About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / filesystems / cramfs.txt




Custom Search

Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	
2		Cramfs - cram a filesystem onto a small ROM
3	
4	cramfs is designed to be simple and small, and to compress things well. 
5	
6	It uses the zlib routines to compress a file one page at a time, and
7	allows random page access.  The meta-data is not compressed, but is
8	expressed in a very terse representation to make it use much less
9	diskspace than traditional filesystems. 
10	
11	You can't write to a cramfs filesystem (making it compressible and
12	compact also makes it _very_ hard to update on-the-fly), so you have to
13	create the disk image with the "mkcramfs" utility.
14	
15	
16	Usage Notes
17	-----------
18	
19	File sizes are limited to less than 16MB.
20	
21	Maximum filesystem size is a little over 256MB.  (The last file on the
22	filesystem is allowed to extend past 256MB.)
23	
24	Only the low 8 bits of gid are stored.  The current version of
25	mkcramfs simply truncates to 8 bits, which is a potential security
26	issue.
27	
28	Hard links are supported, but hard linked files
29	will still have a link count of 1 in the cramfs image.
30	
31	Cramfs directories have no `.' or `..' entries.  Directories (like
32	every other file on cramfs) always have a link count of 1.  (There's
33	no need to use -noleaf in `find', btw.)
34	
35	No timestamps are stored in a cramfs, so these default to the epoch
36	(1970 GMT).  Recently-accessed files may have updated timestamps, but
37	the update lasts only as long as the inode is cached in memory, after
38	which the timestamp reverts to 1970, i.e. moves backwards in time.
39	
40	Currently, cramfs must be written and read with architectures of the
41	same endianness, and can be read only by kernels with PAGE_SIZE
42	== 4096.  At least the latter of these is a bug, but it hasn't been
43	decided what the best fix is.  For the moment if you have larger pages
44	you can just change the #define in mkcramfs.c, so long as you don't
45	mind the filesystem becoming unreadable to future kernels.
46	
47	
48	Memory Mapped cramfs image
49	--------------------------
50	
51	The CRAMFS_MTD Kconfig option adds support for loading data directly from
52	a physical linear memory range (usually non volatile memory like Flash)
53	instead of going through the block device layer. This saves some memory
54	since no intermediate buffering is necessary to hold the data before
55	decompressing.
56	
57	And when data blocks are kept uncompressed and properly aligned, they will
58	automatically be mapped directly into user space whenever possible providing
59	eXecute-In-Place (XIP) from ROM of read-only segments. Data segments mapped
60	read-write (hence they have to be copied to RAM) may still be compressed in
61	the cramfs image in the same file along with non compressed read-only
62	segments. Both MMU and no-MMU systems are supported. This is particularly
63	handy for tiny embedded systems with very tight memory constraints.
64	
65	The location of the cramfs image in memory is system dependent. You must
66	know the proper physical address where the cramfs image is located and
67	configure an MTD device for it. Also, that MTD device must be supported
68	by a map driver that implements the "point" method. Examples of such
69	MTD drivers are cfi_cmdset_0001 (Intel/Sharp CFI flash) or physmap
70	(Flash device in physical memory map). MTD partitions based on such devices
71	are fine too. Then that device should be specified with the "mtd:" prefix
72	as the mount device argument. For example, to mount the MTD device named
73	"fs_partition" on the /mnt directory:
74	
75	$ mount -t cramfs mtd:fs_partition /mnt
76	
77	To boot a kernel with this as root filesystem, suffice to specify
78	something like "root=mtd:fs_partition" on the kernel command line.
79	
80	
81	Tools
82	-----
83	
84	A version of mkcramfs that can take advantage of the latest capabilities
85	described above can be found here:
86	
87	https://github.com/npitre/cramfs-tools
88	
89	
90	For /usr/share/magic
91	--------------------
92	
93	0	ulelong	0x28cd3d45	Linux cramfs offset 0
94	>4	ulelong	x		size %d
95	>8	ulelong	x		flags 0x%x
96	>12	ulelong	x		future 0x%x
97	>16	string	>\0		signature "%.16s"
98	>32	ulelong	x		fsid.crc 0x%x
99	>36	ulelong	x		fsid.edition %d
100	>40	ulelong	x		fsid.blocks %d
101	>44	ulelong	x		fsid.files %d
102	>48	string	>\0		name "%.16s"
103	512	ulelong	0x28cd3d45	Linux cramfs offset 512
104	>516	ulelong	x		size %d
105	>520	ulelong	x		flags 0x%x
106	>524	ulelong	x		future 0x%x
107	>528	string	>\0		signature "%.16s"
108	>544	ulelong	x		fsid.crc 0x%x
109	>548	ulelong	x		fsid.edition %d
110	>552	ulelong	x		fsid.blocks %d
111	>556	ulelong	x		fsid.files %d
112	>560	string	>\0		name "%.16s"
113	
114	
115	Hacker Notes
116	------------
117	
118	See fs/cramfs/README for filesystem layout and implementation notes.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.