clang/docs/ControlFlowIntegrity.rst - llvm-project - Git at Google

 ======================
 Control Flow Integrity
 ======================

 .. toctree::
    :hidden:

    ControlFlowIntegrityDesign

 .. contents::
    :local:

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

 Clang includes an implementation of a number of control flow integrity (CFI)
 schemes, which are designed to abort the program upon detecting certain forms
 of undefined behavior that can potentially allow attackers to subvert the
 program's control flow. These schemes have been optimized for performance,
 allowing developers to enable them in release builds.

 To enable Clang's available CFI schemes, use the flag ``-fsanitize=cfi``.
 You can also enable a subset of available :ref:`schemes <cfi-schemes>`.
 As currently implemented, all schemes rely on link-time optimization (LTO);
 so it is required to specify ``-flto``, and the linker used must support LTO,
 for example via the `gold plugin`_.

 To allow the checks to be implemented efficiently, the program must
 be structured such that certain object files are compiled with CFI
 enabled, and are statically linked into the program. This may preclude
 the use of shared libraries in some cases.

 The compiler will only produce CFI checks for a class if it can infer hidden
 LTO visibility for that class. LTO visibility is a property of a class that
 is inferred from flags and attributes. For more details, see the documentation
 for :doc:`LTO visibility <LTOVisibility>`.

 The ``-fsanitize=cfi-{vcall,nvcall,derived-cast,unrelated-cast}`` flags
 require that a ``-fvisibility=`` flag also be specified. This is because the
 default visibility setting is ``-fvisibility=default``, which would disable
 CFI checks for classes without visibility attributes. Most users will want
 to specify ``-fvisibility=hidden``, which enables CFI checks for such classes.

 Experimental support for :ref:`cross-DSO control flow integrity
 <cfi-cross-dso>` exists that does not require classes to have hidden LTO
 visibility. This cross-DSO support has unstable ABI at this time.

 .. _gold plugin: https://llvm.org/docs/GoldPlugin.html

 .. _cfi-schemes:

 Available schemes
 =================

 Available schemes are:

   -  ``-fsanitize=cfi-cast-strict``: Enables :ref:`strict cast checks
      <cfi-strictness>`.
   -  ``-fsanitize=cfi-derived-cast``: Base-to-derived cast to the wrong
      dynamic type.
   -  ``-fsanitize=cfi-unrelated-cast``: Cast from ``void*`` or another
      unrelated type to the wrong dynamic type.
   -  ``-fsanitize=cfi-nvcall``: Non-virtual call via an object whose vptr is of
      the wrong dynamic type.
   -  ``-fsanitize=cfi-vcall``: Virtual call via an object whose vptr is of the
      wrong dynamic type.
   -  ``-fsanitize=cfi-icall``: Indirect call of a function with wrong dynamic
      type.
   -  ``-fsanitize=cfi-mfcall``: Indirect call via a member function pointer with
      wrong dynamic type.

 You can use ``-fsanitize=cfi`` to enable all the schemes and use
 ``-fno-sanitize`` flag to narrow down the set of schemes as desired.
 For example, you can build your program with
 ``-fsanitize=cfi -fno-sanitize=cfi-nvcall,cfi-icall``
 to use all schemes except for non-virtual member function call and indirect call
 checking.

 Remember that you have to provide ``-flto`` or ``-flto=thin`` if at
 least one CFI scheme is enabled.

 Trapping and Diagnostics
 ========================

 By default, CFI will abort the program immediately upon detecting a control
 flow integrity violation. You can use the :ref:`-fno-sanitize-trap=
 <controlling-code-generation>` flag to cause CFI to print a diagnostic
 similar to the one below before the program aborts.

 .. code-block:: console

     bad-cast.cpp:109:7: runtime error: control flow integrity check for type 'B' failed during base-to-derived cast (vtable address 0x000000425a50)
     0x000000425a50: note: vtable is of type 'A'
      00 00 00 00  f0 f1 41 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  20 5a 42 00
                   ^

 If diagnostics are enabled, you can also configure CFI to continue program
 execution instead of aborting by using the :ref:`-fsanitize-recover=
 <controlling-code-generation>` flag.

 Forward-Edge CFI for Virtual Calls
 ==================================

 This scheme checks that virtual calls take place using a vptr of the correct
 dynamic type; that is, the dynamic type of the called object must be a
 derived class of the static type of the object used to make the call.
 This CFI scheme can be enabled on its own using ``-fsanitize=cfi-vcall``.

