Documentation/filesystems/cramfs.rst

Source file repositories/reference/linux-study-clean/Documentation/filesystems/cramfs.rst

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Documentation/filesystems/cramfs.rst
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Support Tooling And Documentation
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Support Tooling And Documentation: documentation
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Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

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.. SPDX-License-Identifier: GPL-2.0

===========================================
Cramfs - cram a filesystem onto a small ROM
===========================================

cramfs is designed to be simple and small, and to compress things well.

It uses the zlib routines to compress a file one page at a time, and
allows random page access.  The meta-data is not compressed, but is
expressed in a very terse representation to make it use much less
diskspace than traditional filesystems.

You can't write to a cramfs filesystem (making it compressible and
compact also makes it _very_ hard to update on-the-fly), so you have to
create the disk image with the "mkcramfs" utility.


Usage Notes
-----------

File sizes are limited to less than 16MB.

Maximum filesystem size is a little over 256MB.  (The last file on the
filesystem is allowed to extend past 256MB.)

Only the low 8 bits of gid are stored.  The current version of
mkcramfs simply truncates to 8 bits, which is a potential security
issue.

Hard links are not preserved.  mkcramfs deduplicates files with
identical content, but two names for the same on-disk inode in the
source tree become two separate (content-shared) entries in the
image, and cramfs always reports a link count of 1.

Cramfs directories have no ``.`` or ``..`` entries.  Directories (like
every other file on cramfs) always have a link count of 1.  (There's
no need to use -noleaf in ``find``, btw.)

No timestamps are stored in a cramfs, so these default to the epoch
(1970 GMT).  Recently-accessed files may have updated timestamps, but
the update lasts only as long as the inode is cached in memory, after
which the timestamp reverts to 1970, i.e. moves backwards in time.

The on-disk layout is host-endian: the kernel does not swab, and
refuses to mount an image whose endianness does not match the CPU.
For cross-builds, mkcramfs -B / -L forces the output endianness so
that a host of one endianness can produce an image for a target of
the other.

The on-disk block size is fixed at 4096 bytes.  On systems with a
larger PAGE_SIZE you can change the #define in mkcramfs.c, with the
caveat that the resulting image will only be readable on kernels
configured for the same PAGE_SIZE.


Memory Mapped cramfs image
--------------------------

The CRAMFS_MTD Kconfig option adds support for loading data directly from
a physical linear memory range (usually non volatile memory like Flash)
instead of going through the block device layer. This saves some memory
since no intermediate buffering is necessary to hold the data before
decompressing.

And when data blocks are kept uncompressed and properly aligned, they will
automatically be mapped directly into user space whenever possible providing
eXecute-In-Place (XIP) from ROM of read-only segments. Data segments mapped
read-write (hence they have to be copied to RAM) may still be compressed in
the cramfs image in the same file along with non compressed read-only

Annotation

Implementation Notes