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Lectures
Lecture 06 Template
06 - Intro to Linux
Outline
Kernel Development
Configuration
Build Process
Announcements
Reading: MOS 10.2, LDD 1
Kernel Development
After obtaining kernel source (see last lecture).
4 steps for basic development:
Configure kernel features
- a lot of code for different hardware, don't enable all of it
- `.config` file to enable/disable feature flags
make menuconfig
sudo make INSTALL_MOD_STRIP=1 modules_install
sudo make install
make menuconfig
make -j $(nproc)
/lib/modules
-
- compile with max possible cores:
- modules need to be compiled for specific kernel versions
- modules installed separately in
-
-
-
- very long wait (~1.5 hr 4 cores; 5.5 hrs 8 cores emulated)
Compile kernel
Install kernel modules (as root)
Install kernel (as root)
A `.config` file is needed in the kernel directory.
Steps:
Easier to copy the config file for the current
running kernel
config files have thousands of non-obvious
feature flags. It's easier to configure via TUI:
- Each config file for corresponding built
kernels are available in `/boot`
- Select based on current running version
Kernel Module Development
Quiz 1 will be on Friday, first 10 min of class.
10 MCQ online, no cheat sheet.
Don't be late!
PA 2 released, but later portions require updates.
You can still start the first 2 parts after this
lecture.
1)
1)
2)
2)
3)
4)
Copy current kernel config
cp /boot/config-$(uname -r) .config
make menuconfig
make -j $(nproc) all
sudo make INSTALL_MOD_STRIP=1 modules_install
sudo make install
sudo update-grub
/boot
-
-
-
- wait hours...
-
- copies compiled kernel and config to
- select new kernel from GRUB menu
- must reboot to switch kernels or if kernel crashes
-
-
Modify kernel config
Compile kernel
Install kernel modules (as root)
Install kernel (as root)
Update bootloader (distro-dependent)
Reboot with new kernel
1)
2)
4)
3)
5)
6)
7)
Basic configurations are available in `arch/`,
but these are missing a lot of details.
ls arch/x86/configs/
ls /boot/config-*
uname -a
Kernel Module Development
Working with Kernel Modules
Kernel Module Example
Dynamically loadable modules (kernel extensions)
Kernel modules run in kernel-mode.
They can touch everything.
Often used for:
Kernel modules are version specific.
Statically compiled kernel modules
- loaded manually, requires root permissions
- don't need to recompile entire kernel
-> faster kernel-mode development
- "linux kernel module" usually refers to this
- crashes can crash the whole kernel
- potential security risk
- device drivers
- file system drivers
- networking protocols (except TCP/IP)
- compiled as part of kernel
- often necessary for kernel function
Code
Development:
We'll do an example "hello world" module.
To build it, we need to create a Makefile to invoke
the kernel build system, since the module needs to
be linked against a specific kernel version.
In action:
Kernel Module Commands
- loads kernel module by path
- unloads kernel module by name
- lists loaded modules (/proc/modules)
- print info about a kernel module
- description, version, parameters,
authors, etc.
- advanced module loading
- modules from kernel sources are at
/lib/modules/<kversion>/
- handles module dependencies
(we probably won't be using it)
- generates dependency list at
/lib/modules/<kversion>/modules.dep
- remove with `-r` flag
insmod <module_path>
rmmod <module_name>
lsmod
modinfo <module_path>
modprobe <module_name>
Requires Linux kernel to link against
Drastically shortens development cycle
- compiled kernel source tree
- header files and libraries
- quickly load/unload, no reboot required
- no need to recompile huge portions of kernel
Compile
Load
Use
Unload
1)
2)
4)
3)
5)
Available Kernel Functions
Modules are linked against the full kernel
Kernel symbol table available at:
C standard library is not generally available
- kernel's printf equivalent (same function args)
- messages must start with a
- does not print to stdout, but to kernel log
- filtered by log level
- don't forget `\n`, otherwise output buffered.
Kernel Module Libraries
printk
Kernel modules can expose symbols for other
modules to use module functionality.
- KERN_EMERG
- KERN_ALERT
- KERN_CIRT
- KERN_ERR
- KERN_WARNING
- KERN_NOTICE
- KERN_INFO
- KERN_DEBUG
- EXPORT_SYMBOL(name);
- EXPORT_SYMBOL_GPL(name);
- all kernel functions in-scope, including
- global variables
- exported functions
- similar functions available in helper libraries
- /proc/kallsyms
- /boot/System.map-*
int printk(const char *fmt, ...)
log level