Unverified Commit 418556fa authored by David Disseldorp's avatar David Disseldorp Committed by Christian Brauner
Browse files

docs: initramfs: update compression and mtime descriptions 



Update the document to reflect that initramfs didn't replace initrd
following kernel 2.5.x.
The initramfs buffer format now supports many compression types in
addition to gzip, so include them in the grammar section.
c_mtime use is dependent on CONFIG_INITRAMFS_PRESERVE_MTIME.

Signed-off-by: default avatarDavid Disseldorp <ddiss@suse.de>
Link: https://lore.kernel.org/r/20250402033949.852-2-ddiss@suse.de


Reviewed-by: default avatarRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 0af2f6be

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

+21 −13
Original line number Diff line number Diff line
@@ -4,20 +4,18 @@ 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

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 format of

the initramfs buffer format.

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).  One valid version of an

initramfs buffer is thus a single .cpio.gz file.

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::
@@ -25,12 +23,20 @@ grammar, where:: 
	*	is used to indicate "0 or more occurrences of"

	(|)	indicates alternatives

	+	indicates concatenation

	GZIP()	indicates the gzip(1) of the operand

	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_gzip_archive)*

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



	cpio_gzip_archive := GZIP(cpio_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)
@@ -75,6 +81,8 @@ c_chksum 8 bytes Checksum of data field if c_magic is 070702; 
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.