docs/HACKING.md

   1 # Hacking on systemd
   2
   3 We welcome all contributions to systemd. If you notice a bug or a missing
   4 feature, please feel invited to fix it, and submit your work as a GitHub Pull
   5 Request (PR) at https://github.com/systemd/systemd/pull/new.
   6
   7 Please make sure to follow our [Coding Style](CODING_STYLE.md) when submitting patches.
   8 Also have a look at our [Contribution Guidelines](CONTRIBUTING.md).
   9
  10 When adding new functionality, tests should be added. For shared functionality
  11 (in `src/basic/` and `src/shared/`) unit tests should be sufficient. The general
  12 policy is to keep tests in matching files underneath `src/test/`,
  13 e.g. `src/test/test-path-util.c` contains tests for any functions in
  14 `src/basic/path-util.c`. If adding a new source file, consider adding a matching
  15 test executable. For features at a higher level, tests in `src/test/` are very
  16 strongly recommended. If that is not possible, integration tests in `test/` are
  17 encouraged.
  18
  19 Please also have a look at our list of [code quality tools](CODE_QUALITY.md) we have setup for systemd,
  20 to ensure our codebase stays in good shape.
  21
  22 Please always test your work before submitting a PR. For many of the components
  23 of systemd testing is straight-forward as you can simply compile systemd and
  24 run the relevant tool from the build directory.
  25
  26 For some components (most importantly, systemd/PID1 itself) this is not
  27 possible, however. In order to simplify testing for cases like this we provide
  28 a set of `mkosi` build files directly in the source tree. `mkosi` is a tool for
  29 building clean OS images from an upstream distribution in combination with a
  30 fresh build of the project in the local working directory. To make use of this,
  31 please acquire `mkosi` from https://github.com/systemd/mkosi first, unless your
  32 distribution has packaged it already and you can get it from there. After the
  33 tool is installed it is sufficient to type `mkosi` in the systemd project
  34 directory to generate a disk image `image.raw` you can boot either in
  35 `systemd-nspawn` or in an UEFI-capable VM:
  36
  37 ```
  38 # systemd-nspawn -bi image.raw
  39 ```
  40
  41 or:
  42
  43 ```
  44 # qemu-system-x86_64 -enable-kvm -m 512 -smp 2 -bios /usr/share/edk2/ovmf/OVMF_CODE.fd -hda image.raw
  45 ```
  46
  47 Every time you rerun the `mkosi` command a fresh image is built, incorporating
  48 all current changes you made to the project tree.
  49
  50 Alternatively, you may install the systemd version from your git check-out
  51 directly on top of your host system's directory tree. This mostly works fine,
  52 but of course you should know what you are doing as you might make your system
  53 unbootable in case of a bug in your changes. Also, you might step into your
  54 package manager's territory with this. Be careful!
  55
  56 And never forget: most distributions provide very simple and convenient ways to
  57 install all development packages necessary to build systemd. For example, on
  58 Fedora the following command line should be sufficient to install all of
  59 systemd's build dependencies:
  60
  61 ```
  62 # dnf builddep systemd
  63 ```
  64
  65 Putting this all together, here's a series of commands for preparing a patch
  66 for systemd (this example is for Fedora):
  67
  68 ```sh
  69 $ sudo dnf builddep systemd            # install build dependencies
  70 $ sudo dnf install mkosi               # install tool to quickly build images
  71 $ git clone https://github.com/systemd/systemd.git
  72 $ cd systemd
  73 $ vim src/core/main.c                  # or wherever you'd like to make your changes
  74 $ meson build                          # configure the build
  75 $ ninja -C build                       # build it locally, see if everything compiles fine
  76 $ ninja -C build test                  # run some simple regression tests
  77 $ (umask 077; echo 123 > mkosi.rootpw) # set root password used by mkosi
  78 $ sudo mkosi                           # build a test image
  79 $ sudo systemd-nspawn -bi image.raw    # boot up the test image
  80 $ git add -p                           # interactively put together your patch
  81 $ git commit                           # commit it
  82 $ git push REMOTE HEAD:refs/heads/BRANCH
  83                                        # where REMOTE is your "fork" on GitHub
  84                                        # and BRANCH is a branch name.
  85 ```
  86
  87 And after that, head over to your repo on GitHub and click "Compare & pull request"
  88
  89 Happy hacking!
  90
  91
  92 ## Fuzzers
  93
  94 systemd includes fuzzers in `src/fuzz/` that use libFuzzer and are automatically
  95 run by [OSS-Fuzz](https://github.com/google/oss-fuzz) with sanitizers. To add a
  96 fuzz target, create a new `src/fuzz/fuzz-foo.c` file with a `LLVMFuzzerTestOneInput`
  97 function and add it to the list in `src/fuzz/meson.build`.
  98
  99 Whenever possible, a seed corpus and a dictionary should also be added with new
 100 fuzz targets. The dictionary should be named `src/fuzz/fuzz-foo.dict` and the seed
 101 corpus should be built and exported as `$OUT/fuzz-foo_seed_corpus.zip` in
 102 `tools/oss-fuzz.sh`.
 103
 104 The fuzzers can be built locally if you have libFuzzer installed by running
 105 `tools/oss-fuzz.sh`. You should also confirm that the fuzzer runs in the
 106 OSS-Fuzz environment by checking out the OSS-Fuzz repo, and then running
 107 commands like this:
 108
 109 ```
 110 python infra/helper.py build_image systemd
 111 python infra/helper.py build_fuzzers --sanitizer memory systemd ../systemd
 112 python infra/helper.py run_fuzzer systemd fuzz-foo
 113 ```
 114
 115 If you find a bug that impacts the security of systemd, please follow the
 116 guidance in [CONTRIBUTING.md](CONTRIBUTING.md) on how to report a security vulnerability.
 117
 118 For more details on building fuzzers and integrating with OSS-Fuzz, visit:
 119
 120 - https://github.com/google/oss-fuzz/blob/master/docs/new_project_guide.md
 121 - https://llvm.org/docs/LibFuzzer.html
 122 - https://github.com/google/fuzzer-test-suite/blob/master/tutorial/libFuzzerTutorial.md
 123 - https://chromium.googlesource.com/chromium/src/testing/libfuzzer/+/HEAD/efficient_fuzzer.md