What is Address Sanitizer?¶
Address Sanitizer (ASan) is a fast memory error detector that detects
use-after-free and out-of-bound bugs in C/C++ programs. It uses a
compile-time instrumentation to check all reads and writes during the
execution. In addition, the runtime part replaces the
free functions to check dynamically allocated memory. More
information on how ASan works can be found on the Address Sanitizer
A meta bug called asan-maintenance is maintained to keep track of all the bugs found with ASan.
Downloading artifact builds¶
For Linux and Windows users, the easiest way to get Firefox builds with Address Sanitizer is to download a continuous integration asan build of mozilla-central (updated at least daily):
The fuzzing team also offers a tool called
fuzzfetch to download these and many
other CI builds. It makes downloading and unpacking these builds much easier and
can be used not just for fuzzing but for all purposes that require a CI build download.
Afterwards, you can run e.g.
$ python -m fuzzfetch --asan -n firefox-asan
to get the optimized Linux ASan build mentioned above unpacked into a directory called
--os switches can be used to get the other variants listed above.
Creating Try builds¶
If for some reason you can’t use the pre-built binaries mentioned in the previous section (e.g. you want a non-Linux build or you need to test a patch), you can either build Firefox yourself (see the following section) or use the try server to create the customized build for you. Pushing to try requires L1 commit access. If you don’t have this access yet you can request access (see Becoming A Mozilla Committer and Mozilla Commit Access Policy for the requirements).
The tree contains several mozconfig files for creating asan
(the “nightly-asan” files create release builds, whereas the
“debug-asan” files create debug+opt builds). For Linux builds, the
appropriate configuration file is used by the
If you want to create a macOS or Windows build, you’ll need to copy the
appropriate configuration file over the regular debug configuration
before pushing to try. For example:
cp browser/config/mozconfigs/macosx64/debug-asan browser/config/mozconfigs/macosx64/debug
You can then push to Try in the usual way and, once the build is complete, download the appropriate build artifact.
Creating local builds on Windows¶
On Windows, ASan is supported only in 64-bit builds.
mach bootstrap to get an updated clang-cl in your
~/.mozbuild directory, then use the following
export LDFLAGS="clang_rt.asan_dynamic-x86_64.lib clang_rt.asan_dynamic_runtime_thunk-x86_64.lib"
CLANG_LIB_DIR="$(cd ~/.mozbuild/clang/lib/clang/*/lib/windows && pwd)"
If you launch an ASan build under WinDbg, you may see spurious
first-chance Access Violation exceptions. These come from ASan creating
shadow memory pages on demand, and can be ignored. Run
sxi av to
ignore these exceptions. (You will still catch second-chance Access
Violation exceptions if you actually crash.)
LeakSanitizer (LSan) is not supported on Windows.
Creating local builds on Linux or Mac¶
The ASan instrumentation is implemented as an LLVM pass and integrated into Clang. Any clang version that is capable of compiling Firefox has everything needed to do an ASAN build.
Getting the source¶
Using that or any later revision, all you need to do is to get yourself a clone of mozilla-central.
Adjusting the build configuration¶
Create the build configuration file
mozconfig with the following
content in your mozilla-central directory:
# Combined .mozconfig file for ASan on Linux+Mac
# Enable ASan specific code and build workarounds
# These three are required by ASan
# Keep symbols to symbolize ASan traces later
# Settings for an opt build (preferred)
# The -gline-tables-only ensures that all the necessary debug information for ASan
# is present, but the rest is stripped so the resulting binaries are smaller.
ac_add_options --enable-optimize="-O2 -gline-tables-only"
# Settings for a debug+opt build
# MacOSX only: Uncomment and adjust this path to match your SDK
# ac_add_options --with-macos-sdk=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk
You may also need this, as seen in
browser/config/mozconfigs/linux64/nightly-asan (the config file used
for Address Sanitizer builds used for automated testing):
# ASan specific options on Linux
Starting the build process¶
Now you start the build process using the regular
After the build has completed,
./mach run with the usual options for
running in a debugger (
rr, etc.) work fine, as do
--disable-e10s and other options.
Firefox build, the build script below will probably help you to do so.
Execute this script in the
js/src/ subdirectory and pass a directory
name as the first parameter. The build will then be created in a new
subdirectory with that name.
if [ -z $1 ] ; then
echo "usage: $0 <dirname>"
elif [ -d $1 ] ; then
echo "directory $1 already exists"
../configure --enable-debug --enable-optimize --enable-address-sanitizer --disable-jemalloc
Getting Symbols in Address Sanitizer Traces¶
By default, ASan traces are unsymbolized and only print the binary/library and a memory offset instead. In order to get more useful traces, containing symbols, there are two approaches.
