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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.