1 2 Cramfs - cram a filesystem onto a small ROM 3 4cramfs is designed to be simple and small, and to compress things well. 5 6It uses the zlib routines to compress a file one page at a time, and 7allows random page access. The meta-data is not compressed, but is 8expressed in a very terse representation to make it use much less 9diskspace than traditional filesystems. 10 11You can't write to a cramfs filesystem (making it compressible and 12compact also makes it _very_ hard to update on-the-fly), so you have to 13create the disk image with the "mkcramfs" utility. 14 15 16Usage Notes 17----------- 18 19File sizes are limited to less than 16MB. 20 21Maximum filesystem size is a little over 256MB. (The last file on the 22filesystem is allowed to extend past 256MB.) 23 24Only the low 8 bits of gid are stored. The current version of 25mkcramfs simply truncates to 8 bits, which is a potential security 26issue. 27 28Hard links are supported, but hard linked files 29will still have a link count of 1 in the cramfs image. 30 31Cramfs directories have no `.' or `..' entries. Directories (like 32every other file on cramfs) always have a link count of 1. (There's 33no need to use -noleaf in `find', btw.) 34 35No timestamps are stored in a cramfs, so these default to the epoch 36(1970 GMT). Recently-accessed files may have updated timestamps, but 37the update lasts only as long as the inode is cached in memory, after 38which the timestamp reverts to 1970, i.e. moves backwards in time. 39 40Currently, cramfs must be written and read with architectures of the 41same 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 43decided what the best fix is. For the moment if you have larger pages 44you can just change the #define in mkcramfs.c, so long as you don't 45mind the filesystem becoming unreadable to future kernels. 46 47 48Memory Mapped cramfs image 49-------------------------- 50 51The CRAMFS_MTD Kconfig option adds support for loading data directly from 52a physical linear memory range (usually non volatile memory like Flash) 53instead of going through the block device layer. This saves some memory 54since no intermediate buffering is necessary to hold the data before 55decompressing. 56 57And when data blocks are kept uncompressed and properly aligned, they will 58automatically be mapped directly into user space whenever possible providing 59eXecute-In-Place (XIP) from ROM of read-only segments. Data segments mapped 60read-write (hence they have to be copied to RAM) may still be compressed in 61the cramfs image in the same file along with non compressed read-only 62segments. Both MMU and no-MMU systems are supported. This is particularly 63handy for tiny embedded systems with very tight memory constraints. 64 65The location of the cramfs image in memory is system dependent. You must 66know the proper physical address where the cramfs image is located and 67configure an MTD device for it. Also, that MTD device must be supported 68by a map driver that implements the "point" method. Examples of such 69MTD drivers are cfi_cmdset_0001 (Intel/Sharp CFI flash) or physmap 70(Flash device in physical memory map). MTD partitions based on such devices 71are fine too. Then that device should be specified with the "mtd:" prefix 72as 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 77To boot a kernel with this as root filesystem, suffice to specify 78something like "root=mtd:fs_partition" on the kernel command line. 79 80 81Tools 82----- 83 84A version of mkcramfs that can take advantage of the latest capabilities 85described above can be found here: 86 87https://github.com/npitre/cramfs-tools 88 89 90For /usr/share/magic 91-------------------- 92 930 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" 103512 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 115Hacker Notes 116------------ 117 118See fs/cramfs/README for filesystem layout and implementation notes. 119
