From 849369d6c66d3054688672f97d31fceb8e8230fb Mon Sep 17 00:00:00 2001 From: root Date: Fri, 25 Dec 2015 04:40:36 +0000 Subject: initial_commit --- Documentation/filesystems/cramfs.txt | 76 ++++++++++++++++++++++++++++++++++++ 1 file changed, 76 insertions(+) create mode 100644 Documentation/filesystems/cramfs.txt (limited to 'Documentation/filesystems/cramfs.txt') diff --git a/Documentation/filesystems/cramfs.txt b/Documentation/filesystems/cramfs.txt new file mode 100644 index 00000000..31f53f0a --- /dev/null +++ b/Documentation/filesystems/cramfs.txt @@ -0,0 +1,76 @@ + + 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. -- cgit v1.2.3