 For this scheme to work, all translation units containing the definition
 of a virtual member function (whether inline or not), other than members
 of :ref:`ignored <cfi-ignorelist>` types or types with public :doc:`LTO
 visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
 enabled and be statically linked into the program.

 Performance
 -----------

 A performance overhead of less than 1% has been measured by running the
 Dromaeo benchmark suite against an instrumented version of the Chromium
 web browser. Another good performance benchmark for this mechanism is the
 virtual-call-heavy SPEC 2006 xalancbmk.

 Note that this scheme has not yet been optimized for binary size; an increase
 of up to 15% has been observed for Chromium.

 Bad Cast Checking
 =================

 This scheme checks that pointer casts are made to an object of the correct
 dynamic type; that is, the dynamic type of the object must be a derived class
 of the pointee type of the cast. The checks are currently only introduced
 where the class being casted to is a polymorphic class.

 Bad casts are not in themselves control flow integrity violations, but they
 can also create security vulnerabilities, and the implementation uses many
 of the same mechanisms.

 There are two types of bad cast that may be forbidden: bad casts
 from a base class to a derived class (which can be checked with
 ``-fsanitize=cfi-derived-cast``), and bad casts from a pointer of
 type ``void*`` or another unrelated type (which can be checked with
 ``-fsanitize=cfi-unrelated-cast``).

 The difference between these two types of casts is that the first is defined
 by the C++ standard to produce an undefined value, while the second is not
 in itself undefined behavior (it is well defined to cast the pointer back
 to its original type) unless the object is uninitialized and the cast is a
 ``static_cast`` (see C++14 [basic.life]p5).

 If a program as a matter of policy forbids the second type of cast, that
 restriction can normally be enforced. However it may in some cases be necessary
 for a function to perform a forbidden cast to conform with an external API
 (e.g. the ``allocate`` member function of a standard library allocator). Such
 functions may be :ref:`ignored <cfi-ignorelist>`.

 For this scheme to work, all translation units containing the definition
 of a virtual member function (whether inline or not), other than members
 of :ref:`ignored <cfi-ignorelist>` types or types with public :doc:`LTO
 visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
 enabled and be statically linked into the program.

 Non-Virtual Member Function Call Checking
 =========================================

 This scheme checks that non-virtual calls take place using an object of
 the correct dynamic type; that is, the dynamic type of the called object
 must be a derived class of the static type of the object used to make the
 call. The checks are currently only introduced where the object is of a
 polymorphic class type.  This CFI scheme can be enabled on its own using
 ``-fsanitize=cfi-nvcall``.

 For this scheme to work, all translation units containing the definition
 of a virtual member function (whether inline or not), other than members
 of :ref:`ignored <cfi-ignorelist>` types or types with public :doc:`LTO
 visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
 enabled and be statically linked into the program.

 .. _cfi-strictness:

 Strictness
 ----------

 If a class has a single non-virtual base and does not introduce or override
 virtual member functions or fields other than an implicitly defined virtual
 destructor, it will have the same layout and virtual function semantics as
 its base. By default, casts to such classes are checked as if they were made
 to the least derived such class.

 Casting an instance of a base class to such a derived class is technically
 undefined behavior, but it is a relatively common hack for introducing
 member functions on class instances with specific properties that works under
 most compilers and should not have security implications, so we allow it by
 default. It can be disabled with ``-fsanitize=cfi-cast-strict``.

 Indirect Function Call Checking
 ===============================

 This scheme checks that function calls take place using a function of the
 correct dynamic type; that is, the dynamic type of the function must match
 the static type used at the call. This CFI scheme can be enabled on its own
 using ``-fsanitize=cfi-icall``.

 For this scheme to work, each indirect function call in the program, other
 than calls in :ref:`ignored <cfi-ignorelist>` functions, must call a
 function which was either compiled with ``-fsanitize=cfi-icall`` enabled,
 or whose address was taken by a function in a translation unit compiled with
 ``-fsanitize=cfi-icall``.

 If a function in a translation unit compiled with ``-fsanitize=cfi-icall``
 takes the address of a function not compiled with ``-fsanitize=cfi-icall``,
 that address may differ from the address taken by a function in a translation
 unit not compiled with ``-fsanitize=cfi-icall``. This is technically a
 violation of the C and C++ standards, but it should not affect most programs.

