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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.
Note: No public builds are available at this time yet.
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.
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:
# Enable LLVM specific code and build workarounds
# If clang is already in your $PATH, then these can simply be:
# export CC=clang
# export CXX=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:
# Add MSan to our compiler flags
# 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 :)
# These three are required by MSan
# Keep symbols to symbolize MSan traces
# Settings for an opt build
ac_add_options --enable-optimize="-O2 -gline-tables-only"
Starting the build process¶
Now you start the build process using the regular
make -f client.mk
After the build has completed, you can start Firefox from the
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.
if [ -z $1 ] ; then
echo "usage: $0 <dirname>"
elif [ -d $1 ] ; then
echo "directory $1 already exists"
../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.