+++ /dev/null
-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>
--- /dev/null
+=======================
+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.
+++ /dev/null
- 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.
--- /dev/null
+=======================
+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>
--- /dev/null
+:orphan:
+
+===============
+Early Userspace
+===============
+
+.. toctree::
+ :maxdepth: 1
+
+ early_userspace_support
+ buffer-format
+
+.. only:: subproject and html
+
+ Indices
+ =======
+
+ * :ref:`genindex`
A description of the process of mounting the root file system can be
found in:
- Documentation/early-userspace/README
+ Documentation/early-userspace/early_userspace_support.rst
- The old initrd file was a gzipped filesystem image (in some file format,
such as ext2, that needed a driver built into the kernel), while the new
initramfs archive is a gzipped cpio archive (like tar only simpler,
- see cpio(1) and Documentation/early-userspace/buffer-format.txt). The
+ see cpio(1) and Documentation/early-userspace/buffer-format.rst). The
kernel's cpio extraction code is not only extremely small, it's also
__init text and data that can be discarded during the boot process.
set permissions or create device nodes in the new archive. (Note that those
two example "file" entries expect to find files named "init.sh" and "busybox" in
a directory called "initramfs", under the linux-2.6.* directory. See
-Documentation/early-userspace/README for more details.)
+Documentation/early-userspace/early_userspace_support.rst for more details.)
The kernel does not depend on external cpio tools. If you specify a
directory instead of a configuration file, the kernel's build infrastructure
When multiple directories and files are specified then the
initramfs image will be the aggregate of all of them.
- See <file:Documentation/early-userspace/README> for more details.
+ See <file:Documentation/early-userspace/early_userspace_support.rst> for more details.
If you are not sure, leave it blank.
projects / openwrt / staging / blogic.git / commitdiff