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This flag causes the compiler to emit bit set entries for functions as well as runtime bitset checks at indirect call sites. Depends on the new function bitset mechanism. Differential Revision: http://reviews.llvm.org/D11857 llvm-svn: 247238
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216 lines
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======================
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Control Flow Integrity
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======================
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.. toctree::
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:hidden:
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ControlFlowIntegrityDesign
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.. contents::
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:local:
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Introduction
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============
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Clang includes an implementation of a number of control flow integrity (CFI)
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schemes, which are designed to abort the program upon detecting certain forms
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of undefined behavior that can potentially allow attackers to subvert the
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program's control flow. These schemes have been optimized for performance,
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allowing developers to enable them in release builds.
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To enable Clang's available CFI schemes, use the flag ``-fsanitize=cfi``.
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As currently implemented, all of Clang's CFI schemes (``cfi-vcall``,
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``cfi-derived-cast``, ``cfi-unrelated-cast``, ``cfi-nvcall``, ``cfi-icall``)
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rely on link-time optimization (LTO); so it is required to specify
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``-flto``, and the linker used must support LTO, for example via the `gold
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plugin`_. To allow the checks to be implemented efficiently, the program must
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be structured such that certain object files are compiled with CFI enabled,
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and are statically linked into the program. This may preclude the use of
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shared libraries in some cases.
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Clang currently implements forward-edge CFI for member function calls and
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bad cast checking. More schemes are under development.
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.. _gold plugin: http://llvm.org/docs/GoldPlugin.html
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Forward-Edge CFI for Virtual Calls
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----------------------------------
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This scheme checks that virtual calls take place using a vptr of the correct
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dynamic type; that is, the dynamic type of the called object must be a
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derived class of the static type of the object used to make the call.
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This CFI scheme can be enabled on its own using ``-fsanitize=cfi-vcall``.
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For this scheme to work, all translation units containing the definition
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of a virtual member function (whether inline or not), other than members
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of :ref:`blacklisted <cfi-blacklist>` types, must be compiled with
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``-fsanitize=cfi-vcall`` enabled and be statically linked into the program.
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Performance
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~~~~~~~~~~~
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A performance overhead of less than 1% has been measured by running the
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Dromaeo benchmark suite against an instrumented version of the Chromium
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web browser. Another good performance benchmark for this mechanism is the
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virtual-call-heavy SPEC 2006 xalancbmk.
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Note that this scheme has not yet been optimized for binary size; an increase
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of up to 15% has been observed for Chromium.
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Bad Cast Checking
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-----------------
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This scheme checks that pointer casts are made to an object of the correct
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dynamic type; that is, the dynamic type of the object must be a derived class
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of the pointee type of the cast. The checks are currently only introduced
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where the class being casted to is a polymorphic class.
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Bad casts are not in themselves control flow integrity violations, but they
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can also create security vulnerabilities, and the implementation uses many
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of the same mechanisms.
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There are two types of bad cast that may be forbidden: bad casts
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from a base class to a derived class (which can be checked with
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``-fsanitize=cfi-derived-cast``), and bad casts from a pointer of
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type ``void*`` or another unrelated type (which can be checked with
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``-fsanitize=cfi-unrelated-cast``).
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The difference between these two types of casts is that the first is defined
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by the C++ standard to produce an undefined value, while the second is not
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in itself undefined behavior (it is well defined to cast the pointer back
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to its original type).
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If a program as a matter of policy forbids the second type of cast, that
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restriction can normally be enforced. However it may in some cases be necessary
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for a function to perform a forbidden cast to conform with an external API
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(e.g. the ``allocate`` member function of a standard library allocator). Such
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functions may be :ref:`blacklisted <cfi-blacklist>`.
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For this scheme to work, all translation units containing the definition
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of a virtual member function (whether inline or not), other than members
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of :ref:`blacklisted <cfi-blacklist>` types, must be compiled with
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``-fsanitize=cfi-derived-cast`` or ``-fsanitize=cfi-unrelated-cast`` enabled
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and be statically linked into the program.
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Non-Virtual Member Function Call Checking
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-----------------------------------------
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This scheme checks that non-virtual calls take place using an object of
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the correct dynamic type; that is, the dynamic type of the called object
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must be a derived class of the static type of the object used to make the
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call. The checks are currently only introduced where the object is of a
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polymorphic class type. This CFI scheme can be enabled on its own using
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``-fsanitize=cfi-nvcall``.
