Early userspace support
=======================
-Last update: 2003-08-21
+Last update: 2004-12-20 tlh
"Early userspace" is a set of libraries and programs that provide
- 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
- optimised for correctness and small size.
+ optimized for correctness and small size.
The cpio file format used by initramfs is the "newc" (aka "cpio -c")
-format, and is documented in the file "buffer-format.txt". If you
-want to generate your own cpio files directly instead of hacking on
-gen_init_cpio, you will need to short-circuit the build process in
-usr/ so that gen_init_cpio does not get run, then simply pop your own
-initramfs_data.cpio.gz file into place.
-
+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.
+Youre 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 it's 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
+scripts/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 arugemnt to scripts/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 'scripts/gen_initramfs_list.sh -h'.
Where's this all leading?
=========================
git://git.onelab.eu / linux-2.6.git / blobdiff