 Each translation unit compiled with ``-fsanitize=cfi-icall`` must be
 statically linked into the program or shared library, and calls across
 shared library boundaries are handled as if the callee was not compiled with
 ``-fsanitize=cfi-icall``.

 This scheme is currently supported on a limited set of targets: x86,
 x86_64, arm, arch64 and wasm.

 ``-fsanitize-cfi-icall-generalize-pointers``
 --------------------------------------------

 Mismatched pointer types are a common cause of cfi-icall check failures.
 Translation units compiled with the ``-fsanitize-cfi-icall-generalize-pointers``
 flag relax pointer type checking for call sites in that translation unit,
 applied across all functions compiled with ``-fsanitize=cfi-icall``.

 Specifically, pointers in return and argument types are treated as equivalent as
 long as the qualifiers for the type they point to match. For example, ``char*``,
 ``char**``, and ``int*`` are considered equivalent types. However, ``char*`` and
 ``const char*`` are considered separate types.

 ``-fsanitize-cfi-icall-generalize-pointers`` is not compatible with
 ``-fsanitize-cfi-cross-dso``.

 .. _cfi-icall-experimental-normalize-integers:

 ``-fsanitize-cfi-icall-experimental-normalize-integers``
 --------------------------------------------------------

 This option enables normalizing integer types as vendor extended types for
 cross-language LLVM CFI/KCFI support with other languages that can't represent
 and encode C/C++ integer types.

 Specifically, integer types are encoded as their defined representations (e.g.,
 8-bit signed integer, 16-bit signed integer, 32-bit signed integer, ...) for
 compatibility with languages that define explicitly-sized integer types (e.g.,
 i8, i16, i32, ..., in Rust).

 ``-fsanitize-cfi-icall-experimental-normalize-integers`` is compatible with
 ``-fsanitize-cfi-icall-generalize-pointers``.

 This option is currently experimental.

 .. _cfi-canonical-jump-tables:

 ``-fsanitize-cfi-canonical-jump-tables``
 ----------------------------------------

 The default behavior of Clang's indirect function call checker will replace
 the address of each CFI-checked function in the output file's symbol table
 with the address of a jump table entry which will pass CFI checks. We refer
 to this as making the jump table `canonical`. This property allows code that
 was not compiled with ``-fsanitize=cfi-icall`` to take a CFI-valid address
 of a function, but it comes with a couple of caveats that are especially
 relevant for users of cross-DSO CFI:

 - There is a performance and code size overhead associated with each
   exported function, because each such function must have an associated
   jump table entry, which must be emitted even in the common case where the
   function is never address-taken anywhere in the program, and must be used
   even for direct calls between DSOs, in addition to the PLT overhead.

 - There is no good way to take a CFI-valid address of a function written in
   assembly or a language not supported by Clang. The reason is that the code
   generator would need to insert a jump table in order to form a CFI-valid
   address for assembly functions, but there is no way in general for the
   code generator to determine the language of the function. This may be
   possible with LTO in the intra-DSO case, but in the cross-DSO case the only
   information available is the function declaration. One possible solution
   is to add a C wrapper for each assembly function, but these wrappers can
   present a significant maintenance burden for heavy users of assembly in
   addition to adding runtime overhead.

 For these reasons, we provide the option of making the jump table non-canonical
 with the flag ``-fno-sanitize-cfi-canonical-jump-tables``. When the jump
 table is made non-canonical, symbol table entries point directly to the
 function body. Any instances of a function's address being taken in C will
 be replaced with a jump table address.

 This scheme does have its own caveats, however. It does end up breaking
 function address equality more aggressively than the default behavior,
 especially in cross-DSO mode which normally preserves function address
 equality entirely.

 Furthermore, it is occasionally necessary for code not compiled with
 ``-fsanitize=cfi-icall`` to take a function address that is valid
 for CFI. For example, this is necessary when a function's address
 is taken by assembly code and then called by CFI-checking C code. The
 ``__attribute__((cfi_canonical_jump_table))`` attribute may be used to make
 the jump table entry of a specific function canonical so that the external
 code will end up taking an address for the function that will pass CFI checks.

 ``-fsanitize=cfi-icall`` and ``-fsanitize=function``
 ----------------------------------------------------

 This tool is similar to ``-fsanitize=function`` in that both tools check
 the types of function calls. However, the two tools occupy different points
 on the design space; ``-fsanitize=function`` is a developer tool designed
 to find bugs in local development builds, whereas ``-fsanitize=cfi-icall``
 is a security hardening mechanism designed to be deployed in release builds.