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For this scheme to work, all translation units containing the definition
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of a virtual member function (whether inline or not), other than members
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of :ref:`blacklisted <cfi-blacklist>` types, must be compiled with
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``-fsanitize=cfi-nvcall`` enabled and be statically linked into the program.
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.. _cfi-strictness:
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Strictness
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~~~~~~~~~~
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If a class has a single non-virtual base and does not introduce or override
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virtual member functions or fields other than an implicitly defined virtual
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destructor, it will have the same layout and virtual function semantics as
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its base. By default, casts to such classes are checked as if they were made
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to the least derived such class.
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Casting an instance of a base class to such a derived class is technically
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undefined behavior, but it is a relatively common hack for introducing
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member functions on class instances with specific properties that works under
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most compilers and should not have security implications, so we allow it by
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default. It can be disabled with ``-fsanitize=cfi-cast-strict``.
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Indirect Function Call Checking
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-------------------------------
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This scheme checks that function calls take place using a function of the
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correct dynamic type; that is, the dynamic type of the function must match
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the static type used at the call. This CFI scheme can be enabled on its own
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using ``-fsanitize=cfi-icall``.
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For this scheme to work, each indirect function call in the program, other
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than calls in :ref:`blacklisted <cfi-blacklist>` functions, must call a
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function which was either compiled with ``-fsanitize=cfi-icall`` enabled,
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or whose address was taken by a function in a translation unit compiled with
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``-fsanitize=cfi-icall``.
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If a function in a translation unit compiled with ``-fsanitize=cfi-icall``
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takes the address of a function not compiled with ``-fsanitize=cfi-icall``,
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that address may differ from the address taken by a function in a translation
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unit not compiled with ``-fsanitize=cfi-icall``. This is technically a
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violation of the C and C++ standards, but it should not affect most programs.
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Each translation unit compiled with ``-fsanitize=cfi-icall`` must be
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statically linked into the program or shared library, and calls across
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shared library boundaries are handled as if the callee was not compiled with
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``-fsanitize=cfi-icall``.
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This scheme is currently only supported on the x86 and x86_64 architectures.
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``-fsanitize=cfi-icall`` and ``-fsanitize=function``
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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This tool is similar to ``-fsanitize=function`` in that both tools check
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the types of function calls. However, the two tools occupy different points
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on the design space; ``-fsanitize=function`` is a developer tool designed
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to find bugs in local development builds, whereas ``-fsanitize=cfi-icall``
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is a security hardening mechanism designed to be deployed in release builds.
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``-fsanitize=function`` has a higher space and time overhead due to a more
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complex type check at indirect call sites, as well as a need for run-time
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type information (RTTI), which may make it unsuitable for deployment. Because
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of the need for RTTI, ``-fsanitize=function`` can only be used with C++
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programs, whereas ``-fsanitize=cfi-icall`` can protect both C and C++ programs.
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On the other hand, ``-fsanitize=function`` conforms more closely with the C++
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standard and user expectations around interaction with shared libraries;
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the identity of function pointers is maintained, and calls across shared
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library boundaries are no different from calls within a single program or
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shared library.
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.. _cfi-blacklist:
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Blacklist
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---------
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A :doc:`SanitizerSpecialCaseList` can be used to relax CFI checks for certain
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source files, functions and types using the ``src``, ``fun`` and ``type``
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entity types.
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In addition, if a type has a ``uuid`` attribute and the blacklist contains
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the type entry ``attr:uuid``, CFI checks are suppressed for that type. This
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allows all COM types to be easily blacklisted, which is useful as COM types
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are typically defined outside of the linked program.
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.. code-block:: bash
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# Suppress checking for code in a file.
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src:bad_file.cpp
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src:bad_header.h
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# Ignore all functions with names containing MyFooBar.
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fun:*MyFooBar*
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# Ignore all types in the standard library.
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type:std::*
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# Ignore all types with a uuid attribute.
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type:attr:uuid
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Design
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------
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Please refer to the :doc:`design document<ControlFlowIntegrityDesign>`.
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Publications
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------------
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`Control-Flow Integrity: Principles, Implementations, and Applications <http://research.microsoft.com/pubs/64250/ccs05.pdf>`_.
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Martin Abadi, Mihai Budiu, Úlfar Erlingsson, Jay Ligatti.
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`Enforcing Forward-Edge Control-Flow Integrity in GCC & LLVM <http://www.pcc.me.uk/~peter/acad/usenix14.pdf>`_.
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Caroline Tice, Tom Roeder, Peter Collingbourne, Stephen Checkoway,
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Úlfar Erlingsson, Luis Lozano, Geoff Pike.
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