102 lines
4.2 KiB
Plaintext
102 lines
4.2 KiB
Plaintext
Early userspace support
|
|
=======================
|
|
|
|
Last update: 2003-08-21
|
|
|
|
|
|
"Early userspace" is a set of libraries and programs that provide
|
|
various pieces of functionality that are important enough to be
|
|
available while a Linux kernel is coming up, but that don't need to be
|
|
run inside the kernel itself.
|
|
|
|
It consists of several major infrastructure components:
|
|
|
|
- gen_init_cpio, a program that builds a cpio-format archive
|
|
containing a root filesystem image. This archive is compressed, and
|
|
the compressed image is linked into the kernel image.
|
|
- initramfs, a chunk of code that unpacks the compressed cpio image
|
|
midway through the kernel boot process.
|
|
- klibc, a userspace C library, currently packaged separately, that is
|
|
optimised for correctness and small size.
|
|
|
|
The cpio file format used by initramfs is the "newc" (aka "cpio -c")
|
|
format, and is documented in the file "buffer-format.txt". If you
|
|
want to generate your own cpio files directly instead of hacking on
|
|
gen_init_cpio, you will need to short-circuit the build process in
|
|
usr/ so that gen_init_cpio does not get run, then simply pop your own
|
|
initramfs_data.cpio.gz file into place.
|
|
|
|
|
|
Where's this all leading?
|
|
=========================
|
|
|
|
The klibc distribution contains some of the necessary software to make
|
|
early userspace useful. The klibc distribution is currently
|
|
maintained separately from the kernel, but this may change early in
|
|
the 2.7 era (it missed the boat for 2.5).
|
|
|
|
You can obtain somewhat infrequent snapshots of klibc from
|
|
ftp://ftp.kernel.org/pub/linux/libs/klibc/
|
|
|
|
For active users, you are better off using the klibc BitKeeper
|
|
repositories, at http://klibc.bkbits.net/
|
|
|
|
The standalone klibc distribution currently provides three components,
|
|
in addition to the klibc library:
|
|
|
|
- ipconfig, a program that configures network interfaces. It can
|
|
configure them statically, or use DHCP to obtain information
|
|
dynamically (aka "IP autoconfiguration").
|
|
- nfsmount, a program that can mount an NFS filesystem.
|
|
- kinit, the "glue" that uses ipconfig and nfsmount to replace the old
|
|
support for IP autoconfig, mount a filesystem over NFS, and continue
|
|
system boot using that filesystem as root.
|
|
|
|
kinit is built as a single statically linked binary to save space.
|
|
|
|
Eventually, several more chunks of kernel functionality will hopefully
|
|
move to early userspace:
|
|
|
|
- Almost all of init/do_mounts* (the beginning of this is already in
|
|
place)
|
|
- ACPI table parsing
|
|
- Insert unwieldy subsystem that doesn't really need to be in kernel
|
|
space here
|
|
|
|
If kinit doesn't meet your current needs and you've got bytes to burn,
|
|
the klibc distribution includes a small Bourne-compatible shell (ash)
|
|
and a number of other utilities, so you can replace kinit and build
|
|
custom initramfs images that meet your needs exactly.
|
|
|
|
For questions and help, you can sign up for the early userspace
|
|
mailing list at http://www.zytor.com/mailman/listinfo/klibc
|
|
|
|
How does it work?
|
|
=================
|
|
|
|
The kernel has currently 3 ways to mount the root filesystem:
|
|
|
|
a) all required device and filesystem drivers compiled into the kernel, no
|
|
initrd. init/main.c:init() will call prepare_namespace() to mount the
|
|
final root filesystem, based on the root= option and optional init= to run
|
|
some other init binary than listed at the end of init/main.c:init().
|
|
|
|
b) some device and filesystem drivers built as modules and stored in an
|
|
initrd. The initrd must contain a binary '/linuxrc' which is supposed to
|
|
load these driver modules. It is also possible to mount the final root
|
|
filesystem via linuxrc and use the pivot_root syscall. The initrd is
|
|
mounted and executed via prepare_namespace().
|
|
|
|
c) using initramfs. The call to prepare_namespace() must be skipped.
|
|
This means that a binary must do all the work. Said binary can be stored
|
|
into initramfs either via modifying usr/gen_init_cpio.c or via the new
|
|
initrd format, an cpio archive. It must be called "/init". This binary
|
|
is responsible to do all the things prepare_namespace() would do.
|
|
|
|
To remain backwards compatibility, the /init binary will only run if it
|
|
comes via an initramfs cpio archive. If this is not the case,
|
|
init/main.c:init() will run prepare_namespace() to mount the final root
|
|
and exec one of the predefined init binaries.
|
|
|
|
Bryan O'Sullivan <bos@serpentine.com>
|