linux/Documentation/early-userspace/buffer-format.txt
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   1                       initramfs buffer format
   2                       -----------------------
   3
   4                       Al Viro, H. Peter Anvin
   5                      Last revision: 2002-01-13
   6
   7Starting with kernel 2.5.x, the old "initial ramdisk" protocol is
   8getting {replaced/complemented} with the new "initial ramfs"
   9(initramfs) protocol.  The initramfs contents is passed using the same
  10memory buffer protocol used by the initrd protocol, but the contents
  11is different.  The initramfs buffer contains an archive which is
  12expanded into a ramfs filesystem; this document details the format of
  13the initramfs buffer format.
  14
  15The initramfs buffer format is based around the "newc" or "crc" CPIO
  16formats, and can be created with the cpio(1) utility.  The cpio
  17archive can be compressed using gzip(1).  One valid version of an
  18initramfs buffer is thus a single .cpio.gz file.
  19
  20The full format of the initramfs buffer is defined by the following
  21grammar, where:
  22        *       is used to indicate "0 or more occurrences of"
  23        (|)     indicates alternatives
  24        +       indicates concatenation
  25        GZIP()  indicates the gzip(1) of the operand
  26        ALGN(n) means padding with null bytes to an n-byte boundary
  27
  28        initramfs  := ("\0" | cpio_archive | cpio_gzip_archive)*
  29
  30        cpio_gzip_archive := GZIP(cpio_archive)
  31
  32        cpio_archive := cpio_file* + (<nothing> | cpio_trailer)
  33
  34        cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data
  35
  36        cpio_trailer := ALGN(4) + cpio_header + "TRAILER!!!\0" + ALGN(4)
  37
  38
  39In human terms, the initramfs buffer contains a collection of
  40compressed and/or uncompressed cpio archives (in the "newc" or "crc"
  41formats); arbitrary amounts zero bytes (for padding) can be added
  42between members.
  43
  44The cpio "TRAILER!!!" entry (cpio end-of-archive) is optional, but is
  45not ignored; see "handling of hard links" below.
  46
  47The structure of the cpio_header is as follows (all fields contain
  48hexadecimal ASCII numbers fully padded with '0' on the left to the
  49full width of the field, for example, the integer 4780 is represented
  50by the ASCII string "000012ac"):
  51
  52Field name    Field size         Meaning
  53c_magic       6 bytes            The string "070701" or "070702"
  54c_ino         8 bytes            File inode number
  55c_mode        8 bytes            File mode and permissions
  56c_uid         8 bytes            File uid
  57c_gid         8 bytes            File gid
  58c_nlink       8 bytes            Number of links
  59c_mtime       8 bytes            Modification time
  60c_filesize    8 bytes            Size of data field
  61c_maj         8 bytes            Major part of file device number
  62c_min         8 bytes            Minor part of file device number
  63c_rmaj        8 bytes            Major part of device node reference
  64c_rmin        8 bytes            Minor part of device node reference
  65c_namesize    8 bytes            Length of filename, including final \0
  66c_chksum      8 bytes            Checksum of data field if c_magic is 070702;
  67                                 otherwise zero
  68
  69The c_mode field matches the contents of st_mode returned by stat(2)
  70on Linux, and encodes the file type and file permissions.
  71
  72The c_filesize should be zero for any file which is not a regular file
  73or symlink.
  74
  75The c_chksum field contains a simple 32-bit unsigned sum of all the
  76bytes in the data field.  cpio(1) refers to this as "crc", which is
  77clearly incorrect (a cyclic redundancy check is a different and
  78significantly stronger integrity check), however, this is the
  79algorithm used.
  80
  81If the filename is "TRAILER!!!" this is actually an end-of-archive
  82marker; the c_filesize for an end-of-archive marker must be zero.
  83
  84
  85*** Handling of hard links
  86
  87When a nondirectory with c_nlink > 1 is seen, the (c_maj,c_min,c_ino)
  88tuple is looked up in a tuple buffer.  If not found, it is entered in
  89the tuple buffer and the entry is created as usual; if found, a hard
  90link rather than a second copy of the file is created.  It is not
  91necessary (but permitted) to include a second copy of the file
  92contents; if the file contents is not included, the c_filesize field
  93should be set to zero to indicate no data section follows.  If data is
  94present, the previous instance of the file is overwritten; this allows
  95the data-carrying instance of a file to occur anywhere in the sequence
  96(GNU cpio is reported to attach the data to the last instance of a
  97file only.)
  98
  99c_filesize must not be zero for a symlink.
 100
 101When a "TRAILER!!!" end-of-archive marker is seen, the tuple buffer is
 102reset.  This permits archives which are generated independently to be
 103concatenated.
 104
 105To combine file data from different sources (without having to
 106regenerate the (c_maj,c_min,c_ino) fields), therefore, either one of
 107the following techniques can be used:
 108
 109a) Separate the different file data sources with a "TRAILER!!!"
 110   end-of-archive marker, or
 111
 112b) Make sure c_nlink == 1 for all nondirectory entries.
 113
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