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Diffstat (limited to 'Documentation/early-userspace')
| -rw-r--r-- | Documentation/early-userspace/README | 151 | ||||
| -rw-r--r-- | Documentation/early-userspace/buffer-format.txt | 112 |
2 files changed, 263 insertions, 0 deletions
diff --git a/Documentation/early-userspace/README b/Documentation/early-userspace/README new file mode 100644 index 000000000..1e1057958 --- /dev/null +++ b/Documentation/early-userspace/README @@ -0,0 +1,151 @@ +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 --help' 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> diff --git a/Documentation/early-userspace/buffer-format.txt b/Documentation/early-userspace/buffer-format.txt new file mode 100644 index 000000000..e1fd7f9da --- /dev/null +++ b/Documentation/early-userspace/buffer-format.txt @@ -0,0 +1,112 @@ + initramfs buffer format + ----------------------- + + Al Viro, H. Peter Anvin + Last revision: 2002-01-13 + +Starting with kernel 2.5.x, the old "initial ramdisk" protocol is +getting {replaced/complemented} with the new "initial ramfs" +(initramfs) protocol. The initramfs contents is passed using the same +memory buffer protocol used by the initrd protocol, but the contents +is different. The initramfs buffer contains an archive which is +expanded into a ramfs filesystem; this document details the format of +the initramfs buffer format. + +The initramfs buffer format is based around the "newc" or "crc" CPIO +formats, and can be created with the cpio(1) utility. The cpio +archive can be compressed using gzip(1). One valid version of an +initramfs buffer is thus a single .cpio.gz file. + +The full format of the initramfs buffer is defined by the following +grammar, where: + * is used to indicate "0 or more occurrences of" + (|) indicates alternatives + + indicates concatenation + GZIP() indicates the gzip(1) of the operand + ALGN(n) means padding with null bytes to an n-byte boundary + + initramfs := ("\0" | cpio_archive | cpio_gzip_archive)* + + cpio_gzip_archive := GZIP(cpio_archive) + + cpio_archive := cpio_file* + (<nothing> | cpio_trailer) + + cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data + + cpio_trailer := ALGN(4) + cpio_header + "TRAILER!!!\0" + ALGN(4) + + +In human terms, the initramfs buffer contains a collection of +compressed and/or uncompressed cpio archives (in the "newc" or "crc" +formats); arbitrary amounts zero bytes (for padding) can be added +between members. + +The cpio "TRAILER!!!" entry (cpio end-of-archive) is optional, but is +not ignored; see "handling of hard links" below. + +The structure of the cpio_header is as follows (all fields contain +hexadecimal ASCII numbers fully padded with '0' on the left to the +full width of the field, for example, the integer 4780 is represented +by the ASCII string "000012ac"): + +Field name Field size Meaning +c_magic 6 bytes The string "070701" or "070702" +c_ino 8 bytes File inode number +c_mode 8 bytes File mode and permissions +c_uid 8 bytes File uid +c_gid 8 bytes File gid +c_nlink 8 bytes Number of links +c_mtime 8 bytes Modification time +c_filesize 8 bytes Size of data field +c_maj 8 bytes Major part of file device number +c_min 8 bytes Minor part of file device number +c_rmaj 8 bytes Major part of device node reference +c_rmin 8 bytes Minor part of device node reference +c_namesize 8 bytes Length of filename, including final \0 +c_chksum 8 bytes Checksum of data field if c_magic is 070702; + otherwise zero + +The c_mode field matches the contents of st_mode returned by stat(2) +on Linux, and encodes the file type and file permissions. + +The c_filesize should be zero for any file which is not a regular file +or symlink. + +The c_chksum field contains a simple 32-bit unsigned sum of all the +bytes in the data field. cpio(1) refers to this as "crc", which is +clearly incorrect (a cyclic redundancy check is a different and +significantly stronger integrity check), however, this is the +algorithm used. + +If the filename is "TRAILER!!!" this is actually an end-of-archive +marker; the c_filesize for an end-of-archive marker must be zero. + + +*** Handling of hard links + +When a nondirectory with c_nlink > 1 is seen, the (c_maj,c_min,c_ino) +tuple is looked up in a tuple buffer. If not found, it is entered in +the tuple buffer and the entry is created as usual; if found, a hard +link rather than a second copy of the file is created. It is not +necessary (but permitted) to include a second copy of the file +contents; if the file contents is not included, the c_filesize field +should be set to zero to indicate no data section follows. If data is +present, the previous instance of the file is overwritten; this allows +the data-carrying instance of a file to occur anywhere in the sequence +(GNU cpio is reported to attach the data to the last instance of a +file only.) + +c_filesize must not be zero for a symlink. + +When a "TRAILER!!!" end-of-archive marker is seen, the tuple buffer is +reset. This permits archives which are generated independently to be +concatenated. + +To combine file data from different sources (without having to +regenerate the (c_maj,c_min,c_ino) fields), therefore, either one of +the following techniques can be used: + +a) Separate the different file data sources with a "TRAILER!!!" + end-of-archive marker, or + +b) Make sure c_nlink == 1 for all nondirectory entries. |