| 1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677 |
- 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 supported, but hard linked files
- will still have a link count of 1 in the cramfs image.
- 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.
- Currently, cramfs must be written and read with architectures of the
- same endianness, and can be read only by kernels with PAGE_SIZE
- == 4096. At least the latter of these is a bug, but it hasn't been
- decided what the best fix is. For the moment if you have larger pages
- you can just change the #define in mkcramfs.c, so long as you don't
- mind the filesystem becoming unreadable to future kernels.
- For /usr/share/magic
- --------------------
- 0 ulelong 0x28cd3d45 Linux cramfs offset 0
- >4 ulelong x size %d
- >8 ulelong x flags 0x%x
- >12 ulelong x future 0x%x
- >16 string >\0 signature "%.16s"
- >32 ulelong x fsid.crc 0x%x
- >36 ulelong x fsid.edition %d
- >40 ulelong x fsid.blocks %d
- >44 ulelong x fsid.files %d
- >48 string >\0 name "%.16s"
- 512 ulelong 0x28cd3d45 Linux cramfs offset 512
- >516 ulelong x size %d
- >520 ulelong x flags 0x%x
- >524 ulelong x future 0x%x
- >528 string >\0 signature "%.16s"
- >544 ulelong x fsid.crc 0x%x
- >548 ulelong x fsid.edition %d
- >552 ulelong x fsid.blocks %d
- >556 ulelong x fsid.files %d
- >560 string >\0 name "%.16s"
- Hacker Notes
- ------------
- See fs/cramfs/README for filesystem layout and implementation notes.
|
