- initramfs buffer format
- -----------------------
+=======================
+initramfs buffer format
+=======================
- Al Viro, H. Peter Anvin
- Last revision: 2002-01-13
+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 buffer is thus a single .cpio.gz file.
The full format of the initramfs buffer is defined by the following
-grammar, where:
+grammar, where::
+
* is used to indicate "0 or more occurrences of"
(|) indicates alternatives
+ indicates concatenation
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_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.
marker; the c_filesize for an end-of-archive marker must be zero.
-*** Handling of hard links
+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
+=======================
Early userspace support
=======================
the image from specifications.
CPIO ARCHIVE method
+-------------------
You can create a cpio archive that contains the early userspace image.
Your cpio archive should be specified in CONFIG_INITRAMFS_SOURCE and it
combination with a cpio archive.
IMAGE BUILDING method
+---------------------
The kernel build process can also build an early userspace image from
source parts rather than supplying a cpio archive. This method provides
- The old initrd file was a gzipped filesystem image (in some file format,
such as ext2, that needed a driver built into the kernel), while the new
initramfs archive is a gzipped cpio archive (like tar only simpler,
- see cpio(1) and Documentation/early-userspace/buffer-format.txt). The
+ see cpio(1) and Documentation/early-userspace/buffer-format.rst). The
kernel's cpio extraction code is not only extremely small, it's also
__init text and data that can be discarded during the boot process.
set permissions or create device nodes in the new archive. (Note that those
two example "file" entries expect to find files named "init.sh" and "busybox" in
a directory called "initramfs", under the linux-2.6.* directory. See
-Documentation/early-userspace/README for more details.)
+Documentation/early-userspace/early_userspace_support.rst for more details.)
The kernel does not depend on external cpio tools. If you specify a
directory instead of a configuration file, the kernel's build infrastructure
When multiple directories and files are specified then the
initramfs image will be the aggregate of all of them.
- See <file:Documentation/early-userspace/README> for more details.
+ See <file:Documentation/early-userspace/early_userspace_support.rst> for more details.
If you are not sure, leave it blank.