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ChromiumOS Developer Guide

Introduction

This guide describes how to work on ChromiumOS. If you want to help develop ChromiumOS and you're looking for detailed information about how to get started, you're in the right place.

Target audience

The target audience of this guide is anyone who wants to obtain, build, or contribute to ChromiumOS. That includes new developers who are interested in the project and who simply want to browse through the ChromiumOS code, as well as developers who have been working on ChromiumOS for a long time.

Organization & content

This guide describes the common tasks required to develop ChromiumOS. The guide is organized linearly, so that developers who are new to ChromiumOS can follow the tasks in sequence. The tasks are grouped into the following sections:

Typography conventions

Label Commands
(outside) on your build computer, outside the chroot
(device) on your ChromiumOS computer

Beneath the label, the command(s) you should type are prefixed with a generic shell prompt, $ . This distinguishes input from the output of commands, which is not so prefixed.

Modifying this document

If you're a ChromiumOS developer, YOU SHOULD UPDATE THIS DOCUMENT and fix things as appropriate. See README.md for how to update this document. Bias towards action:

Please try to abide by the following guidelines when you modify this document:

More information

This document provides an overview of the tasks required to develop ChromiumOS. After you've learned the basics, check out the links in the Additional information section at the end of this document for tips and tricks, FAQs, and important details (e.g., the ChromiumOS directory structure, using the dev server, etc.).

Finally, if you build a ChromiumOS image, please read this important note about Attribution requirements.

Prerequisites

You must have Linux to develop ChromiumOS. Any recent or up-to-date distribution should work. However, we can't support everyone and their dog's Linux distro, so the only official supported environment is listed below. If you encounter issues with other setups, patches are generally welcomed, but please do not expect us to figure out your distro.

You will have a much nicer time if you also have:

Python

If your system does not have a compatible Python version installed, you'll need to install Python 3.8 or greater.

To check your python version:

(outside)
$ python3 -V

If an error or a version lower than 3.8 is returned, proceed with the rest of the section. If not, skip this section.

If your system supports newer specific Python versions, you can install it. If an incompatible version is present, uninstall it:

(outside)
$ sudo apt-get remove <current Python package>

And install a new version:

(outside)
$ sudo apt-get install python3.9

Install development tools

Some host OS tools are needed to manipulate code, bootstrap the development environment, and run preupload hooks later on.

Install the git revision control system, the curl download helper, and more.

(outside)
# On Ubuntu, make sure to enable the universe repository.
$ sudo add-apt-repository universe
$ sudo apt-get install git gitk git-gui curl xz-utils

These commands also installs git's graphical front end (git gui) and revision history browser (gitk).

Install depot_tools

To get started, follow the initial instructions at install depot_tools. You only need to clone the repo & setup your PATH -- the rest of the document is for browser developers.

This step is required so that you can use the repo to get/sync the source code, as well as other CrOS specific tools.

Tweak your sudoers configuration

You can tweak your sudoers configuration to make sudo request your password less frequently as described in Making sudo a little more permissive.

Set locale

These may not be needed if you are building on a system that you already use, however if you have a clean instance on GCE, you'll need to set a better locale. For example, on Debian on GCE, do:

(outside)
$ sudo apt-get install locales
$ sudo dpkg-reconfigure locales

When running dpkg-reconfigure locales, choose a language with UTF-8, e.g. en_US.UTF-8. For this change to take effect, you will need to log out and back in (closing all terminal windows, tmux/screen sessions, etc.).

Configure git

Setup git now. If you don't do this, you may run into errors/issues later. Replace you@example.com and Your Name with your information:

(outside)
$ git config --global user.email "you@example.com"
$ git config --global user.name "Your Name"

Get credentials to access source repos

Follow the Gerrit guide to get machine access credentials for the source repos.

This will also set up your code review account(s), which you can use to upstream changes back to ChromiumOS. This will be discussed in more detail in the "Making changes to packages whose source code is checked into ChromiumOS git repositories" section.

Double-check that you are running a 64-bit architecture

Run the following command:

(outside)
$ uname -m

You should see the result: x86_64

If you see something else (for example, i686, which means you are on a 32-bit machine or a 64-bit machine running a 32-bit OS) then you won't be able to build ChromiumOS. The project would happily welcome patches to fix this.

Verify that your default file permissions (umask) setting is correct

Sources need to be world-readable to properly function inside the chroot (described later). For that reason, the last digit of your umask should not be higher than 2, e.g. 002 or 022. Many distros have this by default; Ubuntu, for instance, does not. It is essential to put the following line into your ~/.bashrc file before you checkout or sync your sources.

(outside)
$ umask 022

You can verify that this works by creating any file and checking if its permissions are correct.

(outside)
$ touch ~/foo
$ ls -la ~/foo
-rw-r--r-- 1 user group 0 2012-08-30 23:09 /home/user/foo

Get the Source

Decide where your source will live

ChromiumOS developers commonly put their source code in ~/chromiumos. If you feel strongly, put your own source elsewhere, but note that all commands in this document assume that your source code is in ~/chromiumos.

Create the directory for your source code with this command:

(outside)
$ mkdir -p ~/chromiumos

IMPORTANT NOTE: Please ensure the backing filesystem for the ChromiumOS code is compatible, as detailed in the next section.

Compatible Filesystems

Some filesystems are not compatible with the ChromiumOS build system, and as such, the code (and chroot and out directory) should not be hosted on such filesystems. Below lists the known compatibility issues:

Filesystem Compatible
ext4 Yes
nfs No
btrfs Yes, but only without compression or other features

If your home directory is not backed by a compatible filesystem, then wherever you place your source, you can still add a symbolic link to it from your home directory (this is suggested), like so:

(outside)
$ mkdir -p /usr/local/path/to/source/chromiumos
$ ln -s /usr/local/path/to/source/chromiumos ~/chromiumos

Get the source code

ChromiumOS uses repo to sync down source code. repo is a wrapper for the git that helps deal with a large number of git repositories. You already installed repo when you installed depot_tools above.

The repo init command below uses a feature known as sync to stable; the version of the source that you will sync to is 4-10 hours old but is verified by our CI system.

