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