path: root/README.md

xloop - eXtended loop device
The eXtended loop (xloop) device provides a subsystem for the Linux kernel to register custom file format drivers. Those file format drivers enable the possibility to write and read file formats in the kernel space rather than in the user space.
This repository contains the source code for the xloop Linux kernel modules

xloop: eXtended loop device with file format subsystem
xloop_file_format_raw: file format subsystem driver for RAW files
xloop_file_format_qcow: file format subsystem driver for QCOW files (read only)

and the user space utility xlosetup to configure xloop devices. The xloop Linux kernel modules can be built for the following Linux kernel versions and Linux distributions:

Archlinux with Linux kernel 6.1.x, 5.15.x or 5.10.x
Raspberry Pi OS with Linux kernel 5.4.x
Ubuntu 20.04 with Linux kernel 6.1.x, 5.15.x, 5.10.x or 5.4.x
Ubuntu 18.04 with Linux kernel 4.19.x
CentOS 8 with Linux kernel 4.18.x
AlmaLinux 8 with Linux kernel 4.18.x

Build
Preliminaries
A build of the xloop Linux kernel modules and the xlosetup utility requires the installation of the following build tools and libraries under your supported Linux distribution.
Archlinux with Linux kernel 5.15.x or 5.10.x
pacman -S git \
          make \
          cmake \
          gcc \
          clang \
          linux-headers \  # or linux-lts-headers
          libcap \  # required only in Debug build configuration
          dpkg \
          rpm-tools


Raspberry Pi OS with Linux kernel 5.4.x
apt-get install git \
                make \
                cmake \
                gcc \
                clang-format \
                raspberrypi-kernel-headers \
                libcap-dev \  # required only in Debug build configuration
                rpm


Ubuntu 20.04 with Linux kernel 5.4.x
apt-get install git \
                make \
                cmake \
                gcc \
                clang-format \
                linux-headers-generic \
                libcap-dev \  # required only in Debug build configuration
                rpm


Ubuntu 18.04 with Linux kernel 4.19.x
apt-get install git \
                make \
                cmake \
                gcc \
                clang-format \
                linux-headers-generic \
                libcap-dev \  # required only in Debug build configuration
                rpm


CentOS 8 with Linux kernel 4.18.x
yum install git \
            make \
            cmake \
            gcc \
            clang-tools-extra \
            kernel-devel \
            elfutils-libelf-devel \
            libcap-devel \  # required only in Debug build configuration
            rpm-build


AlmaLinux 8 with Linux kernel 4.18.x
yum install git \
            make \
            cmake \
            gcc \
            clang-tools-extra \
            kernel-devel \
            elfutils-libelf-devel \
            libcap-devel \  # required only in Debug build configuration
            rpm-build


Preparation
Before a build takes place, you should create a build directory inside the root folder of the repository. After that, change your working directory to that new directory as follows:
mkdir build
cd build


Configuration
A build of the xloop Linux kernel modules and the xlosetup utility can be configured and customized by the following configuration variables (CMake cache entries):



Variable
Type
Values
Default value
Description




CMAKE_BUILD_TYPE
STRING
{Debug, Release}
Debug
Build configuration of the xloop project.


KERNEL_BUILD_DIR
PATH
{a .. z, A .. Z, /, _, -}
/lib/modules/uname -r/build
Path to Linux kernel modules to compile against.


KERNEL_INSTALL_DIR
PATH
{a .. z, A .. Z, /, _, -}
/lib/modules/uname -r/extra
Path to install Linux kernel modules.


KERNEL_SCRIPTS_DIR
PATH
{a .. z, A .. Z, /, _, -}
/lib/modules/uname -r/build/scripts
Path to Linux kernel scripts directory.


XLOOP_MAJOR
NUMBER
{0 .. 255}
120
Major number for xloop devices.


XLOOP_CTRL_MINOR
NUMBER
{0 .. 255}
15
Minor number for the xloop-control device.


BLK_DEV_XLOOP_MIN_COUNT
NUMBER
{0 .. 255}
8
Number of xloop devices to pre-create at init time.



A value from the range of appropriate values can be assigend to each configuration variable by executing CMake once with the following command pattern:
cmake -D<VARIABLE>=<VALUE> [-D ...] ../.


Debug
In the Debug build configuration, all Linux kernel modules and the utility can be built by calling make:
make


Optionally, the output files can be installed with superuser permissions on the local system using the Makefile target install:
sudo make install
sudo depmod -a


Packages
In the Release build configuration, installation packages can be built by calling the make target package:
make package


This target creates a Debian installation package (*.deb), a RPM installation package (*.rpm) and a compressed archive (*.tar.gz) containing the built xloop Linux kernel modules and the xlosetup utility executable as well as its man page and bash-completion support.
Sources
In the Release build configuration, sources can be built by calling the make target source:
make source


This target creates compressed archives (*_source.tar.gz and *_source.zip) containing the source code of this repository for code distribution purposes.
Debugging
Debugging of the Linux kernel modules and the user space utility requires this project to be built in the Debug configuration.
Linux kernel modules
The Linux kernel modules xloop, xloop_file_fmt_raw and xloop_file_fmt_qcow support the Linux kernel's dynamic debug feature if the Linux kernel is built with the enabled kernel configuration CONFIG_DYNAMIC_DEBUG. The dynamic debug feature allows the printing of customizable debug messages into the Linux kernel's message buffer.
Dynamic debug for the modules can be either enabled at module initialization or during operation. At module initialization, dynamic debug can be enabled by modprobe using the "fake" module parameter dyndbg:
modprobe xloop dyndbg=+pflmt
modprobe xloop_file_fmt_raw dyndbg=+pflmt
modprobe xloop_file_fmt_qcow dyndbg=+pflmt


The module parameter dyndbg customizes the debug messages written into the Linux kernel's message buffer. The specific value +pflmt enables all debug messages in the source code and includes function name (f), line number (l), module name (m) and thread ID (t) for each executed debug statement from the source code.
During operation, debug messages from debug statements in the code can be customized and enabled dynamically as well using the debugfs control file <DEBUG_FS>/dynamic_debug/control where DEBUG_FS is the mount point of a mounted DebugFS, eg. /sys/kernel/debug:
echo "module xloop +pflmt" > <DEBUG_FS>/dynamic_debug/control
echo "module xloop_file_fmt_raw +pflmt" > <DEBUG_FS>/dynamic_debug/control
echo "module xloop_file_fmt_qcow +pflmt" > <DEBUG_FS>/dynamic_debug/control


More information regarding the Linux kernel's dynamic debug feature can be found in the Linux kernel documentation.
User space utility
Built-in debug messages from the user space utility xlosetup can be enabled by setting the following environment variables before any execution of xlosetup:
export XLOOPDEV_DEBUG=all
export LIBSMARTCOLS_DEBUG=all


Development notes
Code style of source code files
The code style of the source code files can be checked by calling the make target lint:
make lint


If some source code files do not meet the project's code style, they can be fixed automatically by calling the make target lint-fix:
make lint-fix