view www/build-stages.html @ 1362:9d7c03814309

Move root's home directory to /home/root, which is A) known writeable space so dropbear can have a .ssh directory if it needs one, B) frees up /root to be a virtfs mount point of the host's root filesystem.
author Rob Landley <rob@landley.net>
date Tue, 24 May 2011 02:06:22 -0500
parents 6da94a74a8a9
children 3a66b5554d1e
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<title>Aboriginal Linux - Build Stages</title>
<body>
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<p>The Aboriginal Linux build scripts are the source code for the Aboriginal
Linux project.  If you would like to build your own cross compiler or target
system image from source, use these build scripts.  They're written in bash
and should be fairly easy to read.</p>

<h2>Quick start</h2>

<p>Run <b>build.sh</b> with no arguments to see a list of targets.  Select
a target, and run <b>build.sh $TARGET</b> with the target name in place of
$TARGET.  When it finishes, run <b>more/dev-environment.sh $TARGET</b> to
boot the resulting system image under QEMU, configured for use as a
development environment.  Type <b>exit</b> to shut down the emulator.</p>

<h2>Overview</h2>

<p>The build runs the following stages, in order:</p>

<ul>
<li><b>download.sh</b> - Download source packages used by the rest of the build.</li>
<li><b>host-tools.sh</b> - Build prerequisites host needs to run remaining stages.</li>
<li><b>simple-cross-compiler.sh</b> - Build cross compiler for selected target architecture.</li>
<li><b>[</b>cross-compiler.sh<b>]</b> - optionally produce a more portable cross compiler (not needed by rest of build).</li>
<li><b>native-compiler.sh</b> - Build native compiler to install/run on target.</li>
<li><b>simple-root-filesystem.sh</b> - Build root filesystem that can boot to a shell prompt.</li>
<li><b>root-filesystem.sh</b> - Combine output of native-compiler and simple-root-filesystem stages into a single directory.</li>
<li><b>root-image.sh</b> - Create ext2/squashfs/initramfs image file from root-filesystem or simple-root-filesystem output.</li>
<li><b>linux-kernel.sh</b> - Build bootable Linux kernel.</li>
<li><b>system-image.sh</b> - Package together filesystem image and kernel with scripts to launch them under an emulator.</li>
</ul>

<p>The top level wrapper script <b>build.sh</b> runs the above stages in order,
but each stage script can also be run individually.  Each of the above
build scripts (except download.sh and host-tools.sh) take a single argument:
the name of the target architecture to build code for.  Run build.sh with no
arguments to see a list of available targets.</p>

<p>Each build stage (except download.sh and host-tools.sh) produces its output
in the "build" directory under a subdirectory named after the script plus the
target.  It also produces a tarball of that directory if the build stage
completed successfully.  (The download.sh script populates the "packages"
directory instead, and host-tools.sh produces its output the directory
"build/host" with no tarball version since those programs are intended to
run locally.)</p>

<p>All downloaded files wind up in the "packages" directory.  Output from
compiles is generated in the "build" directory.  These are the only two
directories the build writes to, and both directories may be deleted and
then recreated by the build scripts.  (The equivalent of "distclean" is
<b>rm -rf build packages</b> from the top level directory.)</p>

<p>None of these scripts need to be run as root -- an explicit design goal of
Aboriginal Linux is that root access on the host is never required.</p>

<h2>Build stages</h2>

<p>The files in the top level directory of the Aboriginal Linux source
are:</p>

<ul>
<li><b>configure</b>
<blockquote>
<p>Contains several variables that can be set to control the build's behavior,
with descriptions of each.  These variables may be set in this file, or
exported as environment variables.</p>

<p>A useful shell syntax to export environment variables for just a single
command, without persistently altering the environment, is to list the
assignments before the command on the same line.  For example:</p>

<blockquote><pre><b>CROSS_HOST=i686 SYSIMAGE_TYPE=ext2 ./build.sh armv5l</b></pre></blockquote>
</li>

<li><b>build.sh $TARGET</b>
<blockquote>
<p>Top level wrapper script which builds a system image for a target,
by calling most of the other scripts listed here in the appropriate
order.  It requires one argument, which is the target platform to build
for.  When run without arguments, build.sh lists available architectures.</p>

<p>This script is just a wrapper, it contains no actual build logic (except
checking some of the configuration variables).</p>
</li>

<li><b>download.sh</b>

<blockquote>
<p>Uses wget to download the source code required by the later build stages,
saving it in the "packages" directory.  It compares the sha1 checksum
of any existing tarballs to an expected value, only downloading new
source tarballs when it needs to.</p>

<p>If a package's primary site is down, it checks a series of fallback mirrors.
The environment variable PREFERRED_MIRROR can insert a new mirror at
the start of the list, which is checked before even the official website.</p>

<p>This script is not target-specific, and only needs to be called once
even when building multiple architectures.</p>
</blockquote>
</li>

<li><b>host-tools.sh</b>

<blockquote>
<p>Sanitizes the host environment by building known versions of needed tools
from source code, then restricting the $PATH to just those tools.  This is
an optional step which can be skipped, but without it the build process
is very brittle.</p>