Public:

(outside)
$ cd ~/chromiumos
$ repo init -u https://chromium.googlesource.com/chromiumos/manifest -b stable
$ repo sync -j4

*** note Note: If you are using public manifest with devices that have restricted binary prebuilts, such as GPU drivers on ARM devices, you'll have to explicitly accept licenses. Read more at https://www.chromium.org/chromium-os/licensing/building-a-distro/

Googlers/internal manifest:

(outside)
$ cd ~/chromiumos
$ repo init -u https://chrome-internal.googlesource.com/chromeos/manifest-internal -b stable
$ repo sync -j4

*** note Note: -j4 tells repo to concurrently sync up to 4 repositories at once. You can adjust the number based on how fast your internet connection is. For the initial sync, it's generally requested that you use no more than 8 concurrent jobs. (For later syncs, when you already have the majority of the source local, using -j16 or so is generally okay.)

*** note Note: If you are on a slow network connection or have low disk space, you can use the -g minilayout option. This starts you out with a minimum amount of source code. This isn't a particularly well tested configuration and has been known to break from time-to-time, so we usually recommend against it.

Sync to stable

As of October 2023, we recommend using stable (instead of tip-of-tree/ToT/main) for development. The stable ref points to a version of the source that is generally 4-10 hours old compared to ToT but is verified by our CI system.

If you have previously run repo init without the -b stable, you can convert an existing checkout to sync to stable:

(outside)
$ repo init -b stable
$ repo sync

The stable ref updates anytime snapshot-orchestrator passes all build and unittest stages for all build targets (it ignores VM/hardware tests so that stable is updated more frequently). stable will not update anytime ToT is broken, so if ToT is broken for a long period of time, stable could be much older than 4-10 hours.

*** note Note: Run repo init -b main to switch back to ToT. You would want to do this if you want to see a change that just landed on ToT and don't want to wait for the stable ref to be updated to include the change you are interested in building off of.

*** note Note: You can also sync to a specific version, for example, R117-15550. Please refer to Switch repo to the snapshot.

Optionally add Google API keys

*** note Note: If you plan to build the image with Chrome, you can skip this step. These keys are only necessary when you build the image with Chromium and need to use features that access Google APIs (signing in, translating web pages, geolocation, etc).

You will need to have keys (see API Keys) either in your include.gypi, or in a file in your home directory called ".googleapikeys". If either of these file are present for step 1 of building (below) they will be included automatically. If you don't have these keys, these features of Chromium will be quietly disabled.

Branch Builds

If you want to build on a branch, pass the branch name to repo init (e.g: repo init -u <URL> [-g minilayout] -b release-R80-12739.B).

When you use repo init you will be asked to confirm your name, email address, and whether you want color in your terminal. This command runs quickly. The repo sync command takes a lot longer.

More info can be found in the working on a branch page.

Make sure you are authorized to access Google Storage (GS) buckets

Building and testing ChromiumOS requires access to Google Storage. This is done via gsutil. Once configured, an authorization key is placed in ~/.boto. Every time you access the chroot via cros_sdk, the .boto file is copied to the chroot. If you run gsutil inside the chroot, it will configure the key in the chroot version of ~/.boto, but every time you re-run cros_sdk, it will overwrite the ~/.boto file in the chroot.

Authenticate for remote Bazel caching with RBE, if applicable

Some ChromiumOS packages use Bazel to build, which can interact with a remote cache such as RBE for faster builds. If your organization (Google, and maybe eventually others) has enabled remote caching in RBE for your Bazel-based builds, you'll need to install the gcloud CLI (https://cloud.google.com/sdk/docs/install, or go/installgcloud for Googlers) and run:

(outside)
$ gcloud auth application-default login
$ gcloud auth application-default set-quota-project chromeos-bot

This will store authentication credentials in ~${HOME}/.config/gcloud that will be mapped into the chroot, and which will allow Bazel executions within your builds to authenticate as you to RBE to access the remote cache it hosts.

More details about configuring remote caching for non-Google organizations is available at bazel_remote_caching.

Building ChromiumOS

Create a chroot

To make sure everyone uses the same exact environment and tools to build ChromiumOS, all building is done inside a chroot. This chroot is its own little world: it contains its own compiler, its own tools (its own copy of bash, its own copy of sudo), etc. Now that you've synced down the source code, you need to create this chroot. Assuming you're already in ~/chromiumos (or wherever your source lives), the command to download and install the chroot is:

(outside)
$ cros_sdk --create

If this does not work, make sure you've added the depot_tools directory to your PATH already (as was needed above with using repo).

This will download and setup a prebuilt chroot from ChromiumOS mirrors (under 400MB). If you prefer to build it from source, or have trouble accessing the servers, use cros_sdk --bootstrap. Note that this will also enter the chroot. If you prefer to build only, use --download.

The command with --bootstrap takes about half an hour to run on a four core machine. It compiles quite a bit of software, which it installs into your chroot, and downloads some additional items (around 300MB). While it is building you will see a regular update of the number of packages left to build. Once the command finishes, the chroot will take up total disk space of a little over 3GB.

The chroot lives by default at ~/chromiumos/chroot. Inside that directory you will find system directories like /usr/bin and /etc. These are local to the chroot and are separate from the system directories on your machine. For example, the chroot has its own version of the ls utility. It will be very similar, but it is actually a different binary than the normal one you use on your machine.

IMPORTANT NOTES:

SIDE NOTES:

Select a board

Building ChromiumOS produces a disk image (usually just called an "image") that can be copied directly onto the boot disk of a computer intended to run ChromiumOS. Depending on the specifics of that computer, you may want different files in the disk image. For example, if your computer has an ARM processor, you'll want to make sure that all executables in the image are compiled for the ARM instruction set. Similarly, if your computer has special hardware, you'll want to include a matching set of device drivers.

Different classes of computers are referred to by ChromiumOS as different target "boards". The following are some example boards:

To list all known boards in your checkout, run this command:

(outside)
$ cros query boards

You need to choose a board for your first build. Be aware that the generic images may not work well (or not at all) when run on official hardware. Don't worry too much about this choice, though – you can always build for another board later. If you want a list of known boards, you can look in ~/chromiumos/src/overlays.

Each command in the build processes takes a --board parameter. To facilitate this, it can be helpful to keep the name of the board in a shell variable. This is not strictly necessary, but if you do this, you can simply copy and paste the commands below into your terminal program. Enter the following inside your shell:

(outside)
$ BOARD=<your pick of board>

This setting only holds while you stay in the chroot. If you leave and come back, you need to specify this setting again.

