124 lines
4.6 KiB
ReStructuredText
124 lines
4.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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===========================================
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Cramfs - cram a filesystem onto a small ROM
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===========================================
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cramfs is designed to be simple and small, and to compress things well.
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It uses the zlib routines to compress a file one page at a time, and
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allows random page access. The meta-data is not compressed, but is
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expressed in a very terse representation to make it use much less
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diskspace than traditional filesystems.
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You can't write to a cramfs filesystem (making it compressible and
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compact also makes it _very_ hard to update on-the-fly), so you have to
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create the disk image with the "mkcramfs" utility.
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Usage Notes
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-----------
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File sizes are limited to less than 16MB.
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Maximum filesystem size is a little over 256MB. (The last file on the
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filesystem is allowed to extend past 256MB.)
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Only the low 8 bits of gid are stored. The current version of
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mkcramfs simply truncates to 8 bits, which is a potential security
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issue.
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Hard links are supported, but hard linked files
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will still have a link count of 1 in the cramfs image.
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Cramfs directories have no ``.`` or ``..`` entries. Directories (like
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every other file on cramfs) always have a link count of 1. (There's
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no need to use -noleaf in ``find``, btw.)
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No timestamps are stored in a cramfs, so these default to the epoch
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(1970 GMT). Recently-accessed files may have updated timestamps, but
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the update lasts only as long as the inode is cached in memory, after
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which the timestamp reverts to 1970, i.e. moves backwards in time.
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Currently, cramfs must be written and read with architectures of the
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same endianness, and can be read only by kernels with PAGE_SIZE
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== 4096. At least the latter of these is a bug, but it hasn't been
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decided what the best fix is. For the moment if you have larger pages
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you can just change the #define in mkcramfs.c, so long as you don't
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mind the filesystem becoming unreadable to future kernels.
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Memory Mapped cramfs image
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--------------------------
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The CRAMFS_MTD Kconfig option adds support for loading data directly from
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a physical linear memory range (usually non volatile memory like Flash)
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instead of going through the block device layer. This saves some memory
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since no intermediate buffering is necessary to hold the data before
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decompressing.
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And when data blocks are kept uncompressed and properly aligned, they will
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automatically be mapped directly into user space whenever possible providing
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eXecute-In-Place (XIP) from ROM of read-only segments. Data segments mapped
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read-write (hence they have to be copied to RAM) may still be compressed in
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the cramfs image in the same file along with non compressed read-only
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segments. Both MMU and no-MMU systems are supported. This is particularly
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handy for tiny embedded systems with very tight memory constraints.
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The location of the cramfs image in memory is system dependent. You must
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know the proper physical address where the cramfs image is located and
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configure an MTD device for it. Also, that MTD device must be supported
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by a map driver that implements the "point" method. Examples of such
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MTD drivers are cfi_cmdset_0001 (Intel/Sharp CFI flash) or physmap
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(Flash device in physical memory map). MTD partitions based on such devices
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are fine too. Then that device should be specified with the "mtd:" prefix
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as the mount device argument. For example, to mount the MTD device named
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"fs_partition" on the /mnt directory::
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$ mount -t cramfs mtd:fs_partition /mnt
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To boot a kernel with this as root filesystem, suffice to specify
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something like "root=mtd:fs_partition" on the kernel command line.
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Tools
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-----
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A version of mkcramfs that can take advantage of the latest capabilities
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described above can be found here:
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https://github.com/npitre/cramfs-tools
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For /usr/share/magic
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--------------------
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===== ======================= =======================
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0 ulelong 0x28cd3d45 Linux cramfs offset 0
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>4 ulelong x size %d
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>8 ulelong x flags 0x%x
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>12 ulelong x future 0x%x
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>16 string >\0 signature "%.16s"
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>32 ulelong x fsid.crc 0x%x
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>36 ulelong x fsid.edition %d
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>40 ulelong x fsid.blocks %d
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>44 ulelong x fsid.files %d
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>48 string >\0 name "%.16s"
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512 ulelong 0x28cd3d45 Linux cramfs offset 512
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>516 ulelong x size %d
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>520 ulelong x flags 0x%x
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>524 ulelong x future 0x%x
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>528 string >\0 signature "%.16s"
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>544 ulelong x fsid.crc 0x%x
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>548 ulelong x fsid.edition %d
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>552 ulelong x fsid.blocks %d
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>556 ulelong x fsid.files %d
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>560 string >\0 name "%.16s"
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===== ======================= =======================
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Hacker Notes
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------------
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See fs/cramfs/README for filesystem layout and implementation notes.
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