Git submodules

A Git submodule is a Git repository inside another Git repository. Chromium project doesn't rely on Git submodules directly. Instead, gclient sync is used to manage Git dependencies.

In 2023Q3, we started to move source of Git dependencies from DEPS files to Git submodules. While we do our best to hide complexities of submodules, some will be exposed.

A quick introduction to Git submoduldes

Git submodules are managed via the combination of .gitmodules files and gitlinks. .gitmodules is a text file that configures submodules, and each submodule entry contains the path to the submodule's worktree and the URL of the submodule. Gitlink is a special type of file in the Git database that tracks a submodule commit.

You can find an example of Git dependency below. Note that gclient-condition is a custom property used by gclient and not git. It's identical to condition in DEPS and the allowed variables are defined in vars = { section of DEPS.


[submodule "third_party/catapult"]
	path = third_party/catapult
	url =
	gclient-condition = checkout_linux

gitlink entry, retrieved using git ls-files -s -- third_party/catapult:

160000 0b39a694c0b61392d1180520ed1c13e390029c41 0       third_party/catapult

Corresponding DEPS entry would look like:

  'third_party/catapult': {
    'url': '',
    'condition': 'checkout_linux',

How to avoid accidental Git submodule updates?

The simplest approach is to always run gclient sync after updated chromium checkout (e.g. after git pull, or git checkout). You can automate that by adding post-checkout hook (example below). To confirm there are no changes, run git status. If you use git commit -a, check the “Changes to be committed” section that shows up in the edit commit message.

Automatically run gclient sync after git pull / git checkout

We need to have Git two hooks: post-checkout and post-merge. In chromium/src directory, edit .git/hooks/post-checkout:


set -u
gclient sync

and set it to be executable: chmod +x .git/hooks/post-checkout. Repeat the same for .git/hooks/post-merge.

More information about githooks can be found here.

Git status shows modified dependencies. What does that mean?

If a submodule is checked out at a different commit than one tracked by its parent, git status in the parent repo will show unstaged changes with “new commits” in parenthesis, such as:

modified: <git deps name> (new commits)

Commands like git commit -a or git add *|.|-A|u WILL include this in your commit and your CL (which is likely NOT what you want).

Instead you can:

# Run gclient sync to sync dependencies
gclient sync
# check git status again

# OR
git add <file> # for each file you want to stage
# Then commit your staged files
git commit -v -m "Fix foo/bar"

NOTE: due to a bug in gclient (, it's possible that gclient left unmanaged git repository. You may need to manually remove those unmanaged repositories.

# Inside chromium/src checkout:
# This ensures that all managed dependencies are in sync:
gclient sync -D
# This moves all unused dependencies to ../unused directory in gclient root
# (just outside of src directory). It then tells git to restore gitlink.
for f in $( git status | grep '(new commits)' | awk '{print $2}' ); do mkdir -p "../unused/`dirname $f`" && mv $f "../unused/$f" && git checkout -- $f; done
# inspect ../unused/ if you'd like, and remove it there's nothing useful there,
# e.g. no non-uploaded commits.

If a submodule has uncommitted changes (i.e. you made some manual changes to the affected submodule), running git status in its parent repo will show them as unstaged changes:

  modified: <git deps name> (modified content)

# or

  modified: <git deps name> (untracked content)

It's not possible to add those changes to the parent repository. You can ignore such status, or you can cd into submodule and address it. E.g. you may delete untracked files (content) or reset modified content to match HEAD.

I accidentally staged Git submodule (not yet committed)

If you accidentally stage a Git submodule, you can unstage it by running git restore --staged <path to submodule>.

I accidentally committed Git submodule

We will need to create either a commit that sets it back to old value, or amend the commit that added it. You can try to run gclient sync to bring the commit back to what is expected. If that doesn't work, you can use gclient setdep -r <path>@<old hash>, run gclient gitmodules to sync all submodules commits back to what is in DEPS, or check detailed instructions in Managing dependencies.

NOTE: setdep for chromium/src is always prefixed with src/. For example, if you are updating v8, the command would be `gclient setdep -r src/v8@.

