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Using third party code can save time and is consistent with our values - no need
to reinvent the wheel! We put all code that isn't written by Chromium developers
into //third_party (even if you end up modifying just a few functions). We do
this to make it easy to track license compliance, security patches, and supply
the right credit and attributions. It also makes it a lot easier for other
projects that embed our code to track what is Chromium licensed and what is
covered by other licenses.
By default, all code should be checked into //third_party, for the reasons given above. Other locations are only appropriate in a few situations and need explicit approval; don't assume that because there's some other directory with third_party in the name it's okay to put new things there.
To make sure the inclusion of a new third_party project makes sense for the Chromium project, you should first obtain Chrome ATL approval. Please include the following information in an email to [email protected]:
- Motivation of your project
- Design docs
- Additional checkout size
- If the increase is significant (e.g., 20+ MB), can we consider limiting the files to be checked in?
- Build time increase
- If the increase is significant (e.g., 30+ seconds), can we consider making this an optional build target?
- Binary size increase on Android (official builds)
- Any 16 KB increase on Android is flagged on the build bots and justification is needed.
- Binary size increase on Windows
- Is this library maintained on all platforms that we will use it on?
- If not, will the Chrome org be expected to maintain this for some or all platforms?
- Does it have any performance / memory implications (esp. on Android)? Was the library designed with intended use on Android?
- Do we really need the library? Is there any alternative such as an existing library already in Chromium? If introducing a library with similar functionality as existing, will it be easy for another developer to understand which should be used where? Will you commit to consolidating uses in Chromium and remove the alternative libraries?
- For desktop (Win/Mac/Linux/ChromeOS), does the dependency introduce closed source components (e.g., binaries, WASM binaries, obfuscated code)? If yes, please reach out to Chrome ATLs.
Googlers can access go/chrome-atls and review existing topics in g/chrome-atls, and can also come to office hours to ask questions.
Rust is allowed for third-party libraries as long as there is a business need, which includes the following:
- The Rust implementation is the best (e.g., speed, memory, lack of bugs) or only existing implementation available for the third-party library.
- The Rust implementation allows the operation to move to a higher privileged process, and this benefits the product by improving on guardrail metrics (e.g. through avoiding process startup, IPC overheads, or C++ memory-unsafety mitigations).
- The Rust implementation can meaningfully reduce our expected risk of (memory/crashes/undefined behavior) bugs, when compared to the existing third-party library and related C++ code required to use the library. We realize assessing risk is quite complex and very nuanced. If this is the criteria by which the third-party library is being added, [email protected] and [email protected] may ask for more data.
Support for third-party libraries written in Rust is in active development. If the library you wish to add is in Rust, reach out to [email protected] first.
The size of Chromium derived executables can impact overall performance of those binaries as they need to run on a wide range of devices including those with extremely limited RAM. Additionally, we have experience from Windows of the binary size impacting successful patch rate of updates as well as constraints from the Android Ecosystem where APKs included in the system image have hard limits on their size due to allocation size of the system partition. For more details and guidelines on size increases see //docs/speed/binary_size/binary_size_explainer.md and Googlers can additionally check go/chrome-binary-size
There are two common ways to depend on third-party code: you can reference a Git repo directly (via entries in the DEPS file) or you can check in a snapshot. The former is preferable in most cases:
- If you are actively developing in the upstream repo, then having the DEPS file include the upstream (that's been mirrored to GoB, see below) can be a way to include those changes into Chromium at a particular revision. The DEPS file will be updated to a new revision when you are ready to "roll" the new version into Chromium. This also avoids duplicate copies of the code showing up in multiple repos leading to contributor confusion.
- This interacts favorably with our upstream tracking automation. We automatically consume the upstream Git hashes and match them against a database of known upstreams to tracking drift between Chromium and upstream sources.
- This makes adding deps that don't need local changes easier. E.g. some of our automation automatically converts non-GN build rules into GN build rules without any additional CLs.
Checking in a snapshot is useful if this is effectively taking on maintenance of an unmaintained project (e.g. an ancient library that we're going to GN-ify that hasn't been updated in years). And, of course, if the code you need isn't in a Git repo, then you have to snapshot.
To include a Node package, add the dependency to the
Node package.json. Make sure to update
the corresponding npm_exclude.txt
and npm_include.txt to make the code
available during checkout.
If the code is in a Git repo that you want to mirror, please file an infra git ticket to get the repo mirrored onto chromium.googlesource.com; we don't allow direct dependencies on non-Google-hosted repositories, so that we can still build if an external repository goes down.
Once the mirror is set up, add an entry to //DEPS so that gclient will pull it in. If the code is only needed on some platforms, add a condition to the deps entry so that developers on other platforms don't pull in things they don't need.
As for specifying the path where the library is fetched, a path like
//third_party/<project_name>/src is highly recommended so that you can put
the file like OWNERS or README.chromium at //third_party/<project_name>. If
you have a wrong path in DEPS and want to change the path of the existing
library in DEPS, please ask the infrastructure team before committing the
change.