 ``-fsanitize=function`` has a higher space and time overhead due to a more
 complex type check at indirect call sites, which may make it unsuitable for
 deployment.

 On the other hand, ``-fsanitize=function`` conforms more closely with the C++
 standard and user expectations around interaction with shared libraries;
 the identity of function pointers is maintained, and calls across shared
 library boundaries are no different from calls within a single program or
 shared library.

 .. _kcfi:

 ``-fsanitize=kcfi``
 -------------------

 This is an alternative indirect call control-flow integrity scheme designed
 for low-level system software, such as operating system kernels. Unlike
 ``-fsanitize=cfi-icall``, it doesn't require ``-flto``, won't result in
 function pointers being replaced with jump table references, and never breaks
 cross-DSO function address equality. These properties make KCFI easier to
 adopt in low-level software. KCFI is limited to checking only function
 pointers, and isn't compatible with executable-only memory.

 ``-fsanitize-kcfi-arity``
 -----------------------------

 For supported targets, this feature extends kCFI by telling the compiler to
 record information about each indirect-callable function's arity (i.e., the
 number of arguments passed in registers) into the binary. Some kernel CFI
 techniques, such as FineIBT, may be able to use this information to provide
 enhanced security.

 Member Function Pointer Call Checking
 =====================================

 This scheme checks that indirect calls via a member function pointer
 take place using an object of the correct dynamic type. Specifically, we
 check that the dynamic type of the member function referenced by the member
 function pointer matches the "function pointer" part of the member function
 pointer, and that the member function's class type is related to the base
 type of the member function. This CFI scheme can be enabled on its own using
 ``-fsanitize=cfi-mfcall``.

 The compiler will only emit a full CFI check if the member function pointer's
 base type is complete. This is because the complete definition of the base
 type contains information that is necessary to correctly compile the CFI
 check. To ensure that the compiler always emits a full CFI check, it is
 recommended to also pass the flag ``-fcomplete-member-pointers``, which
 enables a non-conforming language extension that requires member pointer
 base types to be complete if they may be used for a call.

 For this scheme to work, all translation units containing the definition
 of a virtual member function (whether inline or not), other than members
 of :ref:`ignored <cfi-ignorelist>` types or types with public :doc:`LTO
 visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
 enabled and be statically linked into the program.

 This scheme is currently not compatible with cross-DSO CFI or the
 Microsoft ABI.

 .. _cfi-ignorelist:

 Ignorelist
 ==========

 A :doc:`SanitizerSpecialCaseList` can be used to relax CFI checks for certain
 source files, functions and types using the ``src``, ``fun`` and ``type``
 entity types. Specific CFI modes can be be specified using ``[section]``
 headers.

 .. code-block:: bash

     # Suppress all CFI checking for code in a file.
     src:bad_file.cpp
     src:bad_header.h
     # Ignore all functions with names containing MyFooBar.
     fun:*MyFooBar*
     # Ignore all types in the standard library.
     type:std::*
     # Disable only unrelated cast checks for this function
     [cfi-unrelated-cast]
     fun:*UnrelatedCast*
     # Disable CFI call checks for this function without affecting cast checks
     [cfi-vcall|cfi-nvcall|cfi-icall]
     fun:*BadCall*


 .. _cfi-cross-dso:

 Shared library support
 ======================

 Use **-f[no-]sanitize-cfi-cross-dso** to enable the cross-DSO control
 flow integrity mode, which allows all CFI schemes listed above to
 apply across DSO boundaries. As in the regular CFI, each DSO must be
 built with ``-flto`` or ``-flto=thin``.

 Normally, CFI checks will only be performed for classes that have hidden LTO
 visibility. With this flag enabled, the compiler will emit cross-DSO CFI
 checks for all classes, except for those which appear in the CFI ignorelist
 or which use a ``no_sanitize`` attribute.

 Design
 ======

 Please refer to the :doc:`design document<ControlFlowIntegrityDesign>`.

 Publications
 ============

 `Control-Flow Integrity: Principles, Implementations, and Applications <https://research.microsoft.com/pubs/64250/ccs05.pdf>`_.
 Martin Abadi, Mihai Budiu, Úlfar Erlingsson, Jay Ligatti.

