| |
| 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_CACHE_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. |
