11 MemorySanitizer is a detector of uninitialized reads. It consists of a
12 compiler instrumentation module and a run-time library.
14 Typical slowdown introduced by MemorySanitizer is **3x**.
19 Follow the `clang build instructions <../get_started.html>`_. CMake
25 Simply compile and link your program with ``-fsanitize=memory`` flag.
26 The MemorySanitizer run-time library should be linked to the final
27 executable, so make sure to use ``clang`` (not ``ld``) for the final
28 link step. When linking shared libraries, the MemorySanitizer run-time
29 is not linked, so ``-Wl,-z,defs`` may cause link errors (don't use it
30 with MemorySanitizer). To get a reasonable performance add ``-O1`` or
31 higher. To get meaninful stack traces in error messages add
32 ``-fno-omit-frame-pointer``. To get perfect stack traces you may need
33 to disable inlining (just use ``-O1``) and tail call elimination
34 (``-fno-optimize-sibling-calls``).
36 .. code-block:: console
41 int main(int argc, char** argv) {
49 % clang -fsanitize=memory -fno-omit-frame-pointer -g -O2 umr.cc
51 If a bug is detected, the program will print an error message to
52 stderr and exit with a non-zero exit code. Currently, MemorySanitizer
53 does not symbolize its output by default, so you may need to use a
54 separate script to symbolize the result offline (this will be fixed in
57 .. code-block:: console
60 % projects/compiler-rt/lib/asan/scripts/asan_symbolize.py / < log | c++filt
61 ==30106== WARNING: MemorySanitizer: UMR (uninitialized-memory-read)
62 #0 0x7f45944b418a in main umr.cc:6
63 #1 0x7f45938b676c in __libc_start_main libc-start.c:226
66 By default, MemorySanitizer exits on the first detected error.
68 ``__has_feature(memory_sanitizer)``
69 ------------------------------------
71 In some cases one may need to execute different code depending on
72 whether MemorySanitizer is enabled. :ref:`\_\_has\_feature
73 <langext-__has_feature-__has_extension>` can be used for this purpose.
77 #if defined(__has_feature)
78 # if __has_feature(memory_sanitizer)
79 // code that builds only under MemorySanitizer
83 ``__attribute__((no_sanitize_memory))``
84 -----------------------------------------------
86 Some code should not be checked by MemorySanitizer.
87 One may use the function attribute
88 :ref:`no_sanitize_memory <langext-memory_sanitizer>`
89 to disable uninitialized checks in a particular function.
90 MemorySanitizer may still instrument such functions to avoid false positives.
91 This attribute may not be
92 supported by other compilers, so we suggest to use it together with
93 ``__has_feature(memory_sanitizer)``.
98 MemorySanitizer supports ``src`` and ``fun`` entity types in
99 :doc:`SanitizerSpecialCaseList`, that can be used to relax MemorySanitizer
100 checks for certain source files and functions. All "Use of uninitialized value"
101 warnings will be suppressed and all values loaded from memory will be
102 considered fully initialized.
107 MemorySanitizer can track origins of unitialized values, similar to
108 Valgrind's --track-origins option. This feature is enabled by
109 ``-fsanitize-memory-track-origins`` Clang option. With the code from
112 .. code-block:: console
114 % clang -fsanitize=memory -fsanitize-memory-track-origins -fno-omit-frame-pointer -g -O2 umr.cc
116 % projects/compiler-rt/lib/asan/scripts/asan_symbolize.py / < log | c++filt
117 ==14425== WARNING: MemorySanitizer: UMR (uninitialized-memory-read)
118 ==14425== WARNING: Trying to symbolize code, but external symbolizer is not initialized!
119 #0 0x7f8bdda3824b in main umr.cc:6
120 #1 0x7f8bdce3a76c in __libc_start_main libc-start.c:226
121 raw origin id: 2030043137
122 ORIGIN: heap allocation:
123 #0 0x7f8bdda4034b in operator new[](unsigned long) msan_new_delete.cc:39
124 #1 0x7f8bdda3814d in main umr.cc:4
125 #2 0x7f8bdce3a76c in __libc_start_main libc-start.c:226
128 Origin tracking has proved to be very useful for debugging UMR
129 reports. It slows down program execution by a factor of 1.5x-2x on top
130 of the usual MemorySanitizer slowdown.
132 Handling external code
133 ============================
135 MemorySanitizer requires that all program code is instrumented. This
136 also includes any libraries that the program depends on, even libc.
137 Failing to achieve this may result in false UMR reports.
139 Full MemorySanitizer instrumentation is very difficult to achieve. To
140 make it easier, MemorySanitizer runtime library includes 70+
141 interceptors for the most common libc functions. They make it possible
142 to run MemorySanitizer-instrumented programs linked with
143 uninstrumented libc. For example, the authors were able to bootstrap
144 MemorySanitizer-instrumented Clang compiler by linking it with
145 self-built instrumented libcxx (as a replacement for libstdc++).
147 In the case when rebuilding all program dependencies with
148 MemorySanitizer is problematic, an experimental MSanDR tool can be
149 used. It is a DynamoRio-based tool that uses dynamic instrumentation
150 to avoid false positives due to uninstrumented code. The tool simply
151 marks memory from instrumented libraries as fully initialized. See
152 `http://code.google.com/p/memory-sanitizer/wiki/Running#Running_with_the_dynamic_tool`
153 for more information.
158 MemorySanitizer is supported on
160 * Linux x86\_64 (tested on Ubuntu 10.04 and 12.04);
165 * MemorySanitizer uses 2x more real memory than a native run, 3x with
167 * MemorySanitizer maps (but not reserves) 64 Terabytes of virtual
168 address space. This means that tools like ``ulimit`` may not work as
170 * Static linking is not supported.
171 * Non-position-independent executables are not supported. Therefore, the
172 ``fsanitize=memory`` flag will cause Clang to act as though the ``-fPIE``
173 flag had been supplied if compiling without ``-fPIC``, and as though the
174 ``-pie`` flag had been supplied if linking an executable.
175 * Depending on the version of Linux kernel, running without ASLR may
176 be not supported. Note that GDB disables ASLR by default. To debug
177 instrumented programs, use "set disable-randomization off".
182 MemorySanitizer is an experimental tool. It is known to work on large
183 real-world programs, like Clang/LLVM itself.
188 `http://code.google.com/p/memory-sanitizer <http://code.google.com/p/memory-sanitizer/>`_