153 lines
6.7 KiB
Plaintext
153 lines
6.7 KiB
Plaintext
Early userspace support
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=======================
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Last update: 2004-12-20 tlh
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"Early userspace" is a set of libraries and programs that provide
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various pieces of functionality that are important enough to be
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available while a Linux kernel is coming up, but that don't need to be
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run inside the kernel itself.
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It consists of several major infrastructure components:
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- gen_init_cpio, a program that builds a cpio-format archive
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containing a root filesystem image. This archive is compressed, and
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the compressed image is linked into the kernel image.
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- initramfs, a chunk of code that unpacks the compressed cpio image
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midway through the kernel boot process.
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- klibc, a userspace C library, currently packaged separately, that is
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optimized for correctness and small size.
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The cpio file format used by initramfs is the "newc" (aka "cpio -H newc")
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format, and is documented in the file "buffer-format.txt". There are
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two ways to add an early userspace image: specify an existing cpio
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archive to be used as the image or have the kernel build process build
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the image from specifications.
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CPIO ARCHIVE method
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You can create a cpio archive that contains the early userspace image.
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Your cpio archive should be specified in CONFIG_INITRAMFS_SOURCE and it
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will be used directly. Only a single cpio file may be specified in
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CONFIG_INITRAMFS_SOURCE and directory and file names are not allowed in
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combination with a cpio archive.
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IMAGE BUILDING method
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The kernel build process can also build an early userspace image from
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source parts rather than supplying a cpio archive. This method provides
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a way to create images with root-owned files even though the image was
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built by an unprivileged user.
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The image is specified as one or more sources in
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CONFIG_INITRAMFS_SOURCE. Sources can be either directories or files -
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cpio archives are *not* allowed when building from sources.
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A source directory will have it and all of its contents packaged. The
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specified directory name will be mapped to '/'. When packaging a
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directory, limited user and group ID translation can be performed.
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INITRAMFS_ROOT_UID can be set to a user ID that needs to be mapped to
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user root (0). INITRAMFS_ROOT_GID can be set to a group ID that needs
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to be mapped to group root (0).
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A source file must be directives in the format required by the
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usr/gen_init_cpio utility (run 'usr/gen_init_cpio --help' to get the
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file format). The directives in the file will be passed directly to
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usr/gen_init_cpio.
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When a combination of directories and files are specified then the
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initramfs image will be an aggregate of all of them. In this way a user
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can create a 'root-image' directory and install all files into it.
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Because device-special files cannot be created by a unprivileged user,
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special files can be listed in a 'root-files' file. Both 'root-image'
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and 'root-files' can be listed in CONFIG_INITRAMFS_SOURCE and a complete
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early userspace image can be built by an unprivileged user.
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As a technical note, when directories and files are specified, the
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entire CONFIG_INITRAMFS_SOURCE is passed to
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scripts/gen_initramfs_list.sh. This means that CONFIG_INITRAMFS_SOURCE
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can really be interpreted as any legal argument to
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gen_initramfs_list.sh. If a directory is specified as an argument then
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the contents are scanned, uid/gid translation is performed, and
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usr/gen_init_cpio file directives are output. If a directory is
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specified as an arugemnt to scripts/gen_initramfs_list.sh then the
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contents of the file are simply copied to the output. All of the output
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directives from directory scanning and file contents copying are
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processed by usr/gen_init_cpio.
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See also 'scripts/gen_initramfs_list.sh -h'.
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Where's this all leading?
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=========================
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The klibc distribution contains some of the necessary software to make
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early userspace useful. The klibc distribution is currently
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maintained separately from the kernel, but this may change early in
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the 2.7 era (it missed the boat for 2.5).
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You can obtain somewhat infrequent snapshots of klibc from
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ftp://ftp.kernel.org/pub/linux/libs/klibc/
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For active users, you are better off using the klibc git
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repository, at http://git.kernel.org/?p=libs/klibc/klibc.git
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The standalone klibc distribution currently provides three components,
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in addition to the klibc library:
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- ipconfig, a program that configures network interfaces. It can
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configure them statically, or use DHCP to obtain information
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dynamically (aka "IP autoconfiguration").
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- nfsmount, a program that can mount an NFS filesystem.
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- kinit, the "glue" that uses ipconfig and nfsmount to replace the old
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support for IP autoconfig, mount a filesystem over NFS, and continue
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system boot using that filesystem as root.
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kinit is built as a single statically linked binary to save space.
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Eventually, several more chunks of kernel functionality will hopefully
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move to early userspace:
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- Almost all of init/do_mounts* (the beginning of this is already in
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place)
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- ACPI table parsing
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- Insert unwieldy subsystem that doesn't really need to be in kernel
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space here
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If kinit doesn't meet your current needs and you've got bytes to burn,
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the klibc distribution includes a small Bourne-compatible shell (ash)
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and a number of other utilities, so you can replace kinit and build
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custom initramfs images that meet your needs exactly.
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For questions and help, you can sign up for the early userspace
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mailing list at http://www.zytor.com/mailman/listinfo/klibc
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How does it work?
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=================
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The kernel has currently 3 ways to mount the root filesystem:
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a) all required device and filesystem drivers compiled into the kernel, no
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initrd. init/main.c:init() will call prepare_namespace() to mount the
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final root filesystem, based on the root= option and optional init= to run
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some other init binary than listed at the end of init/main.c:init().
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b) some device and filesystem drivers built as modules and stored in an
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initrd. The initrd must contain a binary '/linuxrc' which is supposed to
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load these driver modules. It is also possible to mount the final root
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filesystem via linuxrc and use the pivot_root syscall. The initrd is
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mounted and executed via prepare_namespace().
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c) using initramfs. The call to prepare_namespace() must be skipped.
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This means that a binary must do all the work. Said binary can be stored
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into initramfs either via modifying usr/gen_init_cpio.c or via the new
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initrd format, an cpio archive. It must be called "/init". This binary
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is responsible to do all the things prepare_namespace() would do.
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To maintain backwards compatibility, the /init binary will only run if it
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comes via an initramfs cpio archive. If this is not the case,
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init/main.c:init() will run prepare_namespace() to mount the final root
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and exec one of the predefined init binaries.
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Bryan O'Sullivan <bos@serpentine.com>
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