Workflows with submodules

Submodules during ‘git status’, ‘git commit’, and ‘git add’

For git status, submodules that show up under Changes not staged for commit can be hidden with git -c diff.ignoreSubmodules=all status

For git commit -a you can exclude all submodules with git -c diff.ignoreSubmodules=all commit -a.

git add does NOT support diff.ignoreSubmodules. Submodules that were hidden from you with git -c diff.ignoreSubmodules=all status would still be staged with git add .|--all|-A and therefore committed with git -c diff.ignoreSubmodules=all commit.

Instead you can run git add ':(exclude,attr:builtin_objectmode=160000)' which will stage all changes except for submodules.

(git assigns 160000 as the objectmode submodules. You can read more about builtin_objectmode and magic pathspecs)

To make these commands shorter, you can create git aliases for them by adding the following to your $HOME/.gitconfig (globally) or src/.git/config file (just chromium/src):

        # 's', 'c', or whatever alias you want for each command
        s = -c diff.ignoreSubmodules=all status
        c = -c diff.ignoreSubmodules=all commit -a
        d = -c diff.ignoreSubmodules=all difftool --dir-diff
        a = add ':(exclude,attr:builtin_objectmode=160000)'

With the above, you can execute these commands by running git s, git c, etc. Or you may also use the pre-commit git hook detailed below.

Submodules during a ‘git rebase-update’

While resolving merge conflicts during a git rebase-update you may see submodules show up in unexpected places.

Submodules under “Changes not staged for commit”

Submodules under this section can be safely ignored. This simply shows that the local commits of these submodules do not match the latest pinned commits fetched from remote. In other words, these submodules have been rolled since your last git rebase-update.

If you use a diff tool like meld you can run: git -c diff.ignoreSubmodules=all difftool --dir-diff to prevent these submodules from showing up in your diff tool.

Submodules under “Unmerged paths”

If Submodules show up under this section it means that new revisions were committed for those submodules (either intentional or unintentionally) and these submodules were also rolled at remote. So now there is a conflict.

If you DID NOT intentionally make any submdoules changes, you should run: gclient gitmodules. This will update the submdoules for you, to match whatever commits are listed in DEPS (which you have just pulled from remote).

If you DID intentionally roll submodules, you can resolve this conflict just by resetting it: gclient setdep -r {path}@{hash}

BETA: Install a hook to help detect unintentional submodule commits

depot_tools provides an opt-in pre-commit hook to detect unintentional submodule changes during git commit and remove them from the commit.

To install the hook: gclient installhooks

If there is an existing pre-commit hook, gclient will instruct you how to update it. If you have already installed this hook, gclient will do nothing.

To uninstall the hook, in chromium/src rm .git/hooks/pre-commit if you have no other hooks. Otherwise update .git/hooks/pre-commit to remove the gclient provided hook.

To bypass this hook run git commit --no-verify (which bypasses all hooks you may have) OR set the following environment variable: SKIP_GITLINK_PRECOMMIT=1 (which bypasses this specific hook).

Note that this is currently and best effort solution and does not guarantee that unintentional commits will always be detected. The team will iterate quickly on this hook to fill in other gaps and behavior is subject to change. Please file an issue for any feedback.


Why do we have Git dependencies in both DEPS and Git submodules?

Lots of Chromium infrastructure already parse DEPS file directly. Instead of a massive switch, it's easier to transition to Git submodules this way. Moreover, unwanted Git submodule updates can be detected and developers can be warned.

How do I manually roll Git submodule?

See the dependencies page.

I got a conflict on a submodule, how do I resolve it?

First, you will need to determine what is the right commit hash. If you accidentally committed a gitlink, which got in the meantime updated, you most likely want to restore the original updated gitlink. You can run gclient gitmodules, which will take care of all unmerged submodule paths, and set it to match DEPS file.

If you prefer to manually resolve it, under git status, you will see “Unmerged paths”. If those are submodules, you want to restore them by running the following command:

git restore --staging <affected path>

How do I see what revision is pinned?

gclient getdep will return whatever commit is pinned for the deps in DEPS (unstaged, staged, or committed). If the repo is using git submodules only (and has no git deps in DEPS) it will return the whatever pinned commit is staged or committed.

gclient getdep -r <path>

If you want to keep your gitlink, then run git add <affected path>.

How can I provide feedback?

Please file a bug under Infra>SDK component.