reboot
rmay@dev-vm:~/$ ls /etc/modprobe.d/ alsa-base.conf blacklist-ath_pci.conf blacklist-firewire.conf blacklist-modem.conf blackli.. amd64-microcode-blacklist.conf blacklist.conf blacklist-framebuffer.conf blacklist-oss.conf intel-mi..
- See aliases, installs, removes
- Ex: blacklist Nuvo driver to allow nvidia proprietary driver to load.
Viewing and Changing Kernel Parameters
/proc/sys and sysctl
The /proc/sys filesystem is not really a fs, it is a memory based fs that is mounted under /proc.
- Remember everything in Linux is treated as a file.
- So we have K module settings (residing in memory) that we can monkey with as a file.
rmay@dev-vm:/$ cat /proc/sys/dev/parport/default/spintime 500
- echo 550 into spintime.
- Reboot clears if there is a problem. So this is temporary.
rmay@dev-vm:/$ sudo sysctl -a | grep parport dev.parport.default.spintime = 550 dev.parport.default.timeslice = 200
sysctl - Set via
sysctl dev.parport.default.spintime = 560
Defaults come from compiled in defaults or sysctl.conf. Edit
/etc/sysctl.conf to make a persistent change.
- Best practice for editing, append to bottom and comment section as user overrides.
Displaying information about system hardware
Lots of utilities for this.
There are a number of devices that are intrinsic to system. Could be a port on the motherboard.
- On system bus, constantly available.
lspci. Bridges (ie PCI bridge), hubs, controller (sata controller), etc.
- On virtual machine, this is weird.
- port/slot/etc, device name, other stuff
- has a stepped verbose modes (-vvv)
- Useful for finding kernel modules associated with devices on system.
Returns Device, DMA, IRQ, IO Ports, scraped from /proc fs Great if you are developing device drivers Not present on old kernels
USB info, gives manufacter and device id, for online lookup.
Device Filesystem udev
/dev filesystem, devices as files.
- In the past, this contained devices attached, and devices that could be attached (Linux system knew about them).
- The udev filesystem is a daemon that probes devices at boot. After boot, the D will add the nessessary files to filesystem.
- Where the files exist/are contained
- How to make changes
- Higher number rules can override lower number rules
- Rule files contain anything that can be passed for/to the loadable module.
UDEVADM Monitors UDEV activity