120 lines
4.7 KiB
ReStructuredText
120 lines
4.7 KiB
ReStructuredText
=======================
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initramfs buffer format
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=======================
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Al Viro, H. Peter Anvin
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Last revision: 2002-01-13
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Starting with kernel 2.5.x, the old "initial ramdisk" protocol is
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getting {replaced/complemented} with the new "initial ramfs"
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(initramfs) protocol. The initramfs contents is passed using the same
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memory buffer protocol used by the initrd protocol, but the contents
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is different. The initramfs buffer contains an archive which is
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expanded into a ramfs filesystem; this document details the format of
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the initramfs buffer format.
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The initramfs buffer format is based around the "newc" or "crc" CPIO
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formats, and can be created with the cpio(1) utility. The cpio
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archive can be compressed using gzip(1). One valid version of an
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initramfs buffer is thus a single .cpio.gz file.
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The full format of the initramfs buffer is defined by the following
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grammar, where::
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* is used to indicate "0 or more occurrences of"
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(|) indicates alternatives
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+ indicates concatenation
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GZIP() indicates the gzip(1) of the operand
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ALGN(n) means padding with null bytes to an n-byte boundary
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initramfs := ("\0" | cpio_archive | cpio_gzip_archive)*
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cpio_gzip_archive := GZIP(cpio_archive)
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cpio_archive := cpio_file* + (<nothing> | cpio_trailer)
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cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data
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cpio_trailer := ALGN(4) + cpio_header + "TRAILER!!!\0" + ALGN(4)
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In human terms, the initramfs buffer contains a collection of
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compressed and/or uncompressed cpio archives (in the "newc" or "crc"
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formats); arbitrary amounts zero bytes (for padding) can be added
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between members.
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The cpio "TRAILER!!!" entry (cpio end-of-archive) is optional, but is
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not ignored; see "handling of hard links" below.
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The structure of the cpio_header is as follows (all fields contain
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hexadecimal ASCII numbers fully padded with '0' on the left to the
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full width of the field, for example, the integer 4780 is represented
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by the ASCII string "000012ac"):
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============= ================== ==============================================
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Field name Field size Meaning
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============= ================== ==============================================
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c_magic 6 bytes The string "070701" or "070702"
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c_ino 8 bytes File inode number
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c_mode 8 bytes File mode and permissions
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c_uid 8 bytes File uid
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c_gid 8 bytes File gid
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c_nlink 8 bytes Number of links
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c_mtime 8 bytes Modification time
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c_filesize 8 bytes Size of data field
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c_maj 8 bytes Major part of file device number
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c_min 8 bytes Minor part of file device number
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c_rmaj 8 bytes Major part of device node reference
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c_rmin 8 bytes Minor part of device node reference
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c_namesize 8 bytes Length of filename, including final \0
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c_chksum 8 bytes Checksum of data field if c_magic is 070702;
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otherwise zero
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============= ================== ==============================================
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The c_mode field matches the contents of st_mode returned by stat(2)
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on Linux, and encodes the file type and file permissions.
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The c_filesize should be zero for any file which is not a regular file
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or symlink.
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The c_chksum field contains a simple 32-bit unsigned sum of all the
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bytes in the data field. cpio(1) refers to this as "crc", which is
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clearly incorrect (a cyclic redundancy check is a different and
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significantly stronger integrity check), however, this is the
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algorithm used.
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If the filename is "TRAILER!!!" this is actually an end-of-archive
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marker; the c_filesize for an end-of-archive marker must be zero.
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Handling of hard links
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======================
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When a nondirectory with c_nlink > 1 is seen, the (c_maj,c_min,c_ino)
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tuple is looked up in a tuple buffer. If not found, it is entered in
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the tuple buffer and the entry is created as usual; if found, a hard
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link rather than a second copy of the file is created. It is not
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necessary (but permitted) to include a second copy of the file
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contents; if the file contents is not included, the c_filesize field
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should be set to zero to indicate no data section follows. If data is
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present, the previous instance of the file is overwritten; this allows
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the data-carrying instance of a file to occur anywhere in the sequence
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(GNU cpio is reported to attach the data to the last instance of a
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file only.)
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c_filesize must not be zero for a symlink.
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When a "TRAILER!!!" end-of-archive marker is seen, the tuple buffer is
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reset. This permits archives which are generated independently to be
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concatenated.
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To combine file data from different sources (without having to
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regenerate the (c_maj,c_min,c_ino) fields), therefore, either one of
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the following techniques can be used:
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a) Separate the different file data sources with a "TRAILER!!!"
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end-of-archive marker, or
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b) Make sure c_nlink == 1 for all nondirectory entries.
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