1======================= 2Early userspace support 3======================= 4 5Last update: 2004-12-20 tlh 6 7 8"Early userspace" is a set of libraries and programs that provide 9various pieces of functionality that are important enough to be 10available while a Linux kernel is coming up, but that don't need to be 11run inside the kernel itself. 12 13It consists of several major infrastructure components: 14 15- gen_init_cpio, a program that builds a cpio-format archive 16 containing a root filesystem image. This archive is compressed, and 17 the compressed image is linked into the kernel image. 18- initramfs, a chunk of code that unpacks the compressed cpio image 19 midway through the kernel boot process. 20- klibc, a userspace C library, currently packaged separately, that is 21 optimized for correctness and small size. 22 23The cpio file format used by initramfs is the "newc" (aka "cpio -H newc") 24format, and is documented in the file "buffer-format.txt". There are 25two ways to add an early userspace image: specify an existing cpio 26archive to be used as the image or have the kernel build process build 27the image from specifications. 28 29CPIO ARCHIVE method 30------------------- 31 32You can create a cpio archive that contains the early userspace image. 33Your cpio archive should be specified in CONFIG_INITRAMFS_SOURCE and it 34will be used directly. Only a single cpio file may be specified in 35CONFIG_INITRAMFS_SOURCE and directory and file names are not allowed in 36combination with a cpio archive. 37 38IMAGE BUILDING method 39--------------------- 40 41The kernel build process can also build an early userspace image from 42source parts rather than supplying a cpio archive. This method provides 43a way to create images with root-owned files even though the image was 44built by an unprivileged user. 45 46The image is specified as one or more sources in 47CONFIG_INITRAMFS_SOURCE. Sources can be either directories or files - 48cpio archives are *not* allowed when building from sources. 49 50A source directory will have it and all of its contents packaged. The 51specified directory name will be mapped to '/'. When packaging a 52directory, limited user and group ID translation can be performed. 53INITRAMFS_ROOT_UID can be set to a user ID that needs to be mapped to 54user root (0). INITRAMFS_ROOT_GID can be set to a group ID that needs 55to be mapped to group root (0). 56 57A source file must be directives in the format required by the 58usr/gen_init_cpio utility (run 'usr/gen_init_cpio -h' to get the 59file format). The directives in the file will be passed directly to 60usr/gen_init_cpio. 61 62When a combination of directories and files are specified then the 63initramfs image will be an aggregate of all of them. In this way a user 64can create a 'root-image' directory and install all files into it. 65Because device-special files cannot be created by a unprivileged user, 66special files can be listed in a 'root-files' file. Both 'root-image' 67and 'root-files' can be listed in CONFIG_INITRAMFS_SOURCE and a complete 68early userspace image can be built by an unprivileged user. 69 70As a technical note, when directories and files are specified, the 71entire CONFIG_INITRAMFS_SOURCE is passed to 72usr/gen_initramfs.sh. This means that CONFIG_INITRAMFS_SOURCE 73can really be interpreted as any legal argument to 74gen_initramfs.sh. If a directory is specified as an argument then 75the contents are scanned, uid/gid translation is performed, and 76usr/gen_init_cpio file directives are output. If a directory is 77specified as an argument to usr/gen_initramfs.sh then the 78contents of the file are simply copied to the output. All of the output 79directives from directory scanning and file contents copying are 80processed by usr/gen_init_cpio. 81 82See also 'usr/gen_initramfs.sh -h'. 83 84Where's this all leading? 85========================= 86 87The klibc distribution contains some of the necessary software to make 88early userspace useful. The klibc distribution is currently 89maintained separately from the kernel. 90 91You can obtain somewhat infrequent snapshots of klibc from 92https://www.kernel.org/pub/linux/libs/klibc/ 93 94For active users, you are better off using the klibc git 95repository, at https://git.kernel.org/?p=libs/klibc/klibc.git 96 97The standalone klibc distribution currently provides three components, 98in addition to the klibc library: 99 100- ipconfig, a program that configures network interfaces. It can 101 configure them statically, or use DHCP to obtain information 102 dynamically (aka "IP autoconfiguration"). 103- nfsmount, a program that can mount an NFS filesystem. 104- kinit, the "glue" that uses ipconfig and nfsmount to replace the old 105 support for IP autoconfig, mount a filesystem over NFS, and continue 106 system boot using that filesystem as root. 107 108kinit is built as a single statically linked binary to save space. 109 110Eventually, several more chunks of kernel functionality will hopefully 111move to early userspace: 112 113- Almost all of init/do_mounts* (the beginning of this is already in 114 place) 115- ACPI table parsing 116- Insert unwieldy subsystem that doesn't really need to be in kernel 117 space here 118 119If kinit doesn't meet your current needs and you've got bytes to burn, 120the klibc distribution includes a small Bourne-compatible shell (ash) 121and a number of other utilities, so you can replace kinit and build 122custom initramfs images that meet your needs exactly. 123 124For questions and help, you can sign up for the early userspace 125mailing list at https://www.zytor.com/mailman/listinfo/klibc 126 127How does it work? 128================= 129 130The kernel has currently 3 ways to mount the root filesystem: 131 132a) all required device and filesystem drivers compiled into the kernel, no 133 initrd. init/main.c:init() will call prepare_namespace() to mount the 134 final root filesystem, based on the root= option and optional init= to run 135 some other init binary than listed at the end of init/main.c:init(). 136 137b) some device and filesystem drivers built as modules and stored in an 138 initrd. The initrd must contain a binary '/linuxrc' which is supposed to 139 load these driver modules. It is also possible to mount the final root 140 filesystem via linuxrc and use the pivot_root syscall. The initrd is 141 mounted and executed via prepare_namespace(). 142 143c) using initramfs. The call to prepare_namespace() must be skipped. 144 This means that a binary must do all the work. Said binary can be stored 145 into initramfs either via modifying usr/gen_init_cpio.c or via the new 146 initrd format, an cpio archive. It must be called "/init". This binary 147 is responsible to do all the things prepare_namespace() would do. 148 149 To maintain backwards compatibility, the /init binary will only run if it 150 comes via an initramfs cpio archive. If this is not the case, 151 init/main.c:init() will run prepare_namespace() to mount the final root 152 and exec one of the predefined init binaries. 153 154Bryan O'Sullivan <bos@serpentine.com> 155