forked from OSchip/llvm-project
342 lines
15 KiB
ReStructuredText
342 lines
15 KiB
ReStructuredText
==========================
|
|
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 <https://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:
|
|
|
|
* ``-fsanitize=...``: print a verbose error report and continue execution (default);
|
|
* ``-fno-sanitize-recover=...``: print a verbose error report and exit the program;
|
|
* ``-fsanitize-trap=...``: 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=builtin``: Passing invalid values to compiler builtins.
|
|
- ``-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 (Darwin/Linux, C++ and x86/x86_64
|
|
only).
|
|
- ``-fsanitize=implicit-unsigned-integer-truncation``,
|
|
``-fsanitize=implicit-signed-integer-truncation``: Implicit conversion from
|
|
integer of larger bit width to smaller bit width, if that results in data
|
|
loss. That is, if the demoted value, after casting back to the original
|
|
width, is not equal to the original value before the downcast.
|
|
The ``-fsanitize=implicit-unsigned-integer-truncation`` handles conversions
|
|
between two ``unsigned`` types, while
|
|
``-fsanitize=implicit-signed-integer-truncation`` handles the rest of the
|
|
conversions - when either one, or both of the types are signed.
|
|
Issues caught by these sanitizers are not undefined behavior,
|
|
but are often unintentional.
|
|
- ``-fsanitize=implicit-integer-sign-change``: Implicit conversion between
|
|
integer types, if that changes the sign of the value. That is, if the the
|
|
original value was negative and the new value is positive (or zero),
|
|
or the original value was positive, and the new value is negative.
|
|
Issues caught by this sanitizer are not undefined behavior,
|
|
but are often unintentional.
|
|
- ``-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=nullability-arg``: Passing null as a function parameter
|
|
which is annotated with ``_Nonnull``.
|
|
- ``-fsanitize=nullability-assign``: Assigning null to an lvalue which
|
|
is annotated with ``_Nonnull``.
|
|
- ``-fsanitize=nullability-return``: Returning null from a function with
|
|
a return type annotated with ``_Nonnull``.
|
|
- ``-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=pointer-overflow``: Performing pointer arithmetic which
|
|
overflows.
|
|
- ``-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, where the
|
|
result of a signed integer computation cannot be represented in its type.
|
|
This includes all the checks covered by ``-ftrapv``, as well as checks for
|
|
signed division overflow (``INT_MIN/-1``), but not checks for
|
|
lossy implicit conversions performed before the computation
|
|
(see ``-fsanitize=implicit-conversion``). Both of these two issues are
|
|
handled by ``-fsanitize=implicit-conversion`` group of checks.
|
|
- ``-fsanitize=unreachable``: If control flow reaches an unreachable
|
|
program point.
|
|
- ``-fsanitize=unsigned-integer-overflow``: Unsigned integer overflow, where
|
|
the result of an unsigned integer computation cannot be represented in its
|
|
type. Unlike signed integer overflow, this is not undefined behavior, but
|
|
it is often unintentional. This sanitizer does not check for lossy implicit
|
|
conversions performed before such a computation
|
|
(see ``-fsanitize=implicit-conversion``).
|
|
- ``-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``, ``implicit-conversion`` and the
|
|
``nullability-*`` group of checks.
|
|
- ``-fsanitize=undefined-trap``: Deprecated alias of
|
|
``-fsanitize=undefined``.
|
|
- ``-fsanitize=implicit-integer-truncation``: Catches lossy integral
|
|
conversions. Enables ``implicit-signed-integer-truncation`` and
|
|
``implicit-unsigned-integer-truncation``.
|
|
- ``-fsanitize=implicit-integer-arithmetic-value-change``: Catches implicit
|
|
conversions that change the arithmetic value of the integer. Enables
|
|
``implicit-signed-integer-truncation`` and ``implicit-integer-sign-change``.
|
|
- ``-fsanitize=implicit-conversion``: Checks for suspicious
|
|
behaviour of implicit conversions. Enables
|
|
``implicit-unsigned-integer-truncation``,
|
|
``implicit-signed-integer-truncation`` and
|
|
``implicit-integer-sign-change``.
|
|
- ``-fsanitize=integer``: Checks for undefined or suspicious integer
|
|
behavior (e.g. unsigned integer overflow).
|
|
Enables ``signed-integer-overflow``, ``unsigned-integer-overflow``,
|
|
``shift``, ``integer-divide-by-zero``,
|
|
``implicit-unsigned-integer-truncation``,
|
|
``implicit-signed-integer-truncation`` and
|
|
``implicit-integer-sign-change``.
|
|
- ``-fsanitize=nullability``: Enables ``nullability-arg``,
|
|
``nullability-assign``, and ``nullability-return``. While violating
|
|
nullability does not have undefined behavior, it is often unintentional,
|
|
so UBSan offers to catch it.
|
|
|
|
Volatile
|
|
--------
|
|
|
|
The ``null``, ``alignment``, ``object-size``, and ``vptr`` checks do not apply
|
|
to pointers to types with the ``volatile`` qualifier.
|
|
|
|
Minimal Runtime
|
|
===============
|
|
|
|
There is a minimal UBSan runtime available suitable for use in production
|
|
environments. This runtime has a small attack surface. It only provides very
|
|
basic issue logging and deduplication, and does not support ``-fsanitize=vptr``
|
|
checking.
|
|
|
|
To use the minimal runtime, add ``-fsanitize-minimal-runtime`` to the clang
|
|
command line options. For example, if you're used to compiling with
|
|
``-fsanitize=undefined``, you could enable the minimal runtime with
|
|
``-fsanitize=undefined -fsanitize-minimal-runtime``.
|
|
|
|
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``.
|
|
|
|
Silencing Unsigned Integer Overflow
|
|
===================================
|
|
To silence reports from unsigned integer overflow, you can set
|
|
``UBSAN_OPTIONS=silence_unsigned_overflow=1``. This feature, combined with
|
|
``-fsanitize-recover=unsigned-integer-overflow``, is particularly useful for
|
|
providing fuzzing signal without blowing up logs.
|
|
|
|
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 operating systems:
|
|
|
|
* Android
|
|
* Linux
|
|
* NetBSD
|
|
* FreeBSD
|
|
* OpenBSD
|
|
* OS X 10.6 onwards
|
|
* Windows
|
|
|
|
The runtime library is relatively portable and platform independent. If the OS
|
|
you need is not listed above, UndefinedBehaviorSanitizer may already work for
|
|
it, or could be made to work with a minor porting effort.
|
|
|
|
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>`_
|