Build the packages for your board

To build all the packages for your board, run the following command:

(outside)
$ cros build-packages --board=${BOARD}

This step is the rough equivalent of make all in a standard Makefile system. This command handles incremental builds; you should run it whenever you change something and need to rebuild it (or after you run repo sync).

Normally, the cros build-packages command builds the stable version of a package (i.e. from committed git sources), unless you are working on a package (with cros workon). If you are working on a package, cros build-packages will build using your local sources. See below for information about cros workon.

SIDE NOTES:

Build a disk image for your board

Once the cros build-packages step is finished, you can build a ChromiumOS-base developer image by running the command below:

(outside)
$ cros build-image --board=${BOARD} --no-enable-rootfs-verification test

The arguments for cros build-image specify what type of build you want. A test image (in the example above) has additional test-specific packages and also accepts incoming SSH connections. It is more convenient to use test images, but developers could also build developer images. A developer image provides a ChromiumOS-based image with additional developer packages. To build it use dev instead of test. If building a test image, the password will be test0000. The --no-enable-rootfs-verification turns off verified boot allowing you to freely modify the root file system. The system is less secure using this flag, however, for rapid development you may want to set this flag. If you would like a more secure, locked-down version of ChromiumOS, then simply remove the --no-enable-rootfs-verification flag. Finally if you want just the pristine ChromiumOS-based image (closest to ChromeOS but not quite the same), pass in base rather than test or dev. Use cros build-image --help for more information.

The image produced by cros build-image will be located in ~/chromiumos/src/build/images/${BOARD}/versionNum/ (where versionNum will actually be a version number). The most recent image produced for a given board will be symlinked to ~/chromiumos/src/build/images/${BOARD}/latest.

At the end of cros build-image's output, it will print a command you can run to start the image in a cros vm virtual machine. It will print something like:

To run the image in a virtual machine, use:
cros vm --start --image-path=../build/images/amd64-generic/R111-15301.0.0-d2023_01_04_103804-a1/chromiumos_test_image.bin --board=amd64-generic

Remember this command for future use; see Running an image in a virtual machine.

IMPORTANT NOTE: It's up to you to delete old builds that you don't need. Every time you run cros build-image, the command creates files that take up to 8GB of space (!).

Installing ChromiumOS on your Device

Put your image on a USB disk

The easiest way to get your image running on your target computer is to put the image on a USB flash disk (sometimes called a USB key), and boot the target computer from the flash disk.

The first step is to disable auto-mounting of USB devices on your build computer as it may corrupt the disk image while it's being written. On systems that use GNOME or Cinnamon, run the following:

(outside)
$ gsettings set org.gnome.desktop.media-handling automount false
$ gsettings set org.gnome.desktop.media-handling automount-open false
$ gsettings set org.cinnamon.desktop.media-handling automount false
$ gsettings set org.cinnamon.desktop.media-handling automount-open false

Next, insert a USB flash disk (8GB or bigger) into your build computer. This disk will be completely erased, so make sure it doesn't have anything important on it. Wait ~10 seconds for the USB disk to register, then type the following command:

(outside)
$ cros flash usb:// ${BOARD}/latest

For more details on using this tool, see the CrOS Flash page.

When the cros flash command finishes, you can simply unplug your USB key and it's ready to boot from.

IMPORTANT NOTE: To emphasize again, cros flash completely replaces the contents of your USB disk. Make sure there is nothing important on your USB disk before you run this command.

SIDE NOTES:

Enter Developer Mode

See the Developer Mode documentation.

Boot from your USB disk

After enabling Developer Mode, you should set your system to boot from USB.

Let the device boot, login and open a shell (or switch to terminal 2 via Ctrl+Alt+F2).

Run the following command:

(device)
$ sudo crossystem

You should see dev_boot_usb equal to 0. Set it to 1 to enable USB boot:

(device)
$ sudo crossystem dev_boot_usb=1
$ sudo crossystem dev_boot_signed_only=0

Now reboot. On the white screen (indicating Developer Mode is enabled), plug-in the USB disk and press Ctrl+U (Debug Button Shortcuts).

Getting to a command prompt on ChromiumOS

You have the ability to login as the chronos user:

  1. After your computer has booted to the ChromiumOS login screen, press [ Ctrl ] [ Alt ] [ F2 ] to get a text-based login prompt. (On a Chromebook keyboard, [ F2 ] will appear as [ ⟳ ] or [ → ].)
  2. Log in with the chronos user and enter the password (usually test0000).

Because you built an image with developer tools, you also have an alternate way to get a terminal prompt. The alternate shell is a little nicer (in the very least, it keeps your screen from dimming on you), even if it is a little harder to get to. To use this alternate shell:

  1. Go through the standard ChromiumOS login screen (you'll need to setup a network, etc.) and get to the web browser. It's OK to login as guest.
  2. Press [ Ctrl ] [ Alt ] [ T ] to get the crosh shell.
  3. Use the shell command to get the shell prompt. NOTE: you don't need to enter the chronos password here, though you will still need the password if you want to use the sudo command.

Installing your ChromiumOS image to your hard disk

Once you've booted from your USB key and gotten to the command prompt, you can install your ChromiumOS image to the hard disk on your computer with this command:

(device)
$ /usr/sbin/chromeos-install

IMPORTANT NOTE: Installing ChromiumOS onto your hard disk will WIPE YOUR HARD DISK CLEAN.

Running an image in a virtual machine

Many times it is easier to simply run ChromiumOS in a virtual machine like QEMU. You can use the cros_vm command to start a VM with the previously built image.

When you start the VM, cros_vm will print out information about how to connect to the running image via SSH and VNC.

For VNC it will normally say VNC server running on ::1:5900 which means it's serving on localhost on the default VNC port (5900). You can connect to localhost with a VNC viewer to connect.

A good VNC client for Linux is the package tigervnc-viewer (available on at least Debian); its command line program is vncviewer. Note that before you use it, you should click Options → Misc → Show dot when no cursor. Other options include the VNC feature in ChromiumIDE's device management, and novnc, which makes the device accessible via a web browser.

***note SIDE NOTES:

Making changes to packages whose source code is checked into ChromiumOS git repositories

Now that you can build and run ChromiumOS, you're ready to start making changes to the code. For further hands on with making changes, you can check out the build codelab.

NOTE: If you skipped to this section without building your own system image, you may run into hard-to-fix dependency problems if you build your own versions of system packages and try to deploy them to a system image that was built by a builder. If you run into trouble, try going through the full Building ChromiumOS process first and installing your own system image.