 `Enforcing Forward-Edge Control-Flow Integrity in GCC & LLVM <http://www.pcc.me.uk/~peter/acad/usenix14.pdf>`_.
 Caroline Tice, Tom Roeder, Peter Collingbourne, Stephen Checkoway,
 Úlfar Erlingsson, Luis Lozano, Geoff Pike.
	======================
	Control Flow Integrity
	======================

	.. toctree::
	:hidden:

	ControlFlowIntegrityDesign

	.. contents::
	:local:

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

	Clang includes an implementation of a number of control flow integrity (CFI)
	schemes, which are designed to abort the program upon detecting certain forms
	of undefined behavior that can potentially allow attackers to subvert the
	program's control flow. These schemes have been optimized for performance,
	allowing developers to enable them in release builds.

	To enable Clang's available CFI schemes, use the flag ``-fsanitize=cfi``.
	You can also enable a subset of available :ref:`schemes <cfi-schemes>`.
	As currently implemented, all schemes rely on link-time optimization (LTO);
	so it is required to specify ``-flto``, and the linker used must support LTO,
	for example via the `gold plugin`_.

	To allow the checks to be implemented efficiently, the program must
	be structured such that certain object files are compiled with CFI
	enabled, and are statically linked into the program. This may preclude
	the use of shared libraries in some cases.

	The compiler will only produce CFI checks for a class if it can infer hidden
	LTO visibility for that class. LTO visibility is a property of a class that
	is inferred from flags and attributes. For more details, see the documentation
	for :doc:`LTO visibility <LTOVisibility>`.

	The ``-fsanitize=cfi-{vcall,nvcall,derived-cast,unrelated-cast}`` flags
	require that a ``-fvisibility=`` flag also be specified. This is because the
	default visibility setting is ``-fvisibility=default``, which would disable
	CFI checks for classes without visibility attributes. Most users will want
	to specify ``-fvisibility=hidden``, which enables CFI checks for such classes.

	Experimental support for :ref:`cross-DSO control flow integrity
	<cfi-cross-dso>` exists that does not require classes to have hidden LTO
	visibility. This cross-DSO support has unstable ABI at this time.

	.. _gold plugin: https://llvm.org/docs/GoldPlugin.html

	.. _cfi-schemes:

	Available schemes
	=================

	Available schemes are:

	- ``-fsanitize=cfi-cast-strict``: Enables :ref:`strict cast checks
	<cfi-strictness>`.
	- ``-fsanitize=cfi-derived-cast``: Base-to-derived cast to the wrong
	dynamic type.
	- ``-fsanitize=cfi-unrelated-cast``: Cast from ``void*`` or another
	unrelated type to the wrong dynamic type.
	- ``-fsanitize=cfi-nvcall``: Non-virtual call via an object whose vptr is of
	the wrong dynamic type.
	- ``-fsanitize=cfi-vcall``: Virtual call via an object whose vptr is of the
	wrong dynamic type.
	- ``-fsanitize=cfi-icall``: Indirect call of a function with wrong dynamic
	type.
	- ``-fsanitize=cfi-mfcall``: Indirect call via a member function pointer with
	wrong dynamic type.

	You can use ``-fsanitize=cfi`` to enable all the schemes and use
	``-fno-sanitize`` flag to narrow down the set of schemes as desired.
	For example, you can build your program with
	``-fsanitize=cfi -fno-sanitize=cfi-nvcall,cfi-icall``
	to use all schemes except for non-virtual member function call and indirect call
	checking.

	Remember that you have to provide ``-flto`` or ``-flto=thin`` if at
	least one CFI scheme is enabled.

	Trapping and Diagnostics
	========================

	By default, CFI will abort the program immediately upon detecting a control
	flow integrity violation. You can use the :ref:`-fno-sanitize-trap=
	<controlling-code-generation>` flag to cause CFI to print a diagnostic
	similar to the one below before the program aborts.

	.. code-block:: console

	bad-cast.cpp:109:7: runtime error: control flow integrity check for type 'B' failed during base-to-derived cast (vtable address 0x000000425a50)
	0x000000425a50: note: vtable is of type 'A'
	00 00 00 00 f0 f1 41 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 20 5a 42 00
	^

	If diagnostics are enabled, you can also configure CFI to continue program
	execution instead of aborting by using the :ref:`-fsanitize-recover=
	<controlling-code-generation>` flag.

	Forward-Edge CFI for Virtual Calls
	==================================

	This scheme checks that virtual calls take place using a vptr of the correct
	dynamic type; that is, the dynamic type of the called object must be a
	derived class of the static type of the object used to make the call.
	This CFI scheme can be enabled on its own using ``-fsanitize=cfi-vcall``.

