Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 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_CACHE_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 For /usr/share/magic 49 -------------------- 50 51 0 ulelong 0x28cd3d45 Linux cramfs offset 0 52 >4 ulelong x size %d 53 >8 ulelong x flags 0x%x 54 >12 ulelong x future 0x%x 55 >16 string >\0 signature "%.16s" 56 >32 ulelong x fsid.crc 0x%x 57 >36 ulelong x fsid.edition %d 58 >40 ulelong x fsid.blocks %d 59 >44 ulelong x fsid.files %d 60 >48 string >\0 name "%.16s" 61 512 ulelong 0x28cd3d45 Linux cramfs offset 512 62 >516 ulelong x size %d 63 >520 ulelong x flags 0x%x 64 >524 ulelong x future 0x%x 65 >528 string >\0 signature "%.16s" 66 >544 ulelong x fsid.crc 0x%x 67 >548 ulelong x fsid.edition %d 68 >552 ulelong x fsid.blocks %d 69 >556 ulelong x fsid.files %d 70 >560 string >\0 name "%.16s" 71 72 73 Hacker Notes 74 ------------ 75 76 See fs/cramfs/README for filesystem layout and implementation notes.