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248 lines
11 KiB
Markdown
248 lines
11 KiB
Markdown
---
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title: Hacking on systemd
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category: Contributing
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layout: default
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---
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# Hacking on systemd
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We welcome all contributions to systemd. If you notice a bug or a missing
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feature, please feel invited to fix it, and submit your work as a GitHub Pull
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Request (PR) at https://github.com/systemd/systemd/pull/new.
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Please make sure to follow our [Coding Style](CODING_STYLE.md) when submitting
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patches. Also have a look at our [Contribution Guidelines](CONTRIBUTING.md).
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When adding new functionality, tests should be added. For shared functionality
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(in `src/basic/` and `src/shared/`) unit tests should be sufficient. The general
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policy is to keep tests in matching files underneath `src/test/`,
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e.g. `src/test/test-path-util.c` contains tests for any functions in
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`src/basic/path-util.c`. If adding a new source file, consider adding a matching
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test executable. For features at a higher level, tests in `src/test/` are very
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strongly recommended. If that is not possible, integration tests in `test/` are
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encouraged.
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Please also have a look at our list of [code quality tools](CODE_QUALITY.md) we
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have setup for systemd, to ensure our codebase stays in good shape.
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Please always test your work before submitting a PR. For many of the components
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of systemd testing is straight-forward as you can simply compile systemd and
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run the relevant tool from the build directory.
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For some components (most importantly, systemd/PID1 itself) this is not
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possible, however. In order to simplify testing for cases like this we provide
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a set of `mkosi` build files directly in the source tree. `mkosi` is a tool for
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building clean OS images from an upstream distribution in combination with a
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fresh build of the project in the local working directory. To make use of this,
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please acquire `mkosi` from https://github.com/systemd/mkosi first, unless your
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distribution has packaged it already and you can get it from there. After the
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tool is installed, symlink the settings file for your distribution of choice
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from .mkosi/ to mkosi.default in the project root directory (note that the
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package manager for this distro needs to be installed on your host system).
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After doing that, it is sufficient to type `mkosi` in the systemd project
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directory to generate a disk image `image.raw` you can boot either in
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`systemd-nspawn` or in an UEFI-capable VM:
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```
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# mkosi boot
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```
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or:
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```
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# mkosi qemu
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```
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Every time you rerun the `mkosi` command a fresh image is built, incorporating
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all current changes you made to the project tree. To save time when rebuilding,
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you can use mkosi's incremental mode (`-i`). This instructs mkosi to build a set
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of cache images that make future builds a lot faster. Note that the `-i` flag
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both instructs mkosi to build cached images if they don't exist yet and to use
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cached images if they already exist so make sure to always specify `-i` if you
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want mkosi to use the cached images.
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If you're going to build mkosi images that use the same distribution and release
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that you're currently using, you can speed up the initial mkosi run by having it
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reuse the host's package cache. To do this, create a mkosi override file in
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mkosi.default.d/ (e.g 20-local.conf) and add the following contents:
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```
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[Packages]
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Cache=<full-path-to-package-manager-cache> # (e.g. /var/cache/dnf)
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```
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If you want to do a local build without mkosi, most distributions also provide
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very simple and convenient ways to install all development packages necessary
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to build systemd. For example, on Fedora the following command line should be
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sufficient to install all of systemd's build dependencies:
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```
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# dnf builddep systemd
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```
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Putting this all together, here's a series of commands for preparing a patch
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for systemd (this example is for Fedora):
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```sh
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$ sudo dnf builddep systemd # install build dependencies
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$ sudo dnf install mkosi # install tool to quickly build images
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$ git clone https://github.com/systemd/systemd.git
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$ cd systemd
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$ vim src/core/main.c # or wherever you'd like to make your changes
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$ meson build # configure the build
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$ meson compile -C build # build it locally, see if everything compiles fine
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$ meson test -C build # run some simple regression tests
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$ ln -s .mkosi/mkosi.fedora mkosi.default # Configure mkosi to build a fedora image
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$ sudo mkosi # build a test image
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$ sudo mkosi boot # boot up the test image
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$ git add -p # interactively put together your patch
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$ git commit # commit it
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$ git push REMOTE HEAD:refs/heads/BRANCH
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# where REMOTE is your "fork" on GitHub
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# and BRANCH is a branch name.
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```
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And after that, head over to your repo on GitHub and click "Compare & pull request"
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Happy hacking!
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## Templating engines in .in files
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Some source files are generated during build. We use two templating engines:
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* meson's `configure_file()` directive uses syntax with `@VARIABLE@`.
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See the
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[Meson docs for `configure_file()`](https://mesonbuild.com/Reference-manual.html#configure_file)
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for details.
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{% raw %}
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* most files are rendered using jinja2, with `{{VARIABLE}}` and `{% if … %}`,
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`{% elif … %}`, `{% else … %}`, `{% endif … %}` blocks. `{# … #}` is a
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jinja2 comment, i.e. that block will not be visible in the rendered
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output. `{% raw %} … `{% endraw %}`{{ '{' }}{{ '% endraw %' }}}` creates a block
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where jinja2 syntax is not interpreted.
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See the
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[Jinja Template Designer Documentation](https://jinja2docs.readthedocs.io/en/stable/templates.html#synopsis)
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for details.
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Please note that files for both template engines use the `.in` extension.
