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).
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/