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