kernel.md

Kernel handling and develpment

Informations

To show the current kernel version:

uname -r

Modules handling

The installed kernel modules are located int the /lib/modules//build directory.

modinfo [-k <kernel version>] <module>  # info about installed module
insmod <mod name>.ko                    # load a module into the kernel
rmmod <mod name>.ko                     # removed a module from the kernel
modprobe <mod name>                     # load an installed module
lsmod                                   # list all the loaded modules

Log handling

The kernel levels are the following:

ID	Level
0	Emergency
1	Alert
2	Critical
3	Error
4	Warning
5	Notice
6	Informations
7	Debug

dmesg is used to show all kernel messages:

dmesg
dmesg -n <log level>                # turn off all levels under it
echo "N" > /proc/sys/kernel/printk  # same as the command before

To emulate printk logging level, echo can be used:

echo "<N> Message" > /dev/msg       # show messages under N logging level

Create modules

A great guide can be found here.

The coding style can be found here.

Makefile for cross compiling

The kernel modules Makefile can be setup for cross compiling as following.

obj-m += hello.o
SYSROOT :=
KDIR := $(SYSROOT)/usr/src/kernel
ARCH := x86
CROSS_COMPILE := i586-poky-linux-

all: build 

build:
   make -C $(KDIR) M=$(PWD) modules

clean:
   make -C $(KDIR) M=$(PWD) clean

Signing kernel modules

A kernel module signing guide can be found here.

Before signing kernel modules, the /usr/src/kernel directory must be configured with the following command:

sudo make -C /usr/src/kernel scripts

Then it's possible to use a Makefile rule for automatic sign, just in case the kernel module must be loaded manually for tests:

sign:
   env LANG=C perl $(KDIR)/scripts/sign-file sha1 \
                   $(KDIR)/signing_key.priv \
                   $(KDIR)/signing_key.x509 \
                   hello.ko

env LANG=C is used to avoid perl warnings.