Keep the tree green

Before you start, take a moment to understand Chromium's source management strategy of "keeping the tree green". For the ChromiumOS project, keeping the tree green means:

  1. Any new commits should not destabilize the build:
    • Images built from the tree should always have basic functionality working.
    • There may be minor functionality not working, and it may be the case, for example, that you will need to use Terminal to fix or work around some of the problems.
  2. If you must introduce unstable changes to the tree (which should happen infrequently), you should use parameterization to hide new, unstable functionality behind a flag that's turned off by default. The ChromiumOS team leaders may need to develop mechanisms for parameterizing different parts of the system (such as the init script).
  3. Internal "dev channel" releases will be produced directly from the tree, with a quick branch to check-point the release version. Any fixes required for a release will be pulled from the tree, avoiding merges back to tree.

This strategy has many benefits, including avoiding separate build trains for parallel development (and the cost of supporting such development), as well as avoiding large, costly merges from forked branches.

SIDE NOTE: "Keep the tree green" means something a bit different for ChromiumOS than for Chromium, which is much further along in its life cycle.

The steps in this section describe how to make changes to a ChromiumOS package whose source is checked into the ChromiumOS source control system. Specifically, this is a package where:

You can see a list of all such packages by running the following command:

(outside)
$ cros workon --board=${BOARD} --all list

Run cros workon start

The first thing you need to do is to mark the package as active. Use the command below, replacing ${PACKAGE_NAME} with your package name (e.g., chromeos-wm):

(outside)
$ cros workon --board=${BOARD} start ${PACKAGE_NAME}

This command:

Run repo sync

After running cros workon, sync down the sources. This is critical if you're using the minilayout, but is probably a good idea in any case to make sure that you're working with the latest code (it'll help avoid merge conflicts later). Run the command below anywhere under your ~/chromiumos directory:

(outside)
$ repo sync

*** note Note: Make sure your umask is set to a supported value (e.g. 022); otherwise, you may end up with bad file permissions in your source tree.

Find out which ebuilds map to which directories

The cros workon tool can help you find out what ebuilds map to each directory. You can view a full list of ebuilds and directories using the following command:

(outside)
$ cros workon --board=${BOARD} --all info

If you want to find out which ebuilds use source code from a specific directory, you can use grep to find them. For example:

(outside)
$ cros workon --board=${BOARD} --all info | grep platform/ec

This returns the following output:

chromeos-base/ec-utils chromiumos/platform/ec src/platform/ec

This tells you the following information:

  1. The name of the ebuild is chromeos-base/ec-utils
  2. The path to the git repository on the server is chromiumos/platform/ec
  3. The path to the source code on your system is src/platform/ec

You can similarly find what source code is associated with a given ebuild by grepping for the ebuild name in the list.

To find out where the ebuild lives:

(outside)
$ cros_sdk equery-${BOARD} which ${PACKAGE_NAME}

As an example, for PACKAGE_NAME=ec-utils, the above command might display:

/home/.../chromiumos/src/third_party/chromiumos-overlay/chromeos-base/ec-utils/ec-utils-9999.ebuild

SIDE NOTE: If you run the same command without running cros workon first, you can see the difference:

/home/.../chromiumos/src/third_party/chromiumos-overlay/chromeos-base/ec-utils/ec-utils-0.0.1-r134.ebuild

Create a branch for your changes

Since ChromiumOS uses repo/git, you should always create a local branch whenever you make changes.

First, find the source directory for the project you just used cros workon on. This isn't directly related to the project name you used with cros workon. (TODO: This isn't very helpful - someone with more experience, actually tell us how to find it reliably? --Meredydd)

cd into that directory, in particular the "files/" directory in which the actual source resides. In the command below, replace ${BRANCH_NAME} with a name that is meaningful to you and that describes your changes (nobody else will see this name):

(outside)
$ repo start ${BRANCH_NAME} .

The branch that this creates will be based on the remote branch (TODO: which one? --Meredydd). If you've made any other local changes, they will not be present in this branch.

Make your changes

You should be able to make your changes to the source code now. To incrementally compile your changes, use either cros_workon_make or emerge-${BOARD}. To use cros_workon_make, run

(outside)
$ cros_sdk cros_workon_make --board=${BOARD} ${PACKAGE_NAME}

This will build your package inside your source directory. Change a single file, and it will rebuild only that file and re-link. If your package contains test binaries, using

(outside)
$ cros_sdk cros_workon_make --board=${BOARD} ${PACKAGE_NAME} --test

will build and run those binaries as well. In case your tests have out-of-package dependencies, you'll first need to run

(outside)
$ cros_sdk USE=test emerge-${BOARD} ${PACKAGE_NAME}

to pull them in.

Call cros_workon_make --help to see other options that are supported.

You probably want to get your changes onto your device now. You need to install the changes you made by using

(outside)
$ cros_sdk cros_workon_make --board=${BOARD} ${PACKAGE_NAME} --install

You can then rebuild an image with cros build-image and reimage your device.

Alternatively, you can build your package using emerge-${BOARD} and quickly install it to the device by using cros deploy.

For example, if you want to build ec-utils to test on your device, use

(outside)
$ cros_sdk emerge-${BOARD} ec-utils

To install the package to the device, use

(outside)
$ cros deploy ${IP} ec-utils

Set your editor

Many of the commands below (in particular git) open up an editor. You probably want to run one of the three commands below depending on your favorite editor.

If you're not a *nix expert, nano is a reasonable editor:

$ export EDITOR='nano'

If you love vi:

$ export EDITOR='vi'

If you love emacs (and don't want an X window to open up every time you do something):

$ export EDITOR='emacs -nw'

You should probably add one of those lines to your .bashrc (or similar file) too.

Submit changes locally

When your changes look good, commit them to your local branch using git. Full documentation of how to use git is beyond the scope of this guide, but you might be able to commit your changes by running something like the command below from the project directory:

(outside)
$ git commit -a

The git commit command brings up a text editor. You should describe your changes, save, and exit the editor. Note that the description you provide is only for your own use. When you upload your changes for code review, the repo upload command grabs all of your previous descriptions, and gives you a chance to edit them.

Upload your changes and get a code review

Check out our Gerrit Workflow guide for details on our review process.

Clean up after you're done with your changes

After you're done with your changes, you're ready to clean up. The most important thing to do is to tell cros workon that you're done by running the following command:

(outside)
$ cros workon --board=${BOARD} stop ${PACKAGE_NAME}

This command tells cros workon to stop forcing the -9999.ebuild and to stop forcing a build from source every time.

