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Running a single binary with UBSAN

UBSAN is a runtime undefined-behavior sanitizer. When a program is built with UBSAN, it will print an error message and fail out if the program does things which are not allowed in C++, such as overflowing a signed integer or exiting a non-void function without returning a value.

What this guide is

Currently, it is not possible to boot a copy of ChromiumOS using UBSAN. However, it is possible to build individual binaries using UBSAN and then run them on a machine which is otherwise running a normal developer image. This can be useful for checking for bugs in an individual program.

This guide describes how to run a single UBSAN-instrumented binary on a test device.

These instructions should also work for running a single binary with the other sanitizers. Just change the USE flag to asan, tsan, etc instead of ubsan.

Restrictions and Caveats

Since the UBSAN build is already broken (does not compile), the build is not checked by CQ, and may break further over time. These directions may break at any time. They worked in March of 2021.

I only tried this on a single board (cyan, which is an X86 architecture). Other boards, other architectures may not work.

I only tried this with anomaly_detector and crash_reporter. Other binaries may not work if their libraries will not compile under UBSAN.

I have not tried this with the Chromium binary. This guide is about the ChromiumOS-specific binaries.


(chroot) $ indicates a command that should be run inside the cros_sdk chroot. localhost # indicates a command that should be run in a root shell on the DUT (test device).


Choose a chroot shell in which you will do all the other compiles below. You must do all compiling in this shell.

(chroot) $ USE=ubsan; export USE

Once you do this, do not attempt any other (non-UBSAN) compiles in this shell! Mixing UBSAN and non-UBSAN compiles in the same build will not end well. I usually recolor the xterm I set USE=ubsan in so I remember to not use it for anything else.

Recompile libraries using UBSAN

In the same shell,

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

This will eventually fail with a compile error, but it will build the libraries first.

Recompile binary using UBSAN

In the same shell, emerge the binary you want. For instance, to compile anomaly_detector, which is in the crash-reporter ebuild file:

(chroot) $ emerge-${BOARD} crash-reporter

This should compile correctly.

Determine which libraries you need

Get the list of libraries that your binary needs using lddtree. For anomaly_detector, this would look like:

(chroot) $ lddtree /build/${BOARD}/usr/bin/anomaly_detector

For anomaly_detector, this might print out:

/build/cyan/usr/bin/anomaly_detector (interpreter => /lib64/ => /usr/lib64/ => /usr/lib64/ => /usr/lib64/
   => /lib64/ => /usr/lib64/ => /usr/lib64/

The names and paths on the right side are the libraries we want.

Transfer binary and libraries

If you haven't yet, make sure your test device has a writable drive:

localhost # /usr/libexec/debugd/helpers/dev_features_rootfs_verification
localhost # reboot

Make a directory where your binary and its libraries will live. (You can't replace the non-UBSAN libraries with the UBSAN libraries; it will crash other parts of the system. So you need a separate location to keep them.)

localhost # mkdir /etc/ubsan_test

Now, scp the UBSAN binaries over. You'll want to transfer the binary over, plus all the libraries we discovered above. For each library, copy-and-paste the name+path, minus the initial /, to get the relative path from the build directory. So for anomaly_detector, given the lddtree output above, we might do:

(chroot) $ cd /build/${BOARD}
(chroot) $ scp usr/bin/anomaly_detector usr/lib64/ \
                usr/lib64/ usr/lib64/ \
                lib64/ usr/lib64/ \
                usr/lib64/ \

Note: If there are any libraries that you can't find, just skip them. The on- device test will default to the non-UBSAN version of any libraries you don't copy over.


Any programs launched by the UBSAN-instrumented binary also need to be compiled using UBSAN, or they will fail when trying to link to the UBSAN-instrumented libraries.

As an example, anomaly_detector launches crash_reporter. So I needed to compile crash_reporter using UBSAN, check what libraries it uses via lddtree, and copy the crash_reporter binary and all of its libraries to /etc/ubsan_test. I also needed to change anomaly_detector's code to launch /etc/ubsan_test/crash_reporter instead of /sbin/crash_reporter.

Run on the test device

Set LD_LIBRARY_PATH to point to the test directory. Tell UBSAN to log to stderr.

For example

localhost # cd /etc/ubsan_test
localhost # LD_LIBRARY_PATH=/etc/ubsan_test UBSAN_OPTIONS="log_path=stderr" \

If you don't set UBSAN_OPTIONS="log_path=stderr", the logs will appear in /var/log/asan (yes, even for UBSAN)

Clean up

Remember to close the shell that has USE=ubsan. Delete the build directory to force all the libraries to recompile without UBSAN.

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

Good hunting, and may all your bugs be shallow!