| 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. |