If you're using the minilayout, doing a cros workon stop will not remove your source code. The code will continue to stay on your hard disk and get synced down.

Making changes to non-cros-workon-able packages

If you want to make changes to something other than packages which source is checked into the ChromiumOS source control system, you can follow the instructions in the previous section, but skip the cros workon step. Note specifically that you still need to run repo start to Create a branch for your changes.

The types of changes that fall into this category include:

Adding small patches to existing packages

When you need to add small patches to existing packages whose source code is not checked into a ChromiumOS git repository (e.g. it comes from portage, and is not a cros workon-able package), you need to do the following:

First, find the package ebuild file under third_party/chromiumos-overlay.

Then, create a patch file from the exact version of the package that is used by the current ebuild. If other patches are already in the ebuild, you'll want to add your patch LAST, and build the patch off of the source that has already had the existing patches applied (either do it by hand, or set FEATURES=noclean and build your patch off of the temp source). Note that patch order is significant, since the ebuild expects each patch line number to be accurate after the previous patch is applied.

Place your patch in the "files" subdir of the directory that contains the ebuild file (e.g. third_party/chromiumos-overlay/dev-libs/mypackage/files/mypackage-1.0.0-my-little-patch.patch).

Then, in the prepare() section of the ebuild (create one if it doesn't exist), add an epatch line:

$ epatch "${FILESDIR}"/${P}-my-little-patch.patch

Lastly, you'll need to bump the revision number in the name of the ebuild file (or symlink) so the build system picks up the change. The current wisdom is that the ebuild file should be symlinked instead of being renamed. For example, if the original ebuild file is "mypackage-1.0.0.ebuild", you should create a "mypackage-1.0.0-r1.ebuild" symbolic link that points at the original ebuild file. If that symlink already exists, create the next higher "rN" symlink.

Making changes to the way that the chroot is constructed

TODO: This section is currently a placeholder, waiting for someone to fill it in. However, a few notes:

Building an individual package

TODO: Document this better, and add the new cros_workon_make.

SIDE NOTE: To build an individual portage package, for a particular board, use emerge-${BOARD}.

For example, if you want to build dash to test on your device:

(outside)
$ cros_sdk emerge-${BOARD} dash

To install the package to the device, see cros deploy.

SIDE NOTE:

Making changes to the Chromium web browser on ChromiumOS

If you want to make modifications to the Chromium web browser and quickly deploy your changes to an already-built ChromiumOS image, see the Simple Chrome Workflow. Or if you have a pending Chromium change that you'd like tested remotely on CrOS's CQ before submitting it, see the chromeos-uprev-tester trybot.

Local Debugging

Debugging both x86 and non-x86 binaries on your workstation

If you build your projects incrementally, write unit tests and use them to drive your development, you may want to debug your code without shipping it over to a running device or VM.

gdb-${BOARD} sets up gdb in your board sysroot and ensures that gdb is using the proper libraries, debug files, etc. for debugging, allowing you to run your target-compiled binaries.

It should already be installed in your chroot. If you do not have the script, update your repository to get the latest changes, then re-build your packages:

(outside)
$ repo sync

(outside)
$ cros build-packages --board=...

This should install gdb-${BOARD} in the /usr/local/bin directory inside the chroot. These board-specific gdb wrapper scripts correctly handle both local and remote debugging (see next section for more information on remote debugging). When used for local debugging, these scripts will run inside a special chroot-inside-your-chroot, rooted in the board's sysroot. For example if you are using gdb-lumpy, it will run inside a chroot based entirely in your /build/lumpy sysroot. The libraries that it will load and use are the libraries in the sysroot, i.e. the target libraries for the board; the gdb binary it will use is the gdb binary in that tree. While debugging with gdb-lumpy (for local debugging), you will not be able to see/access any files outside of the /build/lumpy tree. While for the most part this is very good, as it ensures the correct debugging environment, it does mean that if you want to use this script to debug a lumpy binary, such as a unit test, that you built outside of the /build/lumpy tree, you will need to copy the binary to the /build/lumpy tree first. Also, if you want the debugger to be able to see the source files when debugging, you will need to make sure they exist inside the /build/lumpy tree as well (see example below).

IMPORTANT NOTE 1: Local and remote debugging functionality are combined in this single script. Some of the options shown below only work for remote debugging.

IMPORTANT NOTE 2: When doing local debugging of x86 binaries, they will try to execute on your desktop machine (using the appropriate libraries and gdb binaries). It is possible that for some x86 boards, the binaries may use instructions not understood by your hardware (particularly some vector instructions), in which case you will need to do remote debugging with the actual hardware instead.

IMPORTANT NOTE 3: You can use this script with *some* debugging functionality for local debugging of non-x86 binaries. The script loads QEMU and runs the non-x86 binaries in QEMU. However QEMU has some unfortunate limitations. For example you can "set" breakpoints in the binary (to see what addresses correspond to locations in the source), examine the source or assembly code, and execute the program. But QEMU does not actually hit the breakpoints, so you cannot suspend execution in the middle when running under QEMU. For full debugging functionality with non-x86 binaries, you must debug them remotely running on the correct hardware (see next section on remote debugging). You can see this in the example below, where gdb-daisy does not actually stop at the breakpoint it appears to set, although it does correctly execute the program.

(outside)
$ cros_sdk gdb-daisy -h

usage: cros_gdb [-h]
                [--log-level {fatal,critical,error,warning,notice,info,debug}]
                [--log_format LOG_FORMAT] [--debug] [--nocolor]
                [--board BOARD] [-g GDB_ARGS] [--remote REMOTE] [--pid PID]
                [--remote_pid PID] [--no-ping] [--attach ATTACH_NAME] [--cgdb]
                [binary-to-be-debugged] [args-for-binary-being-debugged]

Wrapper for running gdb.

This handles the fun details like running against the right sysroot, via
QEMU, bind mounts, etc...

positional arguments:
  inf_args              Arguments for gdb to pass to the program being
                        debugged. These are positional and must come at the
                        end of the command line. This will not work if
                        attaching to an already running program.
...

