# AddressSanitizer

- Introduction
- How to Build
- Usage
  - __has_feature(address_sanitizer)
  - __attribute__((no_address_safety_analysis))
- Supported Platforms
- Limitations
- Current Status
- More Information

## Introduction

AddressSanitizer is a fast memory error detector. It consists of a compiler instrumentation module and a run-time library. The tool can detect the following types of bugs:

- Out-of-bounds accesses to heap, stack and globals
- Use-after-free
- Use-after-return (to some extent)
- Double-free, invalid free

Typical slowdown introduced by AddressSanitizer is 2x.

## How to build

Follow the clang build instructions. CMake build is supported.

## Usage

Simply compile and link your program with `-fsanitize=address` flag.
The AddressSanitizer 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 AddressSanitizer run-time is not linked, so `-Wl,-z,defs` may cause link errors (don't use it with AddressSanitizer).
To get a reasonable performance add `-O1` or higher.
To get nicer 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 example_UseAfterFree.cc
    int main(int argc, char **argv) {
      int *array = new int[100];
      delete [] array;
      return array[argc];  // BOOM
    }

    # Compile and link
    % clang -O1 -g -fsanitize=address -fno-omit-frame-pointer example_UseAfterFree.cc

OR

    # Compile
    % clang -O1 -g -fsanitize=address -fno-omit-frame-pointer -c example_UseAfterFree.cc
    # Link
    % clang -g -fsanitize=address example_UseAfterFree.o

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

    % ./a.out 2> log
    % projects/compiler-rt/lib/asan/scripts/asan_symbolize.py / < log | c++filt
    ==9442== ERROR: AddressSanitizer heap-use-after-free on address 0x7f7ddab8c084 at pc 0x403c8c bp 0x7fff87fb82d0 sp 0x7fff87fb82c8
    READ of size 4 at 0x7f7ddab8c084 thread T0
        #0 0x403c8c in main example_UseAfterFree.cc:4
        #1 0x7f7ddabcac4d in __libc_start_main ??:0
    0x7f7ddab8c084 is located 4 bytes inside of 400-byte region [0x7f7ddab8c080,0x7f7ddab8c210)
    freed by thread T0 here:
        #0 0x404704 in operator delete[](void*) ??:0
        #1 0x403c53 in main example_UseAfterFree.cc:4
        #2 0x7f7ddabcac4d in __libc_start_main ??:0
    previously allocated by thread T0 here:
        #0 0x404544 in operator new[](unsigned long) ??:0
        #1 0x403c43 in main example_UseAfterFree.cc:2
        #2 0x7f7ddabcac4d in __libc_start_main ??:0
    ==9442== ABORTING

AddressSanitizer exits on the first detected error. This is by design. One reason: it makes the generated code smaller and faster (both by ~5%). Another reason: this makes fixing bugs unavoidable. With Valgrind, it is often the case that users treat Valgrind warnings as false positives (which they are not) and don't fix them.

### __has_feature(address_sanitizer)

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

    #if defined(__has_feature)
    # if __has_feature(address_sanitizer)
      code that builds only under AddressSanitizer
    # endif
    #endif

### __attribute__((no_address_safety_analysis))

Some code should not be instrumented by AddressSanitizer. One may use the function attribute `no_address_safety_analysis` to disable instrumentation of a particular function. This attribute may not be supported by other compilers, so we suggest to use it together with `__has_feature(address_sanitizer)`. Note: currently, this attribute will be lost if the function is inlined.

## Supported Platforms

AddressSanitizer is supported on

- Linux i386/x86_64 (tested on Ubuntu 10.04 and 12.04).
- MacOS 10.6, 10.7 and 10.8 (i386/x86_64).

Support for Linux ARM (and Android ARM) is in progress (it may work, but is not guaranteed too).

## Limitations

- AddressSanitizer uses more real memory than a native run. Exact overhead depends on the allocations sizes. The smaller the allocations you make the bigger the overhead is.
- AddressSanitizer uses more stack memory. We have seen up to 3x increase.
- On 64-bit platforms AddressSanitizer maps (but not reserves) 16+ Terabytes of virtual address space. This means that tools like `ulimit` may not work as usually expected.
- Static linking is not supported.

## Current Status

AddressSanitizer is fully functional on supported platforms starting from LLVM 3.1. The test suite is integrated into CMake build and can be run with `make check-asan` command.

## More Information

http://code.google.com/p/address-sanitizer.