Using the LLVM Symbolizer (recommended)¶
LLVM ships with a symbolizer binary that ASan will readily use to
immediately output symbolized traces. To use it, just set the
ASAN_SYMBOLIZER_PATH to reflect the location of
llvm-symbolizer binary, before running the process. This
program is usually included in an LLVM distribution. Stacks without
symbols can also be post-processed, see below.
Warning: On OS X, the content sandbox prevents the symbolizer
from running. To use llvm-symbolizer on ASan output from a
content process, the content sandbox must be disabled. This can be
done by setting
MOZ_DISABLE_CONTENT_SANDBOX=1 in your run
environment. Setting this in .mozconfig has no effect.
Post-Processing Traces with asan_symbolize.py¶
Instead of using the llvm-symbolizer binary, you can also pipe the
output through the
asan_symbolize.py script, shipped with LLVM
often included in LLVM distributions. The disadvantage is that the
script will need to use
addr2line to get the symbols, which means
that every library will have to be loaded into memory
(including``libxul``, which takes a bit).
However, in certain situations it makes sense to use this script. For example, if you have/received an unsymbolized trace, then you can still use the script to turn it into a symbolized trace, given that you can get the original binaries that produced the unsymbolized trace. In order for the script to work in such cases, you need to ensure that the paths in the trace point to the actual binaries, or change the paths accordingly.
Since the output of the
asan_symbolize.py script is still mangled,
you might want to pipe the output also through
Troubleshooting / Known problems¶
Cannot specify -o when generating multiple output files¶
If you get the error
cannot specify -o when generating multiple output files" from
elf-hack in your
mozconfig to work around the
Since an issue with -O2/-Os and ASan has been resolved, the regular optimizations used by Firefox should work without any problems. The optimized build has only a barely noticeable speed penalty and seems to be even faster than regular debug builds.
No “AddressSanitizer: libc interceptors initialized” shows after running ./mach run¶
$ ASAN_OPTIONS=verbosity=2 ./mach run
Use the above command instead
“An admin user name and password” is required to enter Developer Mode¶
Please enable Developer mode by:
$ /usr/sbin/DevToolsSecurity -enable
Developer mode is now enabled.
Debugging issues that ASan finds¶
When ASan discovers an issue it will simply print an error message and
exit the app. To stop the app in a debugger before ASan exits it, set a
__asan::ReportGenericError. For more info on using
ASan and debugging issues that it uncovers, see the page Address
sanitizer and a
page on the upstream wiki.
__asan_describe_address(pointer) issued at the debugger prompt or
even directly in the code allows outputting lots of information about
this memory address (thread and stack of allocation, of deallocation,
whether or not it is a bit outside a known buffer, thread and stack of
allocation of this buffer, etc.). This can be useful to understand where
some buffer that is not aligned was allocated, when doing SIMD work, for
rr (Linux x86 only) works great with ASan and combined, this combo allows doing some very powerful debugging strategies.
LeakSanitizer (LSan) is a special execution mode for regular ASan. It
takes advantage of how ASan tracks the set of live blocks at any given
point to print out the allocation stack of any block that is still alive
at shutdown, but is not reachable from the stack, according to a
conservative scan. This is very useful for detecting leaks of things
char* that do not participate in the usual Gecko shutdown
leak detection. LSan is supported on x86_64 Linux and OS X.
LSan is enabled by default in ASan builds, as of more recent versions of
Clang. To make an ASan build not run LSan, set the environment variable
detect_leaks=0 (or add it as an entry to a
:-separated list if it is already set to something). If you want to
enable it when it is not for some reason, set it to 1 instead of 0. If
LSan is enabled and you are using a non-debug build, you will also want
to set the environment variable
ensure that we run shutdown GCs and CCs to avoid spurious leaks.
If an object that is reported by LSan is intentionally never freed, a
symbol can be added to
build/sanitizers/lsan_suppressions.txt to get
LSan to ignore it.
For some more information on LSan, see the Leak Sanitizer wiki page.
A meta bug called lsan is maintained to keep track of all the bugs found with LSan.
Frequently Asked Questions about ASan¶
How does ASan work exactly?¶
More information on how ASan works can be found on the Address Sanitizer wiki.