$ cros_sdk gdb-daisy /bin/grep shebang /bin/ls
15:51:06: INFO: run: file /build/daisy/bin/grep
Reading symbols from /bin/grep...Reading symbols from /usr/lib/debug/bin/grep.debug...done.
done.
(daisy-gdb) b main
Breakpoint 1 at 0x2814: file grep.c, line 2111.
(daisy-gdb) disass main
Dump of assembler code for function main:
   0x00002814 <+0>: ldr.w r2, [pc, #3408] ; 0x3568 <main+3412>
   0x00002818 <+4>: str.w r4, [sp, #-36]!
   0x0000281c <+8>: movs r4, #0
   0x0000281e <+10>: strd r5, r6, [sp, #4]
   0x00002822 <+14>: ldr.w r3, [pc, #3400] ; 0x356c <main+3416>
   0x00002826 <+18>: movs r5, #2
   0x00002828 <+20>: strd r7, r8, [sp, #12]
...
(daisy-gdb) run
Starting program: /bin/grep shebang /bin/ls
qemu: Unsupported syscall: 26
#!/usr/bin/coreutils --coreutils-prog-shebang=ls
qemu: Unsupported syscall: 26
During startup program exited normally.
(daisy-gdb) quit

Note in the example above that, like "regular" gdb when given --args, you can pass the arguments for the program being debugged to the gdb wrapper script just by adding them to the command line after the name of the program being debugged (except that --args isn't needed).

The commands below show how to copy your incrementally-compiled unit test binary and source file(s) to the appropriate sysroot and then start gdb with that binary (using the correct libraries, etc).

(outside)
$ cd ~/chromiumos/chroot/build/lumpy/tmp/portage
$ mkdir shill-test
$ cd shill-test
$ cp <path-to-binary>/shill_unittest .
$ cp <path-to-src>/shill_unittest.cc .
$ cros_sdk gdb-lumpy
(gdb-lumpy) directory /tmp/portage/shill-test # Tell gdb to add /tmp/portage/shill-test to the paths it searches for source files
(gdb-lumpy) file ./shill_unittest

If gdb is still looking for the source file in the wrong directory path, you can use set substitute-path <from> <to> inside gdb to help it find the right path (inside your sysroot) for searching for source files.

Printing stack traces at runtime

See Stack Traces for how to print stack traces at runtime.

Remote Debugging

Setting up remote debugging by hand

If you want to manually run through all the steps necessary to set up your system for remote debugging and start the debugger, see Remote Debugging in ChromiumOS.

Automated remote debugging using gdb-${BOARD} script (gdb-lumpy, gdb-daisy, gdb-parrot, etc)

gdb-${BOARD} is a script that automates many of the steps necessary for setting up remote debugging with gdb. It should already be installed in your chroot. If you do not have the script, update your repository to get the latest changes, then re-build your packages:

(outside)
$ repo sync

(outside)
$ cros build-packages --board=...

This should install gdb_remote in the /usr/bin directory inside the chroot. The gdb-${BOARD} script takes several options. The most important ones are mentioned below.

--gdb_args (-g) are arguments to be passed to gdb itself (rather than to the program gdb is debugging). If multiple arguments are passed, each argument requires a separate -g flag.

E.g gdb-lumpy --remote=123.45.67.765 -g "-core=/tmp/core" -g "-directory=/tmp/source"

--remote is the ip_address or name for your Chromebook, if you are doing remote debugging. If you omit this argument, the assumption is you are doing local debugging in the sysroot on your desktop (see section above). if you are debugging in the VM, then you need to specify either :vm: or localhost:9222.

--pid is the PID of a running process on the remote device to which you want gdb/gdbserver to attach.

--attach is the name of the running process on the remote device to which you want gdb/gdbserver to attach. If you want to attach to the Chrome browser itself, there are three special names you can use: browser will attach to the main browser process; gpu-process will attach to the GPU process; and renderer will attach to the renderer process if there is only one. If there is more than one renderer process --attach=renderer will return a list of the renderer PIDs and stop.

To have gdb/gdbserver start and attach to a new (not already running) binary, give the name of the binary, followed by any arguments for the binary, at the end of the command line:

(outside)
$ cros_sdk gdb-daisy --remote=123.45.67.809 /bin/grep "test" /tmp/myfile

When doing remote debugging you *must* use the --pid or the --attach option, or specify the name of a new binary to start. You cannot start a remote debugging session without having specified the program to debug in one of these three ways.

When you invoke gdb-${BOARD} --remote=..., it will connect to the notebook or VM (automatically setting up port-forwarding on the VM), make sure the port is entered into the iptables, and start up gdbserver, using the correct port and binary, either attaching to the binary (if a remote PID or name was specified) or starting up the binary. It will also start the appropriate version of gdb (for whichever type of board you are debugging) on your desktop and connect the gdb on your desktop to the gdbserver on the remote device.

Edit/Debug cycle

If you want to edit code and debug it on the DUT you can follow this procedure

(outside)
$ cros_sdk CFLAGS="-ggdb" FEATURES="noclean" emerge-${BOARD} -v sys-apps/mosys
$ cros deploy --board=${BOARD} ${IP} sys-apps/mosys
$ cros_sdk gdb-${BOARD} --cgdb --remote "${IP}" \
  -g "--eval-command=directory /build/${BOARD}/tmp/portage/sys-apps/mosys-9999/work/" \
  /usr/sbin/mosys -V

This will build your package with debug symbols (assuming your package respects CFLAGS). We need to use the noclean feature so that we have access to the original sourcecode that was used to build the package. Some packages will generate build artifacts and have different directory structures then the tar/git repo. This ensures all the paths line up correctly and the source code can be located. Ideally we would use the installsources feature, but we don't have support for the debugedit package (yet!). Portage by default will strip the symbols and install the debug symbols in /usr/lib/debug/. gdb-${BOARD} will handle setting up the correct debug symbol path. cros deploy will then update the rootfs on the DUT. We pass the work directory into gdb-${BOARD} so that cgdb can display the sourcecode inline.

Quick primer on cgdb:

Examples of debugging using the gdb-${BOARD} script

Below are three examples of using the board-specific gdb wrapper scripts to start up debugging sessions. The first two examples show connecting to a remote Chromebook. The first one automatically finds the browser's running GPU process, attaches gdbserver to the running process, starts gdb on the desktop, and connects the local gdb to gdbserver. It also shows the user running the bt (backtrace) command after gdb comes up. The second example shows the user specifying the PID of a process on the Chromebook. Again the script attaches gdbserver to the process, starts gdb on the desktop, and connects the two. The third example shows the user connecting to the main browser process in ChromeOS running in a VM on the user's desktop. For debugging the VM, you can use either --remote=:vm: or --remote=localhost:9222 (:vm: gets translated into localhost:9222).

