C

cross-building linux

A process for building a simple linux installation for a different machine architecture than the one you start with. There's a blog, for those who are interested: http://crossbuiltlinux.blogspot.com/

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CROSS-BUILDING LINUX: The Little Blue Linux build process

This project contains instructions for building a complete, minimal,
GNU/Linux system from source code. If you follow these instructions
precisely, what you'll wind up with is a Little Blue Linux system. If
you follow these instructions with modifications or alterations, you'll
wind up with a derivative of Little Blue Linux instead!

The easiest way to use Cross-Building Linux (generally abbreviated CBL)
is to process the files in this project with the `litbuild` program,
which is available as a Ruby Gem. This will produce a human-readable
AsciiDoc version of the instructions, which can then be further
processed to produce HTML or PDF or other formats if you wish. It will
also produce scripts that implement the full set of build instructions;
if you run those scripts, you'll wind up with a Little Blue Linux system
with little or no manual activity needed on your part.

The main point of this README is to walk you through the process of
using litbuild to do those things.

Alternatively, you can just read the files in this project. That's more
difficult, because they are in a format designed to be easy to write and
easy to process programmatically -- it's _not_ designed to be easy to
read! But you can do it if you want to. Start with `sections/cbl.txt`.


GETTING LITBUILD

You can install the litbuild gem from rubygems.org simply by running
`gem install litbuild`.

You can also clone the source code for the litbuild program and package
it as a rubygem yourself. The git repository is located at
http://git.freesa.org/freesa/litbuild and there is a tarfile at
https://repo.freesa.org/cbl/ -- check out the README in that project for
more details. Litbuild requires Ruby at least 2.5 to work.

Once you've got litbuild installed, you can use the `lb` command-line
program to produce the human- and machine-readable versions of the CBL
process as described below.


PRODUCING AND READING THE CROSS-BUILDING LINUX BOOK

Just run `lb cbl` from the directory containing this file. This will
produce a file `cbl.adoc` in the directory `/tmp/build/docs`. If you
want to adjust the parameters used to produce the document, or the
location where it will be generated, you can set environment variables
as described in `narratives/configuration.txt` before running `lb`.

AsciiDoc is a format, similar to markdown, that is designed to be as
readable as plain text files. However, it can also be processed by a
ruby program, `asciidoctor`, to produce a variety of output formats. If
you want a HTML version of the CBL book, just:

  gem install asciidoctor # (if you don't have it installed already)
  cd /tmp/build/docs
  asciidoctor cbl.adoc

And voila, a `cbl.html` file will be produced.

Producing a PDF version of the book is a _little_ more involved:

  gem install asciidoctor # (if it's not already installed)
  gem install asciidoctor-pdf # ibid
  cd /tmp/build/docs
  asciidoctor-pdf cbl.adoc

Other formats are also possible but I'm not addressing them here.


BUILDING LITTLE BLUE LINUX

The architecture we've found to work most conveniently with
QEMU-emulated target systems is ARM (it supports a "virt" machine type
that can be given a lot more memory than is usual for emulated
machines), so that's the suggested architecture to start with. If you
have an actual machine you're targeting with CBL, of course you can use
that one instead! But it might not work as smoothly.

1. Grab all the tarfiles and patch files you need to build the CBL
   system. For your convenience, you can grab `cbl.tar` from
   `http://repo.freesa.org/` to get everything at once (or you can get
   individual files from there, if you prefer; or, for that matter, you
   can get source distribution files from the upstream project sites, if
   you read and understand the IMPORTANT SAFETY TIP below.) Put them in
   `/tmp/cbl-materials`, or in some other location if you prefer.

   IMPORTANT SAFETY TIP: Litbuild presumes that the filename for the
   source tarfiles is the package name, followed by a hyphen, followed by
   the version number, followed by `.tar` and optionally followed by the
   compression suffix `.lz`. (Other compression programs besides lzip
   can be used, but the package-users build script currently only
   handles lzip.) The tarfiles must expand into a directory named
   package name, hyphen, version. (This is the convention used by the GNU
   project, so most packages already conform to it.) When the upstream
   project distribution has some other convention for the filename or
   unpacked path, you need to fix that before you can use it with
   litbuild! All of the source tarfiles on repo.freesa.org have already
   been rejiggered to conform to this convention.

