================
MemorySanitizer
================

.. contents::
   :local:

Introduction
============

MemorySanitizer is a detector of uninitialized reads. It consists of a
compiler instrumentation module and a run-time library.

Typical slowdown introduced by MemorySanitizer is **3x**.

How to build
============

Build LLVM/Clang with `CMake <http://llvm.org/docs/CMake.html>`_.

Usage
=====

Simply compile and link your program with ``-fsanitize=memory`` flag.
The MemorySanitizer run-time library should be linked to the final
executable, so make sure to use ``clang`` (not ``ld``) for the final
link step. When linking shared libraries, the MemorySanitizer run-time
is not linked, so ``-Wl,-z,defs`` may cause link errors (don't use it
with MemorySanitizer). To get a reasonable performance add ``-O1`` or
higher. To get meaninful stack traces in error messages add
``-fno-omit-frame-pointer``. To get perfect stack traces you may need
to disable inlining (just use ``-O1``) and tail call elimination
(``-fno-optimize-sibling-calls``).

.. code-block:: console

    % cat umr.cc
    #include <stdio.h>

    int main(int argc, char** argv) {
      int* a = new int[10];
      a[5] = 0;
      if (a[argc])
        printf("xx\n");
      return 0;
    }

    % clang -fsanitize=memory -fno-omit-frame-pointer -g -O2 umr.cc

If a bug is detected, the program will print an error message to
stderr and exit with a non-zero exit code. Currently, MemorySanitizer
does not symbolize its output by default, so you may need to use a
separate script to symbolize the result offline (this will be fixed in
future).

.. code-block:: console

    % ./a.out
    WARNING: MemorySanitizer: use-of-uninitialized-value
        #0 0x7f45944b418a in main umr.cc:6
        #1 0x7f45938b676c in __libc_start_main libc-start.c:226

By default, MemorySanitizer exits on the first detected error.

``__has_feature(memory_sanitizer)``
------------------------------------

In some cases one may need to execute different code depending on
whether MemorySanitizer is enabled. :ref:`\_\_has\_feature
<langext-__has_feature-__has_extension>` can be used for this purpose.

.. code-block:: c

    #if defined(__has_feature)
    #  if __has_feature(memory_sanitizer)
    // code that builds only under MemorySanitizer
    #  endif
    #endif

``__attribute__((no_sanitize_memory))``
-----------------------------------------------

Some code should not be checked by MemorySanitizer.  One may use the function
attribute `no_sanitize_memory` to disable uninitialized checks in a particular
function.  MemorySanitizer may still instrument such functions to avoid false
positives.  This attribute may not be supported by other compilers, so we
suggest to use it together with ``__has_feature(memory_sanitizer)``.

Blacklist
---------

MemorySanitizer supports ``src`` and ``fun`` entity types in
:doc:`SanitizerSpecialCaseList`, that can be used to relax MemorySanitizer
checks for certain source files and functions. All "Use of uninitialized value"
warnings will be suppressed and all values loaded from memory will be
considered fully initialized.

Report symbolization
====================

MemorySanitizer uses an external symbolizer to print files and line numbers in
reports. Make sure that ``llvm-symbolizer`` binary is in ``PATH``,
or set environment variable ``MSAN_SYMBOLIZER_PATH`` to point to it.

Origin Tracking
===============

MemorySanitizer can track origins of unitialized values, similar to
Valgrind's --track-origins option. This feature is enabled by
``-fsanitize-memory-track-origins=2`` (or simply
``-fsanitize-memory-track-origins``) Clang option. With the code from
the example above,

.. code-block:: console

    % cat umr2.cc
    #include <stdio.h>

    int main(int argc, char** argv) {
      int* a = new int[10];
      a[5] = 0;
      volatile int b = a[argc];
      if (b)
        printf("xx\n");
      return 0;
    }

    % clang -fsanitize=memory -fsanitize-memory-track-origins=2 -fno-omit-frame-pointer -g -O2 umr2.cc
    % ./a.out
    WARNING: MemorySanitizer: use-of-uninitialized-value
        #0 0x7f7893912f0b in main umr2.cc:7
        #1 0x7f789249b76c in __libc_start_main libc-start.c:226

      Uninitialized value was stored to memory at
        #0 0x7f78938b5c25 in __msan_chain_origin msan.cc:484
        #1 0x7f7893912ecd in main umr2.cc:6

      Uninitialized value was created by a heap allocation
        #0 0x7f7893901cbd in operator new[](unsigned long) msan_new_delete.cc:44
        #1 0x7f7893912e06 in main umr2.cc:4

By default, MemorySanitizer collects both allocation points and all
intermediate stores the uninitialized value went through.  Origin
tracking has proved to be very useful for debugging MemorySanitizer
reports. It slows down program execution by a factor of 1.5x-2x on top
of the usual MemorySanitizer slowdown.

Clang option ``-fsanitize-memory-track-origins=1`` enabled a slightly
faster mode when MemorySanitizer collects only allocation points but
not intermediate stores.

Handling external code
============================

MemorySanitizer requires that all program code is instrumented. This
also includes any libraries that the program depends on, even libc.
Failing to achieve this may result in false reports.

Full MemorySanitizer instrumentation is very difficult to achieve. To
make it easier, MemorySanitizer runtime library includes 70+
interceptors for the most common libc functions. They make it possible
to run MemorySanitizer-instrumented programs linked with
uninstrumented libc. For example, the authors were able to bootstrap
MemorySanitizer-instrumented Clang compiler by linking it with
self-built instrumented libc++ (as a replacement for libstdc++).

Supported Platforms
===================

MemorySanitizer is supported on

* Linux x86\_64 (tested on Ubuntu 12.04);

Limitations
===========

* MemorySanitizer uses 2x more real memory than a native run, 3x with
  origin tracking.
* MemorySanitizer maps (but not reserves) 64 Terabytes of virtual
  address space. This means that tools like ``ulimit`` may not work as
  usually expected.
* Static linking is not supported.
* Non-position-independent executables are not supported.  Therefore, the
  ``fsanitize=memory`` flag will cause Clang to act as though the ``-fPIE``
  flag had been supplied if compiling without ``-fPIC``, and as though the
  ``-pie`` flag had been supplied if linking an executable.
* Depending on the version of Linux kernel, running without ASLR may
  be not supported. Note that GDB disables ASLR by default. To debug
  instrumented programs, use "set disable-randomization off".

Current Status
==============

MemorySanitizer is an experimental tool. It is known to work on large
real-world programs, like Clang/LLVM itself.

More Information
================

`http://code.google.com/p/memory-sanitizer <http://code.google.com/p/memory-sanitizer/>`_