mirror of https://gitee.com/openkylin/linux.git
77 lines
2.5 KiB
Plaintext
77 lines
2.5 KiB
Plaintext
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Cramfs - cram a filesystem onto a small ROM
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cramfs is designed to be simple and small, and to compress things well.
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It uses the zlib routines to compress a file one page at a time, and
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allows random page access. The meta-data is not compressed, but is
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expressed in a very terse representation to make it use much less
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diskspace than traditional filesystems.
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You can't write to a cramfs filesystem (making it compressible and
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compact also makes it _very_ hard to update on-the-fly), so you have to
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create the disk image with the "mkcramfs" utility.
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Usage Notes
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-----------
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File sizes are limited to less than 16MB.
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Maximum filesystem size is a little over 256MB. (The last file on the
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filesystem is allowed to extend past 256MB.)
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Only the low 8 bits of gid are stored. The current version of
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mkcramfs simply truncates to 8 bits, which is a potential security
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issue.
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Hard links are supported, but hard linked files
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will still have a link count of 1 in the cramfs image.
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Cramfs directories have no `.' or `..' entries. Directories (like
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every other file on cramfs) always have a link count of 1. (There's
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no need to use -noleaf in `find', btw.)
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No timestamps are stored in a cramfs, so these default to the epoch
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(1970 GMT). Recently-accessed files may have updated timestamps, but
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the update lasts only as long as the inode is cached in memory, after
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which the timestamp reverts to 1970, i.e. moves backwards in time.
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Currently, cramfs must be written and read with architectures of the
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same endianness, and can be read only by kernels with PAGE_CACHE_SIZE
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== 4096. At least the latter of these is a bug, but it hasn't been
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decided what the best fix is. For the moment if you have larger pages
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you can just change the #define in mkcramfs.c, so long as you don't
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mind the filesystem becoming unreadable to future kernels.
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For /usr/share/magic
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--------------------
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0 ulelong 0x28cd3d45 Linux cramfs offset 0
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>4 ulelong x size %d
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>8 ulelong x flags 0x%x
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>12 ulelong x future 0x%x
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>16 string >\0 signature "%.16s"
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>32 ulelong x fsid.crc 0x%x
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>36 ulelong x fsid.edition %d
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>40 ulelong x fsid.blocks %d
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>44 ulelong x fsid.files %d
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>48 string >\0 name "%.16s"
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512 ulelong 0x28cd3d45 Linux cramfs offset 512
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>516 ulelong x size %d
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>520 ulelong x flags 0x%x
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>524 ulelong x future 0x%x
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>528 string >\0 signature "%.16s"
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>544 ulelong x fsid.crc 0x%x
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>548 ulelong x fsid.edition %d
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>552 ulelong x fsid.blocks %d
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>556 ulelong x fsid.files %d
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>560 string >\0 name "%.16s"
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Hacker Notes
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------------
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See fs/cramfs/README for filesystem layout and implementation notes.
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