2. Review the configuration section (`narratives/configuration.txt`) and
   see if the default parameter values are what you want. If they are
   not, override the parameters you want to change in
   `configs/amd64-aarch64.sh`.

3. Get rid of the work directory (default `/tmp/build`), if it exists,
   and generate the CBL build scripts with `source
   configs/amd64-aarch64.sh; lb cbl`.

6. If there is a file `/tmp/build/scripts/lb-sudoers`, ensure that it
   is included in your sudoers file (perhaps by copying it to
   /etc/sudoers.d and chowning it to be root:root with a restrictive
   mode like 0400).

7. Run the script `/tmp/build/scripts/cbl.sh` that was just produced by
   litbuild.

8. Wait a while -- if the host or target system is emulated, this could
   be a long while, perhaps several days! (On my quad-core 64-bit Intel
   laptop, the host-side build takes about 20 hours and a target-side
   build on an Aarch64 "virt" emulated machine with four cores and eight
   GB of RAM takes around eight days) -- to see if you get a COMPLETE
   SUCCESS.


USING THE FRESHLY-BUILT SYSTEM

If you are doing a build with the default configuration -- the host
system is an actual computer, the target system is a QEMU emulated
virtual machine, networking is enabled in the target system -- you'll
need to set up the host system as a DHCP server and gateway for the
final CBL system in order to make it network-accessible.

If the host system is a Little Blue Linux system, the
`setup-virtual-network` blueprint will do this for you. If not, you can
still follow the same basic process as outlined in that blueprint, but
you'll have to make adjustments since the host system probably won't be
managing services with s6-rc.

MISCELLANEOUS NOTES AND PLANS ABOUT CBL:

(As these notes are incorporated (and expanded upon) in the CBL
narrative, they'll be removed from this section.)

- Dependency tracking -- by observing what files were accessed during a
  build, it should be possible to infer what packages that build depends
  on (e.g., do a `touch /tmp/TIMESTAMP && sleep 60` before running a
  build, and  `find / -type f -a -executable -a -anewer /tmp/TIMESTAMP`
  after it's complete). This will be most effective when using package
  users, since looking at the ownership of all the files produced by
  that command will reveal the package responsible for them. So the best
  place to add this feature might be in the package-users build script.

- Bootstrap from C -- You have to have _some_ binary code to start
  with, unless you want to do some hand-compilation and hand-assembly to
  get the initial compiler built. The absolute smallest programming
  language footprint you can start with is the language the operating
  system kernel is written in, so that's the best starting point. That
  means any _other_ programming languages and tools that will wind up on
  a CBL system (go, java, docker, etc) have to be bootstrapped directly
  or indirectly from C or C++. (Or copied in as binaries produced on
  some other system, but that's not something that we're going to
  endorse or support.)

  Since modern GCC is written in C++, this requires starting with GCC
  4.7.x. With that in mind, it might be worthwhile to start with an
  initial GCC 4.7.x in the host-prerequisites section -- unfortunately,
  this would require also using an older glibc (as of this writing,
  latest-stable glibc is 2.27, which requires GCC 4.9 or newer to build)
  and possibly older versions of other packages as well.

- Multiple tool sets. GCC is the default C/C++ toolchain, but LLVM works
  pretty well these days and it would be awesome to be able to use
  either of them as the initial cross-compiler, or as the initial
  host-side compiler. Similarly, glibc is the default C library, but
  musl and uclibc-ng are both perfectly good alternatives at this point.
  It would be a good idea to support multiple tools and libraries!

- Single-arch -- the multi-lib and multi-arch systems used by most
  distributions (and enforced by the current GNU toolchain) are messy
  and confusing, so CBL avoids them except where it is completely
  unavoidable. The multi-target, multi-ABI stuff for gcc results in
  horrible complexity and confusion. The specs file for the final system
  GCC should ideally only have the minimal set of options to support a
  single target and C library. Other (single-arch) toolchains can be set
  up to support other targets.

  An idea under consideration: move all libraries and toolchain-related
  files to subdirectories of a new top-level `/arch` directory
  (completely abandoning the `/lib` directory prescriptions of the FHS).