Bazel Blog

Introducing sandboxfs

sandboxfs is a new project to improve the way sandboxing works in Bazel by making it more efficient and correct. It's experimental and subject to change, but it's available now for you to check out! Read on for details.

Correct builds

As our motto claims, correctness is an integral part of Bazel. To achieve correctness, builds must be hermetic and reproducible, which means that all actions invoked by Bazel should be run in a well-defined environment. In other words: we want actions to run within a sandbox.

Let's consider an example. Think of a cc_library target that specifies as a source. This cc_library rule contains a compilation action that, for this target, turns the source file into a foo.o object file. This action will run clang, and it will need to:

  • read,
  • read all the headers required by,
  • write foo.o, and
  • possibly write some temporary files under /tmp/.

To be confident this build is correct, Bazel must ensure that the action has read-only access to the identified input files and write-only access to the output and temporary files we expect. Otherwise, Bazel cannot know if the compiler went astray and read random files from the system, making future builds inconsistent. One way to achieve these restrictions is by running each action within a sandbox.

Current sandboxing techniques

Today, Bazel uses different technologies to implement the sandboxing of actions. For example, on Linux, Bazel uses PID- and mount-namespaces, and on macOS, Bazel uses symlinks and the sandbox(7) facility. Note that sandboxing is disabled by default.

However, all these approaches have scalability and performance issues. A typical build action requires hundreds, if not thousands, of files and directories to be mapped into the sandbox. Setting these up takes time. On macOS, this approach is especially problematic because Bazel must create thousands of symlinks every time it invokes an action, and this is slow.

To make things worse, these approaches also have correctness issues. If symlinks are used, some tools (e.g. some compilers or linkers) will decide to extract the real path of such symlinks and work off that path. These tools may end up "discovering" and consuming undeclared files that are siblings of the symlink's target.

Enter sandboxfs

To resolve these issues, we came up with the idea of implementing a FUSE file system that allows us to define an arbitrary view of the host's file system under the mount point. We call this approach sandboxfs.

sandboxfs is efficient

The view sandboxfs offers is cheaply configured at mount time. With sandboxfs there is a single system call to configure the mount point versus thousands of symlink creation and deletion system calls.

The view can also be reconfigured cheaply across different actions, avoiding the need to remount the FUSE file system on each action, which would also be costly.

sandboxfs is correct

The view sandboxfs offers is fully virtual, so sandboxfs can enforce arbitrary read-only and read/write access permissions on any file or directory it exposes. Similarly, because the view is virtual, there are no symlinks involved. sandboxfs exposes real files and directories to the actions, so actions cannot reach into the original locations.

Isn't FUSE slow though?

Yes. As you may know, FUSE is slower than a real file system. This difference is because of the overhead of message-passing between the kernel and userspace, and because of all the context switches that are involved in serving a file system operation.

Our hypothesis is that, because builds are generally not I/O bound, the increased cost of I/O within the sandbox will pay for itself when compared to the cost of setting up and tearing down the sandbox for each action. No more creating thousands of symlinks or defining thousands of mount points.

But for now, that's all that it is: a hypothesis. We haven't finished stabilizing the sandboxfs code base, which means we haven't profiled nor tuned it. The integration points with Bazel are still being defined and implemented, which means it's not yet trivial to test-run Bazel's sandboxing with sandboxfs.

But it's here!

Though still in development, there is no reason to keep the code hostage any longer. We are very happy to announce that, as of today, it's now available as an open-source project under the Bazel umbrella! See:

Keep in mind that this project is still an experiment and highly subject to change. In particular, be aware that the command line and the data formats are most certainly going to mutate (for simplicity if anything). But the current code is now sufficient to experiment and play with.

As sandboxfs is now an open-source project, please take a look and report any features you would like to see, any bugs you encounter, and... if you decide to delve into the code and address any of the many TODOs in it, feel free to send us your Pull Requests!


Special credits go to Pallav Agarwal, whom we had the pleasure of hosting over a summer internship in the Google NYC office and who wrote the initial version of sandboxfs.

By Julio Merino