1Early userspace support 2======================= 3 4Last update: 2004-12-20 tlh 5 6 7"Early userspace" is a set of libraries and programs that provide 8various pieces of functionality that are important enough to be 9available while a Linux kernel is coming up, but that don't need to be 10run inside the kernel itself. 11 12It consists of several major infrastructure components: 13 14- gen_init_cpio, a program that builds a cpio-format archive 15 containing a root filesystem image. This archive is compressed, and 16 the compressed image is linked into the kernel image. 17- initramfs, a chunk of code that unpacks the compressed cpio image 18 midway through the kernel boot process. 19- klibc, a userspace C library, currently packaged separately, that is 20 optimized for correctness and small size. 21 22The cpio file format used by initramfs is the "newc" (aka "cpio -H newc") 23format, and is documented in the file "buffer-format.txt". There are 24two ways to add an early userspace image: specify an existing cpio 25archive to be used as the image or have the kernel build process build 26the image from specifications. 27 28CPIO ARCHIVE method 29 30You can create a cpio archive that contains the early userspace image. 31Your cpio archive should be specified in CONFIG_INITRAMFS_SOURCE and it 32will be used directly. Only a single cpio file may be specified in 33CONFIG_INITRAMFS_SOURCE and directory and file names are not allowed in 34combination with a cpio archive. 35 36IMAGE BUILDING method 37 38The kernel build process can also build an early userspace image from 39source parts rather than supplying a cpio archive. This method provides 40a way to create images with root-owned files even though the image was 41built by an unprivileged user. 42 43The image is specified as one or more sources in 44CONFIG_INITRAMFS_SOURCE. Sources can be either directories or files - 45cpio archives are *not* allowed when building from sources. 46 47A source directory will have it and all of its contents packaged. The 48specified directory name will be mapped to '/'. When packaging a 49directory, limited user and group ID translation can be performed. 50INITRAMFS_ROOT_UID can be set to a user ID that needs to be mapped to 51user root (0). INITRAMFS_ROOT_GID can be set to a group ID that needs 52to be mapped to group root (0). 53 54A source file must be directives in the format required by the 55usr/gen_init_cpio utility (run 'usr/gen_init_cpio --help' to get the 56file format). The directives in the file will be passed directly to 57usr/gen_init_cpio. 58 59When a combination of directories and files are specified then the 60initramfs image will be an aggregate of all of them. In this way a user 61can create a 'root-image' directory and install all files into it. 62Because device-special files cannot be created by a unprivileged user, 63special files can be listed in a 'root-files' file. Both 'root-image' 64and 'root-files' can be listed in CONFIG_INITRAMFS_SOURCE and a complete 65early userspace image can be built by an unprivileged user. 66 67As a technical note, when directories and files are specified, the 68entire CONFIG_INITRAMFS_SOURCE is passed to 69scripts/gen_initramfs_list.sh. This means that CONFIG_INITRAMFS_SOURCE 70can really be interpreted as any legal argument to 71gen_initramfs_list.sh. If a directory is specified as an argument then 72the contents are scanned, uid/gid translation is performed, and 73usr/gen_init_cpio file directives are output. If a directory is 74specified as an arugemnt to scripts/gen_initramfs_list.sh then the 75contents of the file are simply copied to the output. All of the output 76directives from directory scanning and file contents copying are 77processed by usr/gen_init_cpio. 78 79See also 'scripts/gen_initramfs_list.sh -h'. 80 81Where's this all leading? 82========================= 83 84The klibc distribution contains some of the necessary software to make 85early userspace useful. The klibc distribution is currently 86maintained separately from the kernel, but this may change early in 87the 2.7 era (it missed the boat for 2.5). 88 89You can obtain somewhat infrequent snapshots of klibc from 90ftp://ftp.kernel.org/pub/linux/libs/klibc/ 91 92For active users, you are better off using the klibc git 93repository, at http://git.kernel.org/?p=libs/klibc/klibc.git 94 95The standalone klibc distribution currently provides three components, 96in addition to the klibc library: 97 98- ipconfig, a program that configures network interfaces. It can 99 configure them statically, or use DHCP to obtain information 100 dynamically (aka "IP autoconfiguration"). 101- nfsmount, a program that can mount an NFS filesystem. 102- kinit, the "glue" that uses ipconfig and nfsmount to replace the old 103 support for IP autoconfig, mount a filesystem over NFS, and continue 104 system boot using that filesystem as root. 105 106kinit is built as a single statically linked binary to save space. 107 108Eventually, several more chunks of kernel functionality will hopefully 109move to early userspace: 110 111- Almost all of init/do_mounts* (the beginning of this is already in 112 place) 113- ACPI table parsing 114- Insert unwieldy subsystem that doesn't really need to be in kernel 115 space here 116 117If kinit doesn't meet your current needs and you've got bytes to burn, 118the klibc distribution includes a small Bourne-compatible shell (ash) 119and a number of other utilities, so you can replace kinit and build 120custom initramfs images that meet your needs exactly. 121 122For questions and help, you can sign up for the early userspace 123mailing list at http://www.zytor.com/mailman/listinfo/klibc 124 125How does it work? 126================= 127 128The kernel has currently 3 ways to mount the root filesystem: 129 130a) all required device and filesystem drivers compiled into the kernel, no 131 initrd. init/main.c:init() will call prepare_namespace() to mount the 132 final root filesystem, based on the root= option and optional init= to run 133 some other init binary than listed at the end of init/main.c:init(). 134 135b) some device and filesystem drivers built as modules and stored in an 136 initrd. The initrd must contain a binary '/linuxrc' which is supposed to 137 load these driver modules. It is also possible to mount the final root 138 filesystem via linuxrc and use the pivot_root syscall. The initrd is 139 mounted and executed via prepare_namespace(). 140 141c) using initramfs. The call to prepare_namespace() must be skipped. 142 This means that a binary must do all the work. Said binary can be stored 143 into initramfs either via modifying usr/gen_init_cpio.c or via the new 144 initrd format, an cpio archive. It must be called "/init". This binary 145 is responsible to do all the things prepare_namespace() would do. 146 147 To maintain backwards compatibility, the /init binary will only run if it 148 comes via an initramfs cpio archive. If this is not the case, 149 init/main.c:init() will run prepare_namespace() to mount the final root 150 and exec one of the predefined init binaries. 151 152Bryan O'Sullivan <bos@serpentine.com> 153