xref: /openbsd/gnu/llvm/clang/docs/AddressSanitizer.rst (revision 12c85518)

================
AddressSanitizer
================

.. contents::
   :local:

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 (clang flag ``-fsanitize-address-use-after-return=(never|runtime|always)`` default: ``runtime``)
    * Enable with: ``ASAN_OPTIONS=detect_stack_use_after_return=1`` (already enabled on Linux).
    * Disable with: ``ASAN_OPTIONS=detect_stack_use_after_return=0``.
* Use-after-scope (clang flag ``-fsanitize-address-use-after-scope``)
* Double-free, invalid free
* Memory leaks (experimental)

Typical slowdown introduced by AddressSanitizer is **2x**.

How to build
============

Build LLVM/Clang with `CMake <https://llvm.org/docs/CMake.html>`_.

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``).

.. code-block:: console

    % 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:

.. code-block:: console

    # 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. AddressSanitizer exits on the first detected error.
This is by design:

* This approach allows AddressSanitizer to produce faster and smaller generated code
  (both by ~5%).
* Fixing bugs becomes unavoidable. AddressSanitizer does not produce
  false alarms. Once a memory corruption occurs, the program is in an inconsistent
  state, which could lead to confusing results and potentially misleading
  subsequent reports.

If your process is sandboxed and you are running on OS X 10.10 or earlier, you
will need to set ``DYLD_INSERT_LIBRARIES`` environment variable and point it to
the ASan library that is packaged with the compiler used to build the
executable. (You can find the library by searching for dynamic libraries with
``asan`` in their name.) If the environment variable is not set, the process will
try to re-exec. Also keep in mind that when moving the executable to another machine,
the ASan library will also need to be copied over.

Symbolizing the Reports
=========================

To make AddressSanitizer symbolize its output
you need to set the ``ASAN_SYMBOLIZER_PATH`` environment variable to point to
the ``llvm-symbolizer`` binary (or make sure ``llvm-symbolizer`` is in your
``$PATH``):

.. code-block:: console

    % ASAN_SYMBOLIZER_PATH=/usr/local/bin/llvm-symbolizer ./a.out
    ==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

If that does not work for you (e.g. your process is sandboxed), you can use a
separate script to symbolize the result offline (online symbolization can be
force disabled by setting ``ASAN_OPTIONS=symbolize=0``):

.. code-block:: console

    % ASAN_OPTIONS=symbolize=0 ./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
    ...

Note that on macOS you may need to run ``dsymutil`` on your binary to have the
file\:line info in the AddressSanitizer reports.

Additional Checks
=================

Initialization order checking
-----------------------------

AddressSanitizer can optionally detect dynamic initialization order problems,
when initialization of globals defined in one translation unit uses
globals defined in another translation unit. To enable this check at runtime,
you should set environment variable
``ASAN_OPTIONS=check_initialization_order=1``.

Note that this option is not supported on macOS.

Stack Use After Return (UAR)
----------------------------

AddressSanitizer can optionally detect stack use after return problems.
This is available by default, or explicitly
(``-fsanitize-address-use-after-return=runtime``).
To disable this check at runtime, set the environment variable
``ASAN_OPTIONS=detect_stack_use_after_return=0``.

Enabling this check (``-fsanitize-address-use-after-return=always``) will
reduce code size.  The code size may be reduced further by completely
eliminating this check (``-fsanitize-address-use-after-return=never``).

To summarize: ``-fsanitize-address-use-after-return=<mode>``
  * ``never``: Completely disables detection of UAR errors (reduces code size).
  * ``runtime``: Adds the code for detection, but it can be disable via the
    runtime environment (``ASAN_OPTIONS=detect_stack_use_after_return=0``).
  * ``always``: Enables detection of UAR errors in all cases. (reduces code
    size, but not as much as ``never``).

Memory leak detection
---------------------

For more information on leak detector in AddressSanitizer, see
:doc:`LeakSanitizer`. The leak detection is turned on by default on Linux,
and can be enabled using ``ASAN_OPTIONS=detect_leaks=1`` on macOS;
however, it is not yet supported on other platforms.

Issue Suppression
=================

AddressSanitizer is not expected to produce false positives. If you see one,
look again; most likely it is a true positive!

