clang/docs/UndefinedBehaviorSanitizer.rst

==========================
UndefinedBehaviorSanitizer
==========================

.. contents::
   :local:

Introduction
============

UndefinedBehaviorSanitizer (UBSan) is a fast undefined behavior detector.
UBSan modifies the program at compile-time to catch various kinds of undefined
behavior during program execution, for example:

* Using misaligned or null pointer
* Signed integer overflow
* Conversion to, from, or between floating-point types which would
  overflow the destination

See the full list of available :ref:`checks <ubsan-checks>` below.

UBSan has an optional run-time library which provides better error reporting.
The checks have small runtime cost and no impact on address space layout or ABI.

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

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

Usage
=====

Use ``clang++`` to compile and link your program with ``-fsanitize=undefined``
flag. Make sure to use ``clang++`` (not ``ld``) as a linker, so that your
executable is linked with proper UBSan runtime libraries. You can use ``clang``
instead of ``clang++`` if you're compiling/linking C code.

.. code-block:: console

  % cat test.cc
  int main(int argc, char **argv) {
    int k = 0x7fffffff;
    k += argc;
    return 0;
  }
  % clang++ -fsanitize=undefined test.cc
  % ./a.out
  test.cc:3:5: runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'

You can enable only a subset of :ref:`checks <ubsan-checks>` offered by UBSan,
and define the desired behavior for each kind of check:

* print a verbose error report and continue execution (default);
* print a verbose error report and exit the program;
* execute a trap instruction (doesn't require UBSan run-time support).

For example if you compile/link your program as:

.. code-block:: console

  % clang++ -fsanitize=signed-integer-overflow,null,alignment -fno-sanitize-recover=null -fsanitize-trap=alignment

the program will continue execution after signed integer overflows, exit after
the first invalid use of a null pointer, and trap after the first use of misaligned
pointer.

.. _ubsan-checks:

Available checks
================

Available checks are:

  -  ``-fsanitize=alignment``: Use of a misaligned pointer or creation
     of a misaligned reference.
  -  ``-fsanitize=bool``: Load of a ``bool`` value which is neither
     ``true`` nor ``false``.
  -  ``-fsanitize=bounds``: Out of bounds array indexing, in cases
     where the array bound can be statically determined.
  -  ``-fsanitize=enum``: Load of a value of an enumerated type which
     is not in the range of representable values for that enumerated
     type.
  -  ``-fsanitize=float-cast-overflow``: Conversion to, from, or
     between floating-point types which would overflow the
     destination.
  -  ``-fsanitize=float-divide-by-zero``: Floating point division by
     zero.
  -  ``-fsanitize=function``: Indirect call of a function through a
     function pointer of the wrong type (Linux, C++ and x86/x86_64 only).
  -  ``-fsanitize=integer-divide-by-zero``: Integer division by zero.
  -  ``-fsanitize=nonnull-attribute``: Passing null pointer as a function
     parameter which is declared to never be null.
  -  ``-fsanitize=null``: Use of a null pointer or creation of a null
     reference.
  -  ``-fsanitize=object-size``: An attempt to potentially use bytes which
     the optimizer can determine are not part of the object being accessed.
     This will also detect some types of undefined behavior that may not
     directly access memory, but are provably incorrect given the size of
     the objects involved, such as invalid downcasts and calling methods on
     invalid pointers. These checks are made in terms of
     ``__builtin_object_size``, and consequently may be able to detect more
     problems at higher optimization levels.
  -  ``-fsanitize=return``: In C++, reaching the end of a
     value-returning function without returning a value.
  -  ``-fsanitize=returns-nonnull-attribute``: Returning null pointer
     from a function which is declared to never return null.
  -  ``-fsanitize=shift``: Shift operators where the amount shifted is
     greater or equal to the promoted bit-width of the left hand side
     or less than zero, or where the left hand side is negative. For a
     signed left shift, also checks for signed overflow in C, and for
     unsigned overflow in C++. You can use ``-fsanitize=shift-base`` or
     ``-fsanitize=shift-exponent`` to check only left-hand side or
     right-hand side of shift operation, respectively.
  -  ``-fsanitize=signed-integer-overflow``: Signed integer overflow,
     including all the checks added by ``-ftrapv``, and checking for
     overflow in signed division (``INT_MIN / -1``).
  -  ``-fsanitize=unreachable``: If control flow reaches
     ``__builtin_unreachable``.
  -  ``-fsanitize=unsigned-integer-overflow``: Unsigned integer
     overflows. Note that unlike signed integer overflow, unsigned integer
     is not undefined behavior. However, while it has well-defined semantics,
     it is often unintentional, so UBSan offers to catch it.
  -  ``-fsanitize=vla-bound``: A variable-length array whose bound
     does not evaluate to a positive value.
  -  ``-fsanitize=vptr``: Use of an object whose vptr indicates that
     it is of the wrong dynamic type, or that its lifetime has not
     begun or has ended. Incompatible with ``-fno-rtti``. Link must
     be performed by ``clang++``, not ``clang``, to make sure C++-specific
     parts of the runtime library and C++ standard libraries are present.