	For this scheme to work, all translation units containing the definition
	of a virtual member function (whether inline or not), other than members
	of :ref:`ignored <cfi-ignorelist>` types or types with public :doc:`LTO
	visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
	enabled and be statically linked into the program.

	Performance
	-----------

	A performance overhead of less than 1% has been measured by running the
	Dromaeo benchmark suite against an instrumented version of the Chromium
	web browser. Another good performance benchmark for this mechanism is the
	virtual-call-heavy SPEC 2006 xalancbmk.

	Note that this scheme has not yet been optimized for binary size; an increase
	of up to 15% has been observed for Chromium.

	Bad Cast Checking
	=================

	This scheme checks that pointer casts are made to an object of the correct
	dynamic type; that is, the dynamic type of the object must be a derived class
	of the pointee type of the cast. The checks are currently only introduced
	where the class being casted to is a polymorphic class.

	Bad casts are not in themselves control flow integrity violations, but they
	can also create security vulnerabilities, and the implementation uses many
	of the same mechanisms.

	There are two types of bad cast that may be forbidden: bad casts
	from a base class to a derived class (which can be checked with
	``-fsanitize=cfi-derived-cast``), and bad casts from a pointer of
	type ``void*`` or another unrelated type (which can be checked with
	``-fsanitize=cfi-unrelated-cast``).

	The difference between these two types of casts is that the first is defined
	by the C++ standard to produce an undefined value, while the second is not
	in itself undefined behavior (it is well defined to cast the pointer back
	to its original type) unless the object is uninitialized and the cast is a
	``static_cast`` (see C++14 [basic.life]p5).

	If a program as a matter of policy forbids the second type of cast, that
	restriction can normally be enforced. However it may in some cases be necessary
	for a function to perform a forbidden cast to conform with an external API
	(e.g. the ``allocate`` member function of a standard library allocator). Such
	functions may be :ref:`ignored <cfi-ignorelist>`.

	For this scheme to work, all translation units containing the definition
	of a virtual member function (whether inline or not), other than members
	of :ref:`ignored <cfi-ignorelist>` types or types with public :doc:`LTO
	visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
	enabled and be statically linked into the program.

	Non-Virtual Member Function Call Checking
	=========================================

	This scheme checks that non-virtual calls take place using an object of
	the correct dynamic type; that is, the dynamic type of the called object
	must be a derived class of the static type of the object used to make the
	call. The checks are currently only introduced where the object is of a
	polymorphic class type. This CFI scheme can be enabled on its own using
	``-fsanitize=cfi-nvcall``.

	For this scheme to work, all translation units containing the definition
	of a virtual member function (whether inline or not), other than members
	of :ref:`ignored <cfi-ignorelist>` types or types with public :doc:`LTO
	visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
	enabled and be statically linked into the program.

	.. _cfi-strictness:

	Strictness
	----------

	If a class has a single non-virtual base and does not introduce or override
	virtual member functions or fields other than an implicitly defined virtual
	destructor, it will have the same layout and virtual function semantics as
	its base. By default, casts to such classes are checked as if they were made
	to the least derived such class.

	Casting an instance of a base class to such a derived class is technically
	undefined behavior, but it is a relatively common hack for introducing
	member functions on class instances with specific properties that works under
	most compilers and should not have security implications, so we allow it by
	default. It can be disabled with ``-fsanitize=cfi-cast-strict``.

	Indirect Function Call Checking
	===============================

	This scheme checks that function calls take place using a function of the
	correct dynamic type; that is, the dynamic type of the function must match
	the static type used at the call. This CFI scheme can be enabled on its own
	using ``-fsanitize=cfi-icall``.

	For this scheme to work, each indirect function call in the program, other
	than calls in :ref:`ignored <cfi-ignorelist>` functions, must call a
	function which was either compiled with ``-fsanitize=cfi-icall`` enabled,
	or whose address was taken by a function in a translation unit compiled with
	``-fsanitize=cfi-icall``.

	If a function in a translation unit compiled with ``-fsanitize=cfi-icall``
	takes the address of a function not compiled with ``-fsanitize=cfi-icall``,
	that address may differ from the address taken by a function in a translation
	unit not compiled with ``-fsanitize=cfi-icall``. This is technically a
	violation of the C and C++ standards, but it should not affect most programs.

	Each translation unit compiled with ``-fsanitize=cfi-icall`` must be
	statically linked into the program or shared library, and calls across
	shared library boundaries are handled as if the callee was not compiled with
	``-fsanitize=cfi-icall``.

