1================ 2MemorySanitizer 3================ 4 5.. contents:: 6 :local: 7 8Introduction 9============ 10 11MemorySanitizer is a detector of uninitialized reads. It consists of a 12compiler instrumentation module and a run-time library. 13 14Typical slowdown introduced by MemorySanitizer is **3x**. 15 16How to build 17============ 18 19Follow the `clang build instructions <../get_started.html>`_. CMake 20build is supported. 21 22Usage 23===== 24 25Simply compile and link your program with ``-fsanitize=memory`` flag. 26The MemorySanitizer run-time library should be linked to the final 27executable, so make sure to use ``clang`` (not ``ld``) for the final 28link step. When linking shared libraries, the MemorySanitizer run-time 29is not linked, so ``-Wl,-z,defs`` may cause link errors (don't use it 30with MemorySanitizer). To get a reasonable performance add ``-O1`` or 31higher. To get meaninful stack traces in error messages add 32``-fno-omit-frame-pointer``. To get perfect stack traces you may need 33to disable inlining (just use ``-O1``) and tail call elimination 34(``-fno-optimize-sibling-calls``). 35 36.. code-block:: console 37 38 % cat umr.cc 39 #include <stdio.h> 40 41 int main(int argc, char** argv) { 42 int* a = new int[10]; 43 a[5] = 0; 44 if (a[argc]) 45 printf("xx\n"); 46 return 0; 47 } 48 49 % clang -fsanitize=memory -fno-omit-frame-pointer -g -O2 umr.cc 50 51If a bug is detected, the program will print an error message to 52stderr and exit with a non-zero exit code. Currently, MemorySanitizer 53does not symbolize its output by default, so you may need to use a 54separate script to symbolize the result offline (this will be fixed in 55future). 56 57.. code-block:: console 58 59 % ./a.out 2>log 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 64 Exiting 65 66By default, MemorySanitizer exits on the first detected error. 67 68``__has_feature(memory_sanitizer)`` 69------------------------------------ 70 71In some cases one may need to execute different code depending on 72whether MemorySanitizer is enabled. :ref:`\_\_has\_feature 73<langext-__has_feature-__has_extension>` can be used for this purpose. 74 75.. code-block:: c 76 77 #if defined(__has_feature) 78 # if __has_feature(memory_sanitizer) 79 // code that builds only under MemorySanitizer 80 # endif 81 #endif 82 83``__attribute__((no_sanitize_memory))`` 84----------------------------------------------- 85 86Some code should not be checked by MemorySanitizer. 87One may use the function attribute 88:ref:`no_sanitize_memory <langext-memory_sanitizer>` 89to disable uninitialized checks in a particular function. 90MemorySanitizer may still instrument such functions to avoid false positives. 91This attribute may not be 92supported by other compilers, so we suggest to use it together with 93``__has_feature(memory_sanitizer)``. Note: currently, this attribute will be 94lost if the function is inlined. 95 96Blacklist 97--------- 98 99MemorySanitizer supports ``src`` and ``fun`` entity types in 100:doc:`SanitizerSpecialCaseList`, that can be used to relax MemorySanitizer 101checks for certain source files and functions. All "Use of uninitialized value" 102warnings will be suppressed and all values loaded from memory will be 103considered fully initialized. 104 105Origin Tracking 106=============== 107 108MemorySanitizer can track origins of unitialized values, similar to 109Valgrind's --track-origins option. This feature is enabled by 110``-fsanitize-memory-track-origins`` Clang option. With the code from 111the example above, 112 113.. code-block:: console 114 115 % clang -fsanitize=memory -fsanitize-memory-track-origins -fno-omit-frame-pointer -g -O2 umr.cc 116 % ./a.out 2>log 117 % projects/compiler-rt/lib/asan/scripts/asan_symbolize.py / < log | c++filt 118 ==14425== WARNING: MemorySanitizer: UMR (uninitialized-memory-read) 119 ==14425== WARNING: Trying to symbolize code, but external symbolizer is not initialized! 120 #0 0x7f8bdda3824b in main umr.cc:6 121 #1 0x7f8bdce3a76c in __libc_start_main libc-start.c:226 122 raw origin id: 2030043137 123 ORIGIN: heap allocation: 124 #0 0x7f8bdda4034b in operator new[](unsigned long) msan_new_delete.cc:39 125 #1 0x7f8bdda3814d in main umr.cc:4 126 #2 0x7f8bdce3a76c in __libc_start_main libc-start.c:226 127 Exiting 128 129Origin tracking has proved to be very useful for debugging UMR 130reports. It slows down program execution by a factor of 1.5x-2x on top 131of the usual MemorySanitizer slowdown. 132 133Handling external code 134============================ 135 136MemorySanitizer requires that all program code is instrumented. This 137also includes any libraries that the program depends on, even libc. 138Failing to achieve this may result in false UMR reports. 139 140Full MemorySanitizer instrumentation is very difficult to achieve. To 141make it easier, MemorySanitizer runtime library includes 70+ 142interceptors for the most common libc functions. They make it possible 143to run MemorySanitizer-instrumented programs linked with 144uninstrumented libc. For example, the authors were able to bootstrap 145MemorySanitizer-instrumented Clang compiler by linking it with 146self-built instrumented libcxx (as a replacement for libstdc++). 147 148In the case when rebuilding all program dependencies with 149MemorySanitizer is problematic, an experimental MSanDR tool can be 150used. It is a DynamoRio-based tool that uses dynamic instrumentation 151to avoid false positives due to uninstrumented code. The tool simply 152marks memory from instrumented libraries as fully initialized. See 153`http://code.google.com/p/memory-sanitizer/wiki/Running#Running_with_the_dynamic_tool` 154for more information. 155 156Supported Platforms 157=================== 158 159MemorySanitizer is supported on 160 161* Linux x86\_64 (tested on Ubuntu 10.04 and 12.04); 162 163Limitations 164=========== 165 166* MemorySanitizer uses 2x more real memory than a native run, 3x with 167 origin tracking. 168* MemorySanitizer maps (but not reserves) 64 Terabytes of virtual 169 address space. This means that tools like ``ulimit`` may not work as 170 usually expected. 171* Static linking is not supported. 172* Non-position-independent executables are not supported. Therefore, the 173 ``fsanitize=memory`` flag will cause Clang to act as though the ``-fPIE`` 174 flag had been supplied if compiling without ``-fPIC``, and as though the 175 ``-pie`` flag had been supplied if linking an executable. 176* Depending on the version of Linux kernel, running without ASLR may 177 be not supported. Note that GDB disables ASLR by default. To debug 178 instrumented programs, use "set disable-randomization off". 179 180Current Status 181============== 182 183MemorySanitizer is an experimental tool. It is known to work on large 184real-world programs, like Clang/LLVM itself. 185 186More Information 187================ 188 189`http://code.google.com/p/memory-sanitizer <http://code.google.com/p/memory-sanitizer/>`_ 190 191