You can also use the following check groups:
  -  ``-fsanitize=undefined``: All of the checks listed above other than
     ``unsigned-integer-overflow``.
  -  ``-fsanitize=undefined-trap``: Deprecated alias of
     ``-fsanitize=undefined``.
  -  ``-fsanitize=integer``: Checks for undefined or suspicious integer
     behavior (e.g. unsigned integer overflow).

Stack traces and report symbolization
=====================================
If you want UBSan to print symbolized stack trace for each error report, you
will need to:

#. Compile with ``-g`` and ``-fno-omit-frame-pointer`` to get proper debug
   information in your binary.
#. Run your program with environment variable
   ``UBSAN_OPTIONS=print_stacktrace=1``.
#. Make sure ``llvm-symbolizer`` binary is in ``PATH``.

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

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

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

You disable UBSan checks for particular functions with
``__attribute__((no_sanitize("undefined")))``. You can use all values of
``-fsanitize=`` flag in this attribute, e.g. if your function deliberately
contains possible signed integer overflow, you can use
``__attribute__((no_sanitize("signed-integer-overflow")))``.

This attribute may not be
supported by other compilers, so consider using it together with
``#if defined(__clang__)``.

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

UndefinedBehaviorSanitizer supports ``src`` and ``fun`` entity types in
:doc:`SanitizerSpecialCaseList`, that can be used to suppress error reports
in the specified source files or functions.

Runtime suppressions
--------------------

Sometimes you can suppress UBSan error reports for specific files, functions,
or libraries without recompiling the code. You need to pass a path to
suppression file in a ``UBSAN_OPTIONS`` environment variable.

.. code-block:: bash

    UBSAN_OPTIONS=suppressions=MyUBSan.supp

You need to specify a :ref:`check <ubsan-checks>` you are suppressing and the
bug location. For example:

.. code-block:: bash

  signed-integer-overflow:file-with-known-overflow.cpp
  alignment:function_doing_unaligned_access
  vptr:shared_object_with_vptr_failures.so

There are several limitations:

* Sometimes your binary must have enough debug info and/or symbol table, so
  that the runtime could figure out source file or function name to match
  against the suppression.
* It is only possible to suppress recoverable checks. For the example above,
  you can additionally pass
  ``-fsanitize-recover=signed-integer-overflow,alignment,vptr``, although
  most of UBSan checks are recoverable by default.
* Check groups (like ``undefined``) can't be used in suppressions file, only
  fine-grained checks are supported.

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

UndefinedBehaviorSanitizer is supported on the following OS:

* Android
* Linux
* FreeBSD
* OS X 10.6 onwards

and for the following architectures:

* i386/x86\_64
* ARM
* AArch64
* PowerPC64
* MIPS/MIPS64

Current Status
==============

UndefinedBehaviorSanitizer is available on selected platforms starting from LLVM
3.3. The test suite is integrated into the CMake build and can be run with
``check-ubsan`` command.

Additional Configuration
========================

UndefinedBehaviorSanitizer adds static check data for each check unless it is
in trap mode. This check data includes the full file name. The option
``-fsanitize-undefined-strip-path-components=N`` can be used to trim this
information. If ``N`` is positive, file information emitted by
UndefinedBehaviorSanitizer will drop the first ``N`` components from the file
path. If ``N`` is negative, the last ``N`` components will be kept.

Example
-------

For a file called ``/code/library/file.cpp``, here is what would be emitted:
* Default (No flag, or ``-fsanitize-undefined-strip-path-components=0``): ``/code/library/file.cpp``
* ``-fsanitize-undefined-strip-path-components=1``: ``code/library/file.cpp``
* ``-fsanitize-undefined-strip-path-components=2``: ``library/file.cpp``
* ``-fsanitize-undefined-strip-path-components=-1``: ``file.cpp``
* ``-fsanitize-undefined-strip-path-components=-2``: ``library/file.cpp``

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

* From LLVM project blog:
  `What Every C Programmer Should Know About Undefined Behavior
  <http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html>`_
* From John Regehr's *Embedded in Academia* blog:
  `A Guide to Undefined Behavior in C and C++
  <http://blog.regehr.org/archives/213>`_