Suppressing Reports in External Libraries
-----------------------------------------
Runtime interposition allows AddressSanitizer to find bugs in code that is
not being recompiled. If you run into an issue in external libraries, we
recommend immediately reporting it to the library maintainer so that it
gets addressed. However, you can use the following suppression mechanism
to unblock yourself and continue on with the testing. This suppression
mechanism should only be used for suppressing issues in external code; it
does not work on code recompiled with AddressSanitizer. To suppress errors
in external libraries, set the ``ASAN_OPTIONS`` environment variable to point
to a suppression file. You can either specify the full path to the file or the
path of the file relative to the location of your executable.

.. code-block:: bash

    ASAN_OPTIONS=suppressions=MyASan.supp

Use the following format to specify the names of the functions or libraries
you want to suppress. You can see these in the error report. Remember that
the narrower the scope of the suppression, the more bugs you will be able to
catch.

.. code-block:: bash

    interceptor_via_fun:NameOfCFunctionToSuppress
    interceptor_via_fun:-[ClassName objCMethodToSuppress:]
    interceptor_via_lib:NameOfTheLibraryToSuppress

Conditional Compilation with ``__has_feature(address_sanitizer)``
-----------------------------------------------------------------

In some cases one may need to execute different code depending on whether
AddressSanitizer 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(address_sanitizer)
    // code that builds only under AddressSanitizer
    #  endif
    #endif

Disabling Instrumentation with ``__attribute__((no_sanitize("address")))``
--------------------------------------------------------------------------

Some code should not be instrumented by AddressSanitizer. One may use
the attribute ``__attribute__((no_sanitize("address")))`` (which has
deprecated synonyms `no_sanitize_address` and
`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)``.

The same attribute used on a global variable prevents AddressSanitizer
from adding redzones around it and detecting out of bounds accesses.


AddressSanitizer also supports
``__attribute__((disable_sanitizer_instrumentation))``. This attribute
works similar to ``__attribute__((no_sanitize("address")))``, but it also
prevents instrumentation performed by other sanitizers.

Suppressing Errors in Recompiled Code (Ignorelist)
--------------------------------------------------

AddressSanitizer supports ``src`` and ``fun`` entity types in
:doc:`SanitizerSpecialCaseList`, that can be used to suppress error reports
in the specified source files or functions. Additionally, AddressSanitizer
introduces ``global`` and ``type`` entity types that can be used to
suppress error reports for out-of-bound access to globals with certain
names and types (you may only specify class or struct types).

You may use an ``init`` category to suppress reports about initialization-order
problems happening in certain source files or with certain global variables.

.. code-block:: bash

    # Suppress error reports for code in a file or in a function:
    src:bad_file.cpp
    # Ignore all functions with names containing MyFooBar:
    fun:*MyFooBar*
    # Disable out-of-bound checks for global:
    global:bad_array
    # Disable out-of-bound checks for global instances of a given class ...
    type:Namespace::BadClassName
    # ... or a given struct. Use wildcard to deal with anonymous namespace.
    type:Namespace2::*::BadStructName
    # Disable initialization-order checks for globals:
    global:bad_init_global=init
    type:*BadInitClassSubstring*=init
    src:bad/init/files/*=init

Suppressing memory leaks
------------------------

Memory leak reports produced by :doc:`LeakSanitizer` (if it is run as a part
of AddressSanitizer) can be suppressed by a separate file passed as

.. code-block:: bash

    LSAN_OPTIONS=suppressions=MyLSan.supp

which contains lines of the form `leak:<pattern>`. Memory leak will be
suppressed if pattern matches any function name, source file name, or
library name in the symbolized stack trace of the leak report. See
`full documentation
<https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer#suppressions>`_
for more details.

Code generation control
=======================

Instrumentation code outlining
------------------------------

By default AddressSanitizer inlines the instrumentation code to improve the
run-time performance, which leads to increased binary size. Using the
(clang flag ``-fsanitize-address-outline-instrumentation` default: ``false``)
flag forces all code instrumentation to be outlined, which reduces the size
of the generated code, but also reduces the run-time performance.

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 of executables is not supported.

Supported Platforms
===================

AddressSanitizer is supported on:

* Linux i386/x86\_64 (tested on Ubuntu 12.04)
* macOS 10.7 - 10.11 (i386/x86\_64)
* iOS Simulator
* Android ARM
* NetBSD i386/x86\_64
* FreeBSD i386/x86\_64 (tested on FreeBSD 11-current)
* Windows 8.1+ (i386/x86\_64)

Ports to various other platforms are in progress.

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.

The Windows port is functional and is used by Chrome and Firefox, but it is not
as well supported as the other ports.

More Information
================

`<https://github.com/google/sanitizers/wiki/AddressSanitizer>`_