This article covers sandboxing in Bazel, installing sandboxfs, and debugging your sandboxing environment.

Sandboxing is a permission restricting strategy that isolates processes from each other or from resources in a system. For Bazel, this means restricting file system access.

Bazel’s file system sandbox runs processes in a working directory that only contains known inputs, such that compilers and other tools don’t see source files they should not access, unless they know the absolute paths to them.

Sandboxing doesn’t hide the host environment in any way. Processes can freely access all files on the file system. However, on platforms that support user namespaces, processes can’t modify any files outside their working directory. This ensures that the build graph doesn’t have hidden dependencies that could affect the reproducibility of the build.

More specifically, Bazel constructs an execroot/ directory for each action, which acts as the action’s work directory at execution time. execroot/ contains all input files to the action and serves as the container for any generated outputs. Bazel then uses an operating-system-provided technique, containers on Linux and sandbox-exec on macOS, to constrain the action within execroot/.

Reasons for sandboxing

  • Without action sandboxing, Bazel will not know if a tool uses undeclared input files (files that are not explicitly listed in the dependencies of an action). When one of the undeclared input files changes, Bazel still believes that the build is up-to-date and won’t rebuild the action-resulting in an incorrect incremental build.

  • Incorrect reuse of cache entries creates problems during remote caching. A bad cache entry in a shared cache affects every developer on the project, and wiping the entire remote cache is not a feasible solution.

  • Sandboxing is closely related to remote execution. If a build works well with sandboxing, it will likely work well with remote execution. Uploading all necessary files (including local tools) can significantly reduce maintenance costs for compile clusters compared to having to install the tools on every machine in the cluster every time you want to try out a new compiler or make a change to an existing tool.


sandboxfs is a FUSE file system that exposes an arbitrary view of the underlying file system without time penalties. Bazel uses sandboxfs to generate execroot/ instantaneously for each action, avoiding the cost of issuing thousands of system calls. Note that further I/O within execroot/ may be slower due to FUSE overhead.

Install sandboxfs

Use the following steps to install sandboxfs and perform a Bazel build with it:


Download and install sandboxfs so that the sandboxfs binary ends up in your PATH.

Run sandboxfs

  1. (macOS-only) Install OSXFUSE.
  2. (macOS-only) Run
       sudo sysctl -w vfs.generic.osxfuse.tunables.allow_other=1

    You will need to do this after installation and after every reboot to ensure core macOS system services work through sandboxfs.

  3. Run a Bazel build with --experimental_use_sandboxfs.
    $bazel build <target> --experimental_use_sandboxfs


If you see local instead of darwin-sandbox or linux-sandbox as an annotation for the actions that are executed, this may mean that sandboxing is disabled. Pass --genrule_strategy=sandboxed --spawn_strategy=sandboxed to enable it.


Follow the strategies below to debug issues with sandboxing.

Deactivated namespaces

On some platforms, such as Google Kubernetes Engine cluster nodes or Debian, user namespaces are deactivated by default due to security concerns. If the /proc/sys/kernel/unprivileged_userns_clone file exists and contains a 0, you can activate user namespaces by running:

   sudo sysctl kernel.unprivileged_userns_clone=1

Rule execution failures

The sandbox may fail to execute rules because of the system setup. If you see a message like namespace-sandbox.c:633: execvp(argv[0], argv): No such file or directory, try to deactivate the sandbox with --strategy=Genrule=local for genrules, and --spawn_strategy=local for other rules.

Detailed debugging for build failures

If your build failed, use --verbose_failures and --sandbox_debug to make Bazel show the exact command it ran when your build failed, including the part that sets up the sandbox.

Example error message:

ERROR: path/to/your/project/BUILD:1:1: compilation of rule
'//path/to/your/project:all' failed:

Sandboxed execution failed, which may be legitimate (e.g. a compiler error),
or due to missing dependencies. To enter the sandbox environment for easier
debugging, run the following command in parentheses. On command failure, a bash
shell running inside the sandbox will then automatically be spawned

namespace-sandbox failed: error executing command
  (cd /some/path && \
  exec env - \
    LANG=en_US \
    PATH=/some/path/bin:/bin:/usr/bin \
    PYTHONPATH=/usr/local/some/path \
  /some/path/namespace-sandbox @/sandbox/root/path/this-sandbox-name.params --
  /some/path/to/your/some-compiler --some-params some-target)

You can now inspect the generated sandbox directory and see which files Bazel created and run the command again to see how it behaves.

Note that Bazel does not delete the sandbox directory when you use --sandbox_debug. Unless you are actively debugging, you should disable --sandbox_debug because it fills up your disk over time.