Starnix is a component that runs Linux binaries on Fuchsia. Currently, starnix is highly experimental and can run only trivial programs.

How to run starnix

Configure your build

In order to run starnix, we need to build //src/proc:

fx set core.x64 --with //src/proc,//src/proc:tests
fx build

Run Fuchsia

Run Fuchsia as normal, for example using fx serve and fx emu -N.

To monitor starnix, look for log messages with the starnix tag:

fx log --tag starnix --hide_metadata --pretty --severity TRACE --select "core/*/starnix*#TRACE"

When running tests, you will need to modify the selector for the logs.

fx log --tag starnix --hide_metadata --pretty --severity TRACE --select "core/test*/*/starnix*#TRACE"

The --select arguments contain the moniker for the starnix instance whose minimum severity log level you want to change. This affects the logs emitted by starnix, as opposed to --severity, which affects which logs are filtered for viewing. The changed log level only persists for the duration of the fx log command.

If you do not care about detailed logging, you can leave out the --severity and just do:

fx log --tag starnix --hide_metadata --pretty

Starnix produces a large amount of logs and this can overload archivist's ability to retain them, instead printing messages like:

[starnix] WARNING: Dropped logs count: 5165

If you see this, you can reduce or eliminate the dropped messages by increasing the value of max_cached_original_bytes the the JSON file `//src/diagnostics/archivist/configs/archivist_config.json' in your local checkout. Increasing by a facfor of 10 seems to work well.

Run a Linux binary

To run a Linux binary, ask starnix to start a component that wraps the binary:

ffx starnix start fuchsia-pkg://

If this is the first time you've used the ffx starnix command, you might need to configure ffx to enable the starnix commands. Attempting to run the start command should provide instructions for enabling the starnix commands.

If everything is working, you should see some log messages like the following:

[00064.846853][33707][33709][starnix, starnix] INFO: main
[00064.847640][33707][33709][starnix, starnix] INFO: start_component: fuchsia-pkg://

Run an interactive Android shell

To run an interactive Android shell, connected to your host machine, run:

ffx starnix shell

Running the ADB Relay

The target daemon 'adbd` is normally started as part of init. If adbd is not already started, it can be started with

ffx starnix start fuchsia-pkg://

The relay between the host and server is started by running the adb starnix plugin in a termainal.

ffx starnix adb

After the plugin is running, connect to it with adb connect on the host.

$ adb connect
$ adb devices -l
List of devices attached         device product:starnix_x86_64 model:starnix_x86_64 device:starnix_x86_64 transport_id:2
emulator-5554          offline transport_id:1

If there is more than one adb device, run commands with the '-sor-t` selectors. For example:

adb -t 2 shell
adb -s logcat

Run a Linux test binary

Linux test binaries can also be run using the Starnix test runner using the standard fx test command:

fx test exit_test --output

You should see output like:

[==========] Running 3 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 3 tests from ExitTest
[ RUN      ] ExitTest.Success
[       OK ] ExitTest.Success (4 ms)
[ RUN      ] ExitTest.Failure
[       OK ] ExitTest.Failure (3 ms)
[ RUN      ] ExitTest.CloseFds

If you set the log level to TRACE (e.g., fx log --severity TRACE --select "core/test*/*/starnix*#TRACE"), you should see the system call handling in the device logs:

[629.603][starnix][D] 1[/data/tests/exit_test] wait4(0x3, 0x1c48095b950, 0x0, 0x0, 0x10, 0x10)
[629.603][starnix][D] 3[/data/tests/exit_test] prctl(0x53564d41, 0x0, 0x700d5ea000, 0x3000, 0x3a506c7a34b, 0xc06913ece9)
[629.603][starnix][D] 3[/data/tests/exit_test] -> 0x0
[629.604][starnix][D] 3[/data/tests/exit_test] exit_group(0x1, 0x3, 0x2b18e3464f8, 0x3000, 0x3a506c7a34b, 0xc06913ece9)
[629.604][starnix][I] exit_group: pid=3 exit_code=1


Writing in-process unit tests

Decorate your test function with the #[::fuchsia::test] macro instead of the standard #[test] macro. #[::fuchsia::test] will initialize logging so that failing tests can be debugged more easily.

Running the in-process unit tests

Starnix also has in-process unit tests that can interact with its internals during the test. To run those tests, use the following command:

fx test starnix-tests

Using a locally built syscalls test binary

The syscalls_test test runs a prebuilt binary that has been built with the Android NDK. You can substitute your own prebuilt binary using the starnix_syscalls_test_label GN argument:

fx set core.x64 --args 'starnix_syscalls_test_label="//local/starnix/syscalls"' --with //src/proc,//src/proc:tests

Build your syscalls binary and put the file in //local/starnix/syscalls. (If you are building using the Google-internal build system, be sure to specific the --config=android_x86_64 build flag to build an NDK binary.)

You can then build and run your test as usual:

fx build
fx test syscalls_test

Updating Starnix when running prebuilt tests

When working on the Starnix runner to make behavioral changes that affect prebuilt tests such as the GVisor tests, re-running the test does not implicitly fetch and start a new runner. In these cases it is necessary to explicitly stop the runner so that it is updated on next component launch.

The following snippet performs those stops:

for moniker in /core/{starnix_runner,starnix_manager,test_manager/{starnix_test_runner,starnix_unit_test_runner}}; do
  ffx component stop -r $moniker