# Clang 12 documentation

## MemorySanitizer

- Introduction

- How to build

- Usage

  - `__has_feature(memory_sanitizer)`

  - `__attribute__((no_sanitize("memory")))`

  - Blacklist

- Report symbolization

- Origin Tracking

- Use-after-destruction detection

- Handling external code

- Supported Platforms

- Limitations

- Current Status

- More Information

## 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.

## 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 meaningful 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`).

    % 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.

    % ./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. If you find the error report hard to understand, try enabling origin tracking.

### `__has_feature(memory_sanitizer)`

In some cases one may need to execute different code depending on whether MemorySanitizer is enabled. __has_feature can be used for this purpose.

    #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 Sanitizer special case list, 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 uninitialized 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,

    % 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 and increases memory overhead.

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

## Use-after-destruction detection

You can enable experimental use-after-destruction detection in MemorySanitizer. After invocation of the destructor, the object will be considered no longer readable, and using underlying memory will lead to error reports in runtime.

This feature is still experimental, in order to enable it at runtime you need to:

1. Pass addition Clang option `-fsanitize-memory-use-after-dtor` during compilation.

2. Set environment variable MSAN_OPTIONS=poison_in_dtor=1 before running the program.

## 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. For the same reason you may need to replace all inline assembly code that writes to memory with a pure C/C++ code.

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 the following OS:

- Linux

- NetBSD

- FreeBSD

## 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.

- Older versions of MSan (LLVM 3.7 and older) didn’t work with non-position-independent executables, and could fail on some Linux kernel versions with disabled ASLR. Refer to documentation for older versions for more details.

- MemorySanitizer might be incompatible with position-independent executables from FreeBSD 13 but there is a check done at runtime and throws a warning in this case.

## Current Status

MemorySanitizer is known to work on large real-world programs (like Clang/LLVM itself) that can be recompiled from source, including all dependent libraries.

## More Information

https://github.com/google/sanitizers/wiki/MemorySanitizer