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About
*****
SquashFS is a highly compressed, read only file system often used as a root fs
on embedded devices, live systems or simply as a compressed archive format.
Think of it as a .tar.gz that you can mount (or XZ, LZO, LZ4, ZSTD).
The filesystem itself and the user space tooling were originally developed by
Phillip Lougher with third party contributions that have accumulated over time.
Unfortunately, the original user space tooling does not support a lot of
standard use cases, the source code of the tools is in a pretty deteriorated
state and apparently no longer maintained.
This package contains the source code of a complete rewrite of the user space
tools that attempt to address many of the problems of the old tools:
- Reproducible SquashFS images, i.e. deterministic packing without
any local time stamps.
- Linux `gen_init_cpio` like file listing for micro managing the
file system contents, permissions, and ownership without having to replicate
the file system (and especially permissions) locally.
- Support for SELinux contexts file (see selabel_file(5)) to generate
SELinux labels.
- Structured and (hopefully) more readable source code that should be better
maintainable in the long run.
In addition to that, tools have been added to directly convert a tar archive
into a SquashFS filesystem image and back. This allows for using existing
tools can work on tar archives seamlessly on SquashFS images.
The tools in this package have different names, so they can be installed
together with the existing tools:
- `gensquashfs` can be used to produce SquashFS images from `gen_init_cpio`
like file listings or simply pack an input directory.
- `rdsquashfs` can be used to inspect and unpack SquashFS images.
- `sqfs2tar` can turn a SquashFS image into a tar ball, written to stdout.
- `tar2sqfs` can turn a tar ball (read from stdin) into a SquashFS image.
Limitations
***********
At the moment, the following things still require some work:
- more testing (extensive unit testing for libsqfs still needed)
- extended attributes
- gensquashfs supports SELinux labeling only
- rdsquashfs and sqfs2tar ignore them entirely
- entire key value blocks are deduplicated, but individual value
deduplication ("out of line" storage) isn't implemented.
- hard links (not implemented; tar2sqfs turns them into symlinks)
- File deduplication (not implemented; do we even need this?)
- NFS export tables (not implemented yet)
Future plans
************
In addition to the above, the following things would be really nice to
have eventually:
- A tool for merging multiple images into one
- A tool for splitting an image
- A diff tool
- Diff of the directory tree of two images
- Diff of the file meta data in two images
- File level diffs
- Combinations of the above in a still human readable form
- [IN PROGRESS] A *complete* specification of the on-disk format and all the
arbitrary checks enforced by the kernel.
- Patching kernel and user space to support SquashFS on top of UBI
- Patching kernel and user space to support ACLs
Copyright & License
*******************
The source code in this package has been written by me, David Oberhollenzer,
in 2019 and is released under the terms and conditions of the GNU General
Public License version 3 or later.
To the best of my knowledge, no code has been copied over from the original
SquashFS tools. The kernel documentation, the kernel headers and this web site
have been used as main sources for understanding SquashFS:
https://dr-emann.github.io/squashfs/
Some additional information (such as xattr implementation) has been gathered
from various mailing lists and other web sources.
Compressor implementations are primarily based on the documentation of the
compression libraries.
The existing unsquashfs tool and kernel implementation were used for trial and
error testing during development.
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