Memory Sanitizer

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What is Memory Sanitizer?

Memory Sanitizer (MSan) is a fast detector used for uninitialized memory in C/C++ programs. It uses a compile-time instrumentation to ensure that all memory access at runtime uses only memory that has been initialized. Unlike most other sanitizers, MSan can easily cause false positives if not all libraries are instrumented. This happens because MSan is not able to observe memory initialization in uninstrumented libraries. More information on MSan can be found on the Memory Sanitizer wiki.

Public Builds

Note: No public builds are available at this time yet.

Manual Build

Build prerequisites

Note: MemorySanitizer requires 64-bit Linux to work. Other platforms/operating systems are not supported.


The MSan instrumentation is implemented as an LLVM pass and integrated into Clang. As MSan is one of the newer sanitizers, we recommend using a recent Clang version, such as Clang 3.7+.

You can find precompiled binaries for LLVM/Clang on the LLVM releases page.

Building Firefox


Warning: Running Firefox with MemorySanitizer would require all external dependencies to be built with MemorySanitizer as well. To our knowledge, this has never been attempted yet, so the build configuration provided here is untested and without an appropriately instrumented userland, it will cause false positives.

Getting the source

If you don’t have a source code repository clone yet, you need 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:

mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/objdir-ff-msan

# Enable LLVM specific code and build workarounds
ac_add_options --enable-memory-sanitizer
# If clang is already in your $PATH, then these can simply be:
#   export CC=clang
#   export CXX=clang++
export CC="/path/to/clang"
export CXX="/path/to/clang++"

# llvm-symbolizer displays much more complete backtraces when data races are detected.
# If it's not already in your $PATH, then uncomment this next line:
#export LLVM_SYMBOLIZER="/path/to/llvm-symbolizer"

# Add MSan to our compiler flags
export CFLAGS="-fsanitize=memory"
export CXXFLAGS="-fsanitize=memory"

# Additionally, we need the MSan flag during linking. Normally, our C/CXXFLAGS would
# be used during linking as well but there is at least one place in our build where
# our CFLAGS are not added during linking.
# Note: The use of this flag causes Clang to automatically link the MSan runtime :)
export LDFLAGS="-fsanitize=memory"

# These three are required by MSan
ac_add_options --disable-jemalloc
ac_add_options --disable-crashreporter
ac_add_options --disable-elf-hack

# Keep symbols to symbolize MSan traces
ac_add_options --enable-debug-symbols
ac_add_options --disable-install-strip

# Settings for an opt build
ac_add_options --enable-optimize="-O2 -gline-tables-only"
ac_add_options --disable-debug

Starting the build process

Now you start the build process using the regular make -f command.

Starting Firefox

After the build has completed, you can start Firefox from the objdir as usual.

Building the JavaScript shell

Note: Unlike Firefox itself, the JavaScript shell does not require an instrumented userland. Calls to external libraries like zlib are handled with special annotations inside the engine.


Warning: Certain technologies used inside the JavaScript engine are incompatible with MSan and must be disabled at runtime to prevent false positives. This includes the JITs and asm.js. Therefore always make sure to run with ``–no-ion –no-baseline –no-asmjs –no-native-regexp``.

If you want to build only the JavaScript shell instead of doing a full Firefox build, the build script below will probably help you to do so. Before using it, you must, of course, adjust the path name for LLVM_ROOT to match your setup. Once you have adjusted everything, 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.

#! /bin/sh

if [ -z $1 ] ; then
    echo "usage: $0 <dirname>"
elif [ -d $1 ] ; then
    echo "directory $1 already exists"
    mkdir $1
    cd $1
    CC="$LLVM_ROOT/build/bin/clang" \
    CXX="$LLVM_ROOT/build/bin/clang++" \
    CFLAGS="-fsanitize=memory" \
    CXXFLAGS="-fsanitize=memory" \
    LDFLAGS="-fsanitize=memory" \
            ../configure --enable-debug --enable-optimize --enable-memory-sanitizer --disable-jemalloc --enable-posix-nspr-emulation
    make -j 8

Using LLVM Symbolizer for faster/better traces

By default, MSan traces are not symbolized.

LLVM ships with the symbolizer binary llvm-symbolize that MSan will readily use to immediately output symbolized traces if the program is found on the PATH. If your llvm-symbolizer lives outside the PATH, you can set the MSAN_SYMBOLIZER_PATH environment variable to point to your symbolizer binary.