Lastly, add the new directory to Chromium's //third_party/.gitignore, so that
it won't show up as untracked files when you run git status on the main
repository.
If you are checking in a snapshot, please describe the source in the
README.chromium file, described below. For security reasons, please retrieve
the code as securely as you can, using HTTPS and GPG signatures if available.
If retrieving a tarball, please do not check the tarball itself into the tree,
but do list the source and the SHA-512 hash (for verification) in the
README.chromium and Change List. The SHA-512 hash can be computed via
sha512sum or openssl dgst -sha512. If retrieving from a git
repository, please list the revision that the code was pulled from.
If you are checking the files in directly, you do not need an entry in DEPS
and do not need to modify //third_party/.gitignore.
_Accessible to Googlers only. Non-Googlers can email one of the people in third_party/OWNERS for help.
See Moving large files to Google Storage
Your OWNERS file must either list two Chromium developer accounts as the first
two lines or include a file: directive to an OWNERS file within the
third_party directory that itself conforms to this criterion. This will ensure
accountability for maintenance of the code over time. While there isn't always
an ideal or obvious set of people that should go in OWNERS, this is critical for
first-line triage of any issues that crop up in the code.
As an OWNER, you're expected to:
- Remove the dependency when/if it is no longer needed
- Update the dependency when a security or stability bug is fixed upstream
- Help ensure the Chrome feature that uses the dependency continues to use the dependency in the best way, as the feature and the dependency change over time.
You need a README.chromium file with information about the project from which you're re-using code. See //third_party/README.chromium.template for a list of fields to include. A presubmit check will check this has the right format.
README.chromium files contain a field indicating whether the package is security-critical or not. A package is security-critical if it is compiled into the product and does any of the following:
- Accepts untrustworthy inputs from the internet
- Parses or interprets complex input formats
- Sends data to internet servers
- Collects new data
- Influences or sets security-related policy (including the user experience)
One of the fields is CPEPrefix. This is used by Chromium and Google systems to spot known upstream security vulnerabilities, and ensure we merge the fixes into our third-party copy. These systems are not foolproof, so as the OWNER, it's up to you to keep an eye out rather than solely relying on these automated systems. But, adding CPEs decreases the chances of us missing vulnerabilities, so they should always be added if possible.
The CPE is a common format shared across the industry; you can look up the CPE for your package here. Please use CPE format 2.2. When searching for a CPE, you may find that there is not yet a CPE for the specific upstream version you're using. This is normal, as CPEs are typically allocated only when a vulnerability is found. You should follow the version number convention such that, when that does occur in future, we'll be notified. If no CPE is available, please specify "unknown".
If you're using a patched or modified version which is halfway between two public versions, please "round downwards" to the lower of the public versions (it's better for us to be notified of false-positive vulnerabilities than false-negatives).
You need a LICENSE file. Example: //third_party/libjpeg/LICENSE.
Run //tools/licenses/licenses.py scan; this will complain about incomplete or missing
data for third_party checkins. We use licenses.py credits to generate the
about:credits page in Google Chrome builds.
If the library will never be shipped as a part of Chrome (e.g. build-time tools, testing tools), make sure to set "License File" as "NOT_SHIPPED" so that the license is not included in about:credits page (more on this below).
All third party additions and substantive changes like re-licensing need the following sign-offs. Some of these are accessible to Googlers only. Non-Googlers can email one of the people in //third_party/OWNERS for help.
- Make sure you have the approval from Chrome ATLs as mentioned above.
- Get [email protected] (or [email protected], Google-only) approval. Email the list with relevant details and a link to the CL. Third party code is a hot spot for security vulnerabilities. When adding a new package that could potentially carry security risk, make sure to highlight risk to [email protected]. You may be asked to add a README.security or, in dangerous cases, README.SECURITY.URGENTLY file.
- Add [email protected] as a reviewer on your change. This will trigger an automatic round-robin assignment to a reviewer who will check licensing matters. These reviewers may not be able to +1 a change so look for verbal approval in the comments. (This list does not receive or deliver email, so only use it as a reviewer, not for other communication. Internally, see cl/221704656 for details about how this is configured.). If you have questions about the third-party process, ask one of the //third_party/OWNERS instead.
- Lastly, if all other steps are complete, get a positive code review from a member of //third_party/OWNERS to land the change.
Please send separate emails to the ATLs and [email protected]. You can skip the ATL review and [email protected] when you are only moving existing directories in Chromium to //third_party/.
Subsequent changes don't normally require third-party-owners or security approval; you can modify the code as much as you want. When you update code, be mindful of security-related mailing lists for the project and relevant CVE to update your package.
As we said at the beginning, it is important that Chrome displays the right credit and attributions for all of the third_party code we use.
To view this in chrome, you can open chrome://credits.
That page displays a resource embedded in the browser as part of the //components/resources/components_resources.grd GRIT file; the actual HTML text is generated in the //components/resources:about_credits build target using a template from the output of the //tools/licenses/licenses.py script. Assuming you've followed the rules above to ensure that you have the proper LICENSE file and it passes the checks, it'll be included automatically.