Example 1:

(outside)
$ cros_sdk gdb-lumpy --remote=123.45.67.809 --attach=gpu-process

14:50:07: INFO: run: ping -c 1 -w 20 123.45.67.809
14:50:09: INFO: run: file /build/lumpy/opt/google/chrome/chrome
14:50:10: INFO: run: x86_64-cros-linux-gnu-gdb --quiet '--eval-command=set sysroot /build/lumpy' '--eval-command=set solib-absolute-prefix /build/lumpy' '--eval-command=set solib-search-path /build/lumpy' '--eval-command=set debug-file-directory /build/lumpy/usr/lib/debug' '--eval-command=set prompt (lumpy-gdb) ' '--eval-command=file /build/lumpy/opt/google/chrome/chrome' '--eval-command=target remote localhost:38080'
Reading symbols from /build/lumpy/opt/google/chrome/chrome...Reading symbols from/build/lumpy/usr/lib/debug/opt/google/chrome/chrome.debug...done.
(lumpy-gdb) bt
#0  0x00007f301fad56ad in poll () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007f3020d5787c in g_main_context_poll (priority=2147483647, n_fds=3,   fds=0xdce10719840, timeout=-1, context=0xdce1070ddc0) at gmain.c:3584
#2  g_main_context_iterate (context=context@entry=0xdce1070ddc0,block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>)  at gmain.c:3285
#3  0x00007f3020d5798c in g_main_context_iteration (context=0xdce1070ddc0may_block=1) at gmain.c:3351
#4  0x00007f30226a4c1a in base::MessagePumpGlib::Run (this=0xdce10718580, delegate=<optimized out>) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/base/message_loop/message_pump_glib.cc:309
#5  0x00007f30226666ef in base::RunLoop::Run (this=this@entry=0x7fff72271af0) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/base/run_loop.cc:55
#6  0x00007f302264e165 in base::MessageLoop::Run (this=this@entry=0x7fff72271db0) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/base/message_loop/message_loop.cc:307
#7  0x00007f30266bc847 in content::GpuMain (parameters=...) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/content/gpu/gpu_main.cc:365
#8  0x00007f30225cedee in content::RunNamedProcessTypeMain (process_type=..., main_function_params=..., delegate=0x7fff72272380 at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/content/app/content_main_runner.cc:385
#9  0x00007f30225cef3a in content::ContentMainRunnerImpl::Run (this=0xdce106fef50) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/content/app/content_main_runner.cc:763
#10 0x00007f30225cd551 in content::ContentMain (params=...) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/content/app/content_main.cc:19
#11 0x00007f3021fef02a in ChromeMain (argc=21, argv=0x7fff722724b8) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/chrome/app/chrome_main.cc:66
#12 0x00007f301fa0bf40 in __libc_start_main (main=0x7f3021fee760 <main(int, char const**)>, argc=21, argv=0x7fff722724b8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>,stack_end=0x7fff722724a8) at libc-start.c:292
#13 0x00007f3021feee95 in _start ()
(lumpy-gdb)

Example 2:

(outside)
$ cros_sdk gdb-daisy --pid=626 --remote=123.45.98.765
14:50:07: INFO: run: ping -c 1 -w 20 123.45.98.765
14:50:09: INFO: run: file /build/daisy/usr/sbin/cryptohomed
14:50:10: INFO: run: armv7a-cros-linux-gnueabi-gdb --quiet '--eval-command=set sysroot /build/daisy' '--eval-command=set solib-absolute-prefix /build/daisy' '--eval-command=set solib-search-path /build/daisy' '--eval-command=set debug-file-directory /build/daisy/usr/lib/debug' '--eval-command=set prompt (daisy-gdb) ' '--eval-command=file /build/daisy/usr/sbin/cryptohomed' '--eval-command=target remote localhost:38080'
Reading symbols from /build/daisy/usr/sbin/cryptohomed...Reading symbols from/build/daisy/usr/lib/debug/usr/bin/cryptohomed.debug...done.
(daisy-gdb)

Example 3:

(outside)
$ cros_sdk gdb-lumpy --remote=:vm: --attach=browser
15:18:28: INFO: run: ping -c 1 -w 20 localhost
15:18:31: INFO: run: file /build/lumpy/opt/google/chrome/chrome
15:18:33: INFO: run: x86_64-cros-linux-gnu-gdb --quiet '--eval-command=setsysroot /build/lumpy' '--eval-command=set solib-absolute-prefix /build/lumpy' '--eval-command=set solib-search-path /build/lumpy' '--eval-command=set debug-file-directory /build/lumpy/usr/lib/debug' '--eval-command=set prompt (lumpy-gdb) ' '--eval-command=file /build/lumpy/opt/google/chrome/chrome' '--eval-command=target remote localhost:48062'
Reading symbols from /build/lumpy/opt/google/chrome/chrome...Reading symbols from /build/lumpy/usr/lib/debug/opt/google/chrome/chrome.debug...done.
done.
Remote debugging using localhost:48062
...
(lumpy-gdb)

If you find problems with the board-specific gdb scripts, please file a bug (crbug.com/new) and add 'build-toolchain' as one of the labels in the bug.

Troubleshooting

I lost my developer tools on the stateful partition, can I get them back?

This happens sometimes because the security system likes to wipe out the stateful partition and a lot of developer tools are in /usr/local/bin. You may be able restore some of them using the dev-install tool. See the dev-install docs for more information about its "Base Image" workflow.

Disabling Enterprise Enrollment

Some devices may be configured with a policy that only allows logging in with enterprise credentials, which will prevent you from logging in with a non-enterprise Google account (e.g., foo@gmail.com). To disable the enterprise enrollment setting:

***note NOTE: The enterprise policy can also prevent transitioning to Developer Mode, in which case you won't be able to perform the above commands.

Running Tests

ChromiumOS integration (or "functional") tests are written using the Tast or Autotest frameworks.

Set up SSH connection between chroot and DUT

To run automated tests on your DUT, you first need to set up SSH connection between chroot on your workstation and the DUT. See this document for how to set it up.

For Googlers

If you are a Google engineer using a corp workstation, you may be required some extra settings, depending on where your DUT is.