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## Developer and release modes
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In the default meson configuration (`-Dmode=developer`), certain checks are
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enabled that are suitable when hacking on systemd (such as internal
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documentation consistency checks). Those are not useful when compiling for
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distribution and can be disabled by setting `-Dmode=release`.
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## Fuzzers
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systemd includes fuzzers in `src/fuzz/` that use libFuzzer and are automatically
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run by [OSS-Fuzz](https://github.com/google/oss-fuzz) with sanitizers.
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To add a fuzz target, create a new `src/fuzz/fuzz-foo.c` file with a `LLVMFuzzerTestOneInput`
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function and add it to the list in `src/fuzz/meson.build`.
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Whenever possible, a seed corpus and a dictionary should also be added with new
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fuzz targets. The dictionary should be named `src/fuzz/fuzz-foo.dict` and the seed
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corpus should be built and exported as `$OUT/fuzz-foo_seed_corpus.zip` in
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`tools/oss-fuzz.sh`.
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The fuzzers can be built locally if you have libFuzzer installed by running
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`tools/oss-fuzz.sh`. You should also confirm that the fuzzer runs in the
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OSS-Fuzz environment by checking out the OSS-Fuzz repo, and then running
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commands like this:
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```
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python infra/helper.py build_image systemd
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python infra/helper.py build_fuzzers --sanitizer memory systemd ../systemd
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python infra/helper.py run_fuzzer systemd fuzz-foo
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```
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If you find a bug that impacts the security of systemd, please follow the
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guidance in [CONTRIBUTING.md](CONTRIBUTING.md) on how to report a security vulnerability.
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For more details on building fuzzers and integrating with OSS-Fuzz, visit:
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- [Setting up a new project - OSS-Fuzz](https://google.github.io/oss-fuzz/getting-started/new-project-guide/)
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- [Tutorials - OSS-Fuzz](https://google.github.io/oss-fuzz/reference/useful-links/#tutorials)
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## mkosi + clangd
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[clangd](https://clangd.llvm.org/) is a language server that provides code completion, diagnostics and more
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right in your editor of choice (with the right plugin installed). When using mkosi, we can run clangd in the
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mkosi build container to avoid needing to build systemd on the host machine just to make clangd work. To
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achieve this, create a script with the following contents in systemd's project directory on the host:
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```sh
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#!/usr/bin/env sh
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tee mkosi-clangd.build > /dev/null << EOF
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#!/usr/bin/env sh
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exec clangd \\
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--compile-commands-dir=/root/build \\
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--path-mappings=\\
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"\\
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$(pwd)=/root/src,\\
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$(pwd)/mkosi.builddir=/root/build,\\
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$(pwd)/mkosi.includedir=/usr/include,\\
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$(pwd)/mkosi.installdir=/root/dest\\
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" \\
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--header-insertion=never
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EOF
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chmod +x mkosi-clangd.build
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exec sudo mkosi --source-file-transfer=mount --incremental --skip-final-phase --build-script mkosi-clangd.build build
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```
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Next, mark the script as executable and point your editor plugin to use this script to start clangd. For
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vscode's clangd extension, this is done via setting the `clangd.path` option to the path of the
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mkosi-clangd.sh script.
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To be able to navigate to include files of systemd's dependencies, we need to make the /usr/include folder of
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the build image available on the host. mkosi supports this by setting the `IncludeDirectory` option in
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mkosi's config. The easiest way to set the option is to create a file 20-local.conf in mkosi.default.d/ and
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add the following contents:
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```
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[Packages]
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IncludeDirectory=mkosi.includedir
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```
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This will make the contents of /usr/include available in mkosi.includedir in the systemd project directory.
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We already configured clangd to map any paths in /usr/include in the build image to mkosi.includedir/ on the
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host in the mkosi-clangd.sh script.
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We also need to make sure clangd is installed in the build image. To have mkosi install clangd in the build
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image, edit the 20-local.conf file we created earlier and add the following contents under the `[Packages]`
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section:
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```
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BuildPackages=<clangd-package>
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```
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Note that the exact package containing clangd will differ depending on the distribution used. Some
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distributions have a separate clangd package, others put the clangd binary in a clang-tools-extra package and
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some bundle clangd in the clang package.
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Because mkosi needs to run as root, we also need to make sure we can enter the root password when the editor
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plugin tries to run the mkosi-clangd.sh script. To be able to enter the root password in non-interactive
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scripts, we use an askpass provider. This is a program that sudo will launch if it detects it's being
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executed from a non-interactive shell so that the root password can still be entered. There are multiple
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implementations such as gnome askpass and KDE askpass. Install one of the askpass packages your distro
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provides and set the `SUDO_ASKPASS` environment variable to the path of the askpass binary you want to use.
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If configured correctly, a window will appear when your editor plugin tries to run the mkosi-clangd.sh script
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allowing you to enter the root password.
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Due to a bug in btrfs, it's currently impossible to mount two mkosi btrfs images at the same time. Because of
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this, trying to do a regular build while the clangd image is running will fail. To circumvent this, use ext4
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instead of btrfs for the images by adding the following contents to 20-local.conf:
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```
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[Output]
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Format=gpt_ext4
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```
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Finally, to ensure clangd starts up quickly in the editor, run an incremental build with mkosi to make sure
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the cached images are initialized (`mkosi -i`).
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Now, your editor will start clangd in the mkosi build image and all of clangd's features will work as
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expected.
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