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diff --git a/Documentation/early-userspace/early_userspace_support.rst b/Documentation/early-userspace/early_userspace_support.rst new file mode 100644 index 000000000000..3deefb34046b --- /dev/null +++ b/Documentation/early-userspace/early_userspace_support.rst @@ -0,0 +1,154 @@ +======================= +Early userspace support +======================= + +Last update: 2004-12-20 tlh + + +"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 + optimized for correctness and small size. + +The cpio file format used by initramfs is the "newc" (aka "cpio -H newc") +format, and is documented in the file "buffer-format.txt". There are +two ways to add an early userspace image: specify an existing cpio +archive to be used as the image or have the kernel build process build +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 +will be used directly. Only a single cpio file may be specified in +CONFIG_INITRAMFS_SOURCE and directory and file names are not allowed in +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 +a way to create images with root-owned files even though the image was +built by an unprivileged user. + +The image is specified as one or more sources in +CONFIG_INITRAMFS_SOURCE. Sources can be either directories or files - +cpio archives are *not* allowed when building from sources. + +A source directory will have it and all of its contents packaged. The +specified directory name will be mapped to '/'. When packaging a +directory, limited user and group ID translation can be performed. +INITRAMFS_ROOT_UID can be set to a user ID that needs to be mapped to +user root (0). INITRAMFS_ROOT_GID can be set to a group ID that needs +to be mapped to group root (0). + +A source file must be directives in the format required by the +usr/gen_init_cpio utility (run 'usr/gen_init_cpio -h' to get the +file format). The directives in the file will be passed directly to +usr/gen_init_cpio. + +When a combination of directories and files are specified then the +initramfs image will be an aggregate of all of them. In this way a user +can create a 'root-image' directory and install all files into it. +Because device-special files cannot be created by a unprivileged user, +special files can be listed in a 'root-files' file. Both 'root-image' +and 'root-files' can be listed in CONFIG_INITRAMFS_SOURCE and a complete +early userspace image can be built by an unprivileged user. + +As a technical note, when directories and files are specified, the +entire CONFIG_INITRAMFS_SOURCE is passed to +usr/gen_initramfs_list.sh. This means that CONFIG_INITRAMFS_SOURCE +can really be interpreted as any legal argument to +gen_initramfs_list.sh. If a directory is specified as an argument then +the contents are scanned, uid/gid translation is performed, and +usr/gen_init_cpio file directives are output. If a directory is +specified as an argument to usr/gen_initramfs_list.sh then the +contents of the file are simply copied to the output. All of the output +directives from directory scanning and file contents copying are +processed by usr/gen_init_cpio. + +See also 'usr/gen_initramfs_list.sh -h'. + +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. + +You can obtain somewhat infrequent snapshots of klibc from +https://www.kernel.org/pub/linux/libs/klibc/ + +For active users, you are better off using the klibc git +repository, at http://git.kernel.org/?p=libs/klibc/klibc.git + +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 maintain 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> |