Tast

Tast is a Go-based integration testing framework with a focus on speed, ease-of-use, and maintainability. Existing Autotest-based tests that run on the ChromeOS Commit Queue are being ported to Tast and decommissioned as of 2018 Q4. Please strongly consider using Tast when writing new integration tests (but be aware that not all functionality provided by Autotest is available in Tast; for example, tests that use multiple devices simultaneously when running are not currently supported).

Here are some starting points for learning more about Tast:

Please contact the public tast-users mailing list if you have questions.

Autotest

Autotest is a Python-based integration testing framework; the codebase is also responsible for managing the ChromeOS lab that is used for hardware testing. ChromiumOS-specific Autotest information is available in the Autotest User Documentation.

Additional Autotest documentation:

Creating a normal image that has been modified for test

See Creating an image that has been modified for test for information about modifying a normal system image so that integration tests can be run on it.

Creating a recovery image that has been modified for test

After building a test image using cros build-image test as described above, you may wish to encapsulate it within a recovery image:

(outside)
$ cros_sdk ./mod_image_for_recovery.sh \
    --board=${BOARD} \
    --nominimize_image \
    --image ~/chromiumos/src/build/images/${BOARD}/latest/chromiumos_test_image.bin \
    --to ~/chromiumos/src/build/images/${BOARD}/latest/recovery_test_image.bin

If desired, you may specify a custom kernel with --kernel_image ${RECOVERY_KERNEL}.

You can write this recovery image out to the USB device like so:

(outside)
$ cros flash usb:// ~/chromiumos/src/build/images/${BOARD}/latest/recovery_test_image.bin

Note that there are some downsides to this approach which you should keep in mind.

Additional information

Entering a shell in the chroot

For commands which need executed in the SDK, we've previously prefixed all them with cros_sdk in this guide. However, you may find that getting a shell in the chroot is useful for debugging or some other advanced workflow. You can do so by running cros_sdk without any arguments:

(outside)
$ cros_sdk

This command will probably prompt you for your password for the sudo command (entering the chroot requires root privileges). Once the command finishes, that terminal is in the chroot and you'll be in the ~/chromiumos/src/scripts directory, where most build commands live. In the chroot you can only see a subset of the filesystem on your machine. However, through some trickery (bind mounts), you will have access to the whole src directory from within the chroot – this is so that you can build the software within the chroot.

Note in particular that the src/scripts directory is the same src/scripts directory found within the ChromiumOS directory you were in before you entered the chroot, even though it looks like a different location. That's because when you enter the chroot, the ~/chromiumos directory in the chroot is mounted such that it points to the main ChromiumOS directory ~/chromiumos outside the chroot. That means that changes that you make to the source code outside of the chroot immediately take effect inside the chroot.

Calling this will also install a chroot, if you don't have one yet, for example by not following the above.

While in the chroot you will see a special "(cr)" prompt to remind you that you are there:

(cr) ((...)) johnnyrotten@flyingkite ~/chromiumos/src/scripts $

You generally cannot run programs on your filesystem from within the chroot. For example, if you are using eclipse as an IDE, or gedit to edit a text file, you will need to run those programs outside the chroot. For lightweight editing, vi (not vim) and nano are available.

SIDE NOTES:

Inspecting a disk image

To inspect a disk image, such as one built with the cros build-image command as in section Build a disk image for your board, first launch an interactive Bash shell within the SDK chroot:

(outside)
$ cros_sdk

Then, mount the image and inspect the /tmp/m directory (here [...] denotes parts of the output omitted for brevity; (inside) denotes any commands executed within the SDK Bash shell):

(inside)
$ ./mount_gpt_image.sh --board=${BOARD} --safe --most_recent
[...]
[...] Image specified by [...] mounted at /tmp/m successfully.

If you built a test image, also make sure to add -i chromiumos_test_image.bin to this command.

The --safe option ensures you do not make accidental changes to the Root FS.

When you're done, unmount the image with:

(inside)
$ ./mount_gpt_image.sh --board=${BOARD} -u

Then exit the SDK chroot Bash shell.

Optionally, you can unpack the partition as separate files and mount them directly:

(outside)
$ cd ~/chromiumos/src/build/images/${BOARD}/latest
$ ./unpack_partitions.sh chromiumos_image.bin
$ mkdir -p rootfs
$ sudo mount -o loop,ro part_3 rootfs

This will do a loopback mount of the rootfs from your image to the location ~/chromiumos/src/build/images/${BOARD}/latest/rootfs in your chroot.

If you built with --no-enable-rootfs-verification you can omit the ro option to mount it read write.

If you built an x86 ChromiumOS image, you can probably even try chrooting into the image:

(outside)
$ sudo chroot ~/chromiumos/src/build/images/${BOARD}/latest/rootfs

This is a little hacky (the ChromiumOS rootfs isn't really designed to be a chroot for your host machine), but it seems to work pretty well. Don't forget to exit this chroot when you're done.

When you're done, unmount the root filesystem:

(outside)
$ sudo umount ~/chromiumos/src/build/images/${BOARD}/latest/rootfs

Working on SDK Packages

SDK packages are normally automatically updated from tarball when you enter the SDK, and there's generally no need to manually update SDK packages yourself, unless you're actually working on a change which makes a change to the SDK.

When making a change to the SDK, you must know that your change won't be generally deployed until the next tarball is generated, roughly every 12 hours. Thus, if your SDK change is needed for building board packages, you should separate the change into two CLs, and land the change which incorporates the SDK package change after the tarball uprevs.

If the package you're working on is a cros-workon package, you'll want to workon the package first:

(outside)
$ cros workon --host chromeos-base/my-package

Then, to test your SDK package changes locally, you can manually emerge the package:

(outside)
$ cros_sdk sudo emerge -guj --deep --newuse chromeos-base/my-package

Alternatively, you may update all the SDK packages, including your package:

(outside)
$ cros_sdk update_chroot --force

--force is required to say, "yes, I really want to update my SDK from the current pinned version." Historically, developers may have been used to calling update_chroot manually, even if they didn't have SDK package changes they made that they want to incorporate, and we added --force to train developers that they generally shouldn't need to call this command manually.

Documentation on this site

You now understand the basics of building, running, modifying, and testing ChromiumOS, but you've still got a lot to learn. Here are links to a few other pages on the chromium.org site that you are likely to find helpful (somewhat ordered by relevance):

External documentation

Below are a few links to external sites that you might also find helpful (somewhat ordered by relevance):