| ================ |
| 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 |
| ============ |
| |
| Follow the `clang build instructions <../get_started.html>`_. CMake |
| build is supported. |
| |
| 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 |
| :ref:`no_sanitize_memory <langext-memory_sanitizer>` |
| 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`` Clang option. With the code from |
| the example above, |
| |
| .. code-block:: console |
| |
| % clang -fsanitize=memory -fsanitize-memory-track-origins -fno-omit-frame-pointer -g -O2 umr.cc |
| % ./a.out |
| WARNING: MemorySanitizer: use-of-uninitialized-value |
| #0 0x7f7893912f0b in main umr2.cc:6 |
| #1 0x7f789249b76c in __libc_start_main libc-start.c:226 |
| |
| 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 |
| |
| 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. |
| |
| MemorySanitizer can provide even more information with |
| ``-fsanitize-memory-track-origins=2`` flag. In this mode reports |
| include information about intermediate stores the uninitialized value went |
| through. |
| |
| .. 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 |
| |
| |
| 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 libcxx (as a replacement for libstdc++). |
| |
| In the case when rebuilding all program dependencies with |
| MemorySanitizer is problematic, an experimental MSanDR tool can be |
| used. It is a DynamoRio-based tool that uses dynamic instrumentation |
| to avoid false positives due to uninstrumented code. The tool simply |
| marks memory from instrumented libraries as fully initialized. See |
| `http://code.google.com/p/memory-sanitizer/wiki/Running#Running_with_the_dynamic_tool` |
| for more information. |
| |
| 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/>`_ |
| |