	This scheme is currently supported on a limited set of targets: x86,
	x86_64, arm, arch64 and wasm.

	``-fsanitize-cfi-icall-generalize-pointers``
	--------------------------------------------

	Mismatched pointer types are a common cause of cfi-icall check failures.
	Translation units compiled with the ``-fsanitize-cfi-icall-generalize-pointers``
	flag relax pointer type checking for call sites in that translation unit,
	applied across all functions compiled with ``-fsanitize=cfi-icall``.

	Specifically, pointers in return and argument types are treated as equivalent as
	long as the qualifiers for the type they point to match. For example, ``char*``,
	``char*``, and ``int`` are considered equivalent types. However, ``char*`` and
	``const char*`` are considered separate types.

	``-fsanitize-cfi-icall-generalize-pointers`` is not compatible with
	``-fsanitize-cfi-cross-dso``.

	.. _cfi-icall-experimental-normalize-integers:

	``-fsanitize-cfi-icall-experimental-normalize-integers``
	--------------------------------------------------------

	This option enables normalizing integer types as vendor extended types for
	cross-language LLVM CFI/KCFI support with other languages that can't represent
	and encode C/C++ integer types.

	Specifically, integer types are encoded as their defined representations (e.g.,
	8-bit signed integer, 16-bit signed integer, 32-bit signed integer, ...) for
	compatibility with languages that define explicitly-sized integer types (e.g.,
	i8, i16, i32, ..., in Rust).

	``-fsanitize-cfi-icall-experimental-normalize-integers`` is compatible with
	``-fsanitize-cfi-icall-generalize-pointers``.

	This option is currently experimental.

	.. _cfi-canonical-jump-tables:

	``-fsanitize-cfi-canonical-jump-tables``
	----------------------------------------

	The default behavior of Clang's indirect function call checker will replace
	the address of each CFI-checked function in the output file's symbol table
	with the address of a jump table entry which will pass CFI checks. We refer
	to this as making the jump table `canonical`. This property allows code that
	was not compiled with ``-fsanitize=cfi-icall`` to take a CFI-valid address
	of a function, but it comes with a couple of caveats that are especially
	relevant for users of cross-DSO CFI:

	- There is a performance and code size overhead associated with each
	exported function, because each such function must have an associated
	jump table entry, which must be emitted even in the common case where the
	function is never address-taken anywhere in the program, and must be used
	even for direct calls between DSOs, in addition to the PLT overhead.

	- There is no good way to take a CFI-valid address of a function written in
	assembly or a language not supported by Clang. The reason is that the code
	generator would need to insert a jump table in order to form a CFI-valid
	address for assembly functions, but there is no way in general for the
	code generator to determine the language of the function. This may be
	possible with LTO in the intra-DSO case, but in the cross-DSO case the only
	information available is the function declaration. One possible solution
	is to add a C wrapper for each assembly function, but these wrappers can
	present a significant maintenance burden for heavy users of assembly in
	addition to adding runtime overhead.

	For these reasons, we provide the option of making the jump table non-canonical
	with the flag ``-fno-sanitize-cfi-canonical-jump-tables``. When the jump
	table is made non-canonical, symbol table entries point directly to the
	function body. Any instances of a function's address being taken in C will
	be replaced with a jump table address.

	This scheme does have its own caveats, however. It does end up breaking
	function address equality more aggressively than the default behavior,
	especially in cross-DSO mode which normally preserves function address
	equality entirely.

	Furthermore, it is occasionally necessary for code not compiled with
	``-fsanitize=cfi-icall`` to take a function address that is valid
	for CFI. For example, this is necessary when a function's address
	is taken by assembly code and then called by CFI-checking C code. The
	``__attribute__((cfi_canonical_jump_table))`` attribute may be used to make
	the jump table entry of a specific function canonical so that the external
	code will end up taking an address for the function that will pass CFI checks.

	``-fsanitize=cfi-icall`` and ``-fsanitize=function``
	----------------------------------------------------

	This tool is similar to ``-fsanitize=function`` in that both tools check
	the types of function calls. However, the two tools occupy different points
	on the design space; ``-fsanitize=function`` is a developer tool designed
	to find bugs in local development builds, whereas ``-fsanitize=cfi-icall``
	is a security hardening mechanism designed to be deployed in release builds.