<p>This "airlock" step serves a similar purpose to the temporary system built
by Linux From Scratch's chapter 5, isolating the new system from variations
in the host.  It also acts as an early check that the resulting system
images offer a sufficient development environment to rebuild themselves
from source, because the host tool versions used to build them in the first
place are the same ones the scripts install into the target root filesystem.</p>

<p>This script populates the "build/host" directory, which is automatically
<p>used by later stages if it exists.  It is not target specific, and only
<p>needs to be run once when building multiple architectures.</p>
</blockquote>
</li>

<li><b>simple-cross-compiler.sh $TARGET</b>

<blockquote>
<p>Creates a cross compiler for the selected target architecture, built from
gcc, binutils, uClibc, and the Linux kernel headers.  This compiler runs
on the host and produces programs that run on the target.</p>

<p>This compiler is sufficient to build a system image for the target, but
isn't as powerful as the compilers created by cross-compiler.sh or
native-compiler.sh.  (It doesn't include thread support, uClibc++, or the
shared version of libgcc.)</p>
</blockquote>
</li>

<li><b>cross-compiler.sh $TARGET</b>

<blockquote>
<p>This optional step creates a more full-featured cross compiler, with
thread support, uClibc++, and the shared version of libgcc.  This is
not required to build a system image, but the prebuilt binary compilers
shipped in the downloads/binaries directory are built this way.</p>

<p>The build.sh wrapper script only calls this stage if the config variable
CROSS_HOST is set, indicating which host architecture to build for.
(For PC hardware, i686 is a good value, since most 64 bit PCs can run 32 bit
code.)</p>

<p>This compiler is statically linked against uClibc, for maximum
portability.  (You can set BUILD_STATIC=none to dynamically link instead,
but then have to install uClibc's shared libraries on the host.)</p>
</blockquote>
</li>

<li><b>native-compiler.sh $TARGET</b>

<blockquote>
<p>This step creates a compiler for the selected target, using one
or more of the existing simple cross compilers.  The compiler it produces
runs on the target and produces programs that also run on the target.</p>

<p>By default this compiler is statically linked so you can add it to an
existing target root filesystem.  Use BUILD_STATIC=none to disable this.,/p>

<p>This compiler includes binutils, gcc, make, bash, and distcc.  Because
it's a native compiler, the executable names do not have any prefixes the
way the cross compilers do.  (I.E. just "ld" instead of "$TARGET-ld".)</p>
</blockquote>
</li>

<li><b>simple-root-filesystem.sh $TARGET</b>

<blockquote>
<p>Creates a root filesystem (with uCLibc, BusyBox, and an init script)
that contains just enough infrastructure to boot up to a shell prompt.</p>

<p>The config variable SIMPLE_ROOT_OVERLAY can supplement this stage.</p>
</blockquote>
</li>

<li><b>root-filesystem.sh $TARGET</b>
<blockquote>
<p>Combines the simple root filesystem and native compiler into a single
root filesystem, producing a bootable root filesystem with development
tools.</p>
</blockquote>
</li>

<li><b>root-image.sh $TARGET</b>
<blockquote>
<p>Creates a filesystem image from the root-filesystem
(or simple-root-filesystem) directory.  The default is squashfs, but
the config variable SYSIMAGE_TYPE can also select ext2 or initramfs formats.</p>
</blockquote>
</li>

<li><b>linux-kernel.sh $TARGET</b>
<blockquote>
<p>Builds a bootable Linux kernel for the target, generally configured for use
with QEMU.</p>

<p>The kernel configuration combines the sources/baseconfig-linux settings
(which are the same for each $TARGET) with the target-specific LINUX_CONFIG
entries from sources/targets/$TARGET/settings.  This configuration mechanism
essentially starts with "allnoconfig" and then switches on each mentioned
symbol, resolving dependencies as it goes, just as if you opened up menuconfig
and set that list of symbols by hand.</p>

<p>Alternately, you could put your own kernel config in
"sources/targets/$TARGET/miniconfig-linux".  Or just build your own kernel.</p>
</blockquote>
</li>

<li><b>system-image.sh $TARGET</b>
<blockquote>
<p>Combines the output of root-image and linux-kernel, and adds emulator
launch scripts.</p>

<p>For more information, see the <a href=prebuilt-binaries.html>the binary
docs</a></p>
</blockquote>
</li>
</ul>

<p>The sources/more directory contains additional scripts the user can run,
but which are not called from build.sh.  This directory contains the external
user interfaces the user can call directly which are not build stages.
See more/README in the Aboriginal source code for details.</p>

<!--
  run-from-build.sh ARCH

    Boot up a system image under its emulator, with full native development
    environment options (a 2 gigabyte /dev/hdb mounted on /home and distcc
    calling out to the appropriate cross compiler).

    Note that targets with the hw- prefix are aimed at actual hardware, and do
    not have an emulator configured.  Hardware targets are derived from an
    existing architecture, repackaging the other architecture's root filesystem
    with a different Linux kernel configuration.
-->

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