Documentation/driver-api/early-userspace/buffer-format.rst

=======================
initramfs buffer format
=======================

Al Viro, H. Peter Anvin

With kernel 2.5.x, the old "initial ramdisk" protocol was complemented
with an "initial ramfs" protocol.  The initramfs content is passed
using the same memory buffer protocol used by initrd, but the content
is different.  The initramfs buffer contains an archive which is
expanded into a ramfs filesystem; this document details 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), or any other algorithm provided
via CONFIG_DECOMPRESS_*.  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 gzip compression of the operand
	BZIP2()	indicates bzip2 compression of the operand
	LZMA()	indicates lzma compression of the operand
	XZ()	indicates xz compression of the operand
	LZO()	indicates lzo compression of the operand
	LZ4()	indicates lz4 compression of the operand
	ZSTD()	indicates zstd compression of the operand
	ALGN(n)	means padding with null bytes to an n-byte boundary

	initramfs := ("\0" | cpio_archive | cpio_compressed_archive)*

	cpio_compressed_archive := (GZIP(cpio_archive) | BZIP2(cpio_archive)
		| LZMA(cpio_archive) | XZ(cpio_archive) | LZO(cpio_archive)
		| LZ4(cpio_archive) | ZSTD(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.

c_mtime is ignored unless CONFIG_INITRAMFS_PRESERVE_MTIME=y is set.

The c_filesize should be zero for any file which is not a regular file
or symlink.

c_namesize may account for more than one trailing '\0', as long as the
value doesn't exceed PATH_MAX.  This can be useful for ensuring that a
subsequent file data segment is aligned, e.g. to a filesystem block
boundary.

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
concatenated.

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.