	``-fsanitize=function`` has a higher space and time overhead due to a more
	complex type check at indirect call sites, which may make it unsuitable for
	deployment.

	On the other hand, ``-fsanitize=function`` conforms more closely with the C++
	standard and user expectations around interaction with shared libraries;
	the identity of function pointers is maintained, and calls across shared
	library boundaries are no different from calls within a single program or
	shared library.

	.. _kcfi:

	``-fsanitize=kcfi``
	-------------------

	This is an alternative indirect call control-flow integrity scheme designed
	for low-level system software, such as operating system kernels. Unlike
	``-fsanitize=cfi-icall``, it doesn't require ``-flto``, won't result in
	function pointers being replaced with jump table references, and never breaks
	cross-DSO function address equality. These properties make KCFI easier to
	adopt in low-level software. KCFI is limited to checking only function
	pointers, and isn't compatible with executable-only memory.

	``-fsanitize-kcfi-arity``
	-----------------------------

	For supported targets, this feature extends kCFI by telling the compiler to
	record information about each indirect-callable function's arity (i.e., the
	number of arguments passed in registers) into the binary. Some kernel CFI
	techniques, such as FineIBT, may be able to use this information to provide
	enhanced security.

	Member Function Pointer Call Checking
	=====================================

	This scheme checks that indirect calls via a member function pointer
	take place using an object of the correct dynamic type. Specifically, we
	check that the dynamic type of the member function referenced by the member
	function pointer matches the "function pointer" part of the member function
	pointer, and that the member function's class type is related to the base
	type of the member function. This CFI scheme can be enabled on its own using
	``-fsanitize=cfi-mfcall``.

	The compiler will only emit a full CFI check if the member function pointer's
	base type is complete. This is because the complete definition of the base
	type contains information that is necessary to correctly compile the CFI
	check. To ensure that the compiler always emits a full CFI check, it is
	recommended to also pass the flag ``-fcomplete-member-pointers``, which
	enables a non-conforming language extension that requires member pointer
	base types to be complete if they may be used for a call.

	For this scheme to work, all translation units containing the definition
	of a virtual member function (whether inline or not), other than members
	of :ref:`ignored <cfi-ignorelist>` types or types with public :doc:`LTO
	visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
	enabled and be statically linked into the program.

	This scheme is currently not compatible with cross-DSO CFI or the
	Microsoft ABI.

	.. _cfi-ignorelist:

	Ignorelist
	==========

	A :doc:`SanitizerSpecialCaseList` can be used to relax CFI checks for certain
	source files, functions and types using the ``src``, ``fun`` and ``type``
	entity types. Specific CFI modes can be be specified using ``[section]``
	headers.

	.. code-block:: bash

	# Suppress all CFI checking for code in a file.
	src:bad_file.cpp
	src:bad_header.h
	# Ignore all functions with names containing MyFooBar.
	fun:MyFooBar
	# Ignore all types in the standard library.
	type:std::*
	# Disable only unrelated cast checks for this function
	[cfi-unrelated-cast]
	fun:UnrelatedCast
	# Disable CFI call checks for this function without affecting cast checks
	[cfi-vcall\|cfi-nvcall\|cfi-icall]
	fun:BadCall


	.. _cfi-cross-dso:

	Shared library support
	======================

	Use -f[no-]sanitize-cfi-cross-dso to enable the cross-DSO control
	flow integrity mode, which allows all CFI schemes listed above to
	apply across DSO boundaries. As in the regular CFI, each DSO must be
	built with ``-flto`` or ``-flto=thin``.

	Normally, CFI checks will only be performed for classes that have hidden LTO
	visibility. With this flag enabled, the compiler will emit cross-DSO CFI
	checks for all classes, except for those which appear in the CFI ignorelist
	or which use a ``no_sanitize`` attribute.

	Design
	======

	Please refer to the :doc:`design document<ControlFlowIntegrityDesign>`.

	Publications
	============

	`Control-Flow Integrity: Principles, Implementations, and Applications <https://research.microsoft.com/pubs/64250/ccs05.pdf>`_.
	Martin Abadi, Mihai Budiu, Úlfar Erlingsson, Jay Ligatti.

	`Enforcing Forward-Edge Control-Flow Integrity in GCC & LLVM <http://www.pcc.me.uk/~peter/acad/usenix14.pdf>`_.
	Caroline Tice, Tom Roeder, Peter Collingbourne, Stephen Checkoway,
	Úlfar Erlingsson, Luis Lozano, Geoff Pike.