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<section id="pnacl-undefined-behavior">
<h1 id="pnacl-undefined-behavior">PNaCl Undefined Behavior</h1>
<div class="contents local" id="contents" style="display: none">
<ul class="small-gap">
<li><a class="reference internal" href="#overview" id="id2">Overview</a></li>
<li><a class="reference internal" href="#specification" id="id3">Specification</a></li>
<li><p class="first"><a class="reference internal" href="#behavior-in-pnacl-bitcode" id="id4">Behavior in PNaCl Bitcode</a></p>
<ul class="small-gap">
<li><a class="reference internal" href="#well-defined" id="id5">Well-Defined</a></li>
<li><p class="first"><a class="reference internal" href="#not-well-defined" id="id6">Not Well-Defined</a></p>
<ul class="small-gap">
<li><a class="reference internal" href="#potentially-fixable" id="id7">Potentially Fixable</a></li>
<li><a class="reference internal" href="#floating-point" id="id8">Floating-Point</a></li>
<li><a class="reference internal" href="#simd-vectors" id="id9">SIMD Vectors</a></li>
<li><a class="reference internal" href="#hard-to-fix" id="id10">Hard to Fix</a></li>
</ul>
</li>
</ul>
</li>
</ul>

</div><h2 id="overview"><span id="undefined-behavior"></span>Overview</h2>
<p>C and C++ undefined behavior allows efficient mapping of the source
language onto hardware, but leads to different behavior on different
platforms.</p>
<p>PNaCl exposes undefined behavior in the following ways:</p>
<ul class="small-gap">
<li><p class="first">The Clang frontend and optimizations that occur on the developer&#8217;s
machine determine what behavior will occur, and it will be specified
deterministically in the <em>pexe</em>. All targets will observe the same
behavior. In some cases, recompiling with a newer PNaCl SDK version
will either:</p>
<ul class="small-gap">
<li>Reliably emit the same behavior in the resulting <em>pexe</em>.</li>
<li>Change the behavior that gets specified in the <em>pexe</em>.</li>
</ul>
</li>
<li><p class="first">The behavior specified in the <em>pexe</em> relies on PNaCl&#8217;s bitcode,
runtime or CPU architecture vagaries.</p>
<ul class="small-gap">
<li>In some cases, the behavior using the same PNaCl translator version
on different architectures will produce different behavior.</li>
<li>Sometimes runtime parameters determine the behavior, e.g. memory
allocation determines which out-of-bounds accesses crash versus
returning garbage.</li>
<li>In some cases, different versions of the PNaCl translator
(i.e. after a Chrome update) will compile the code differently and
cause different behavior.</li>
<li>In some cases, the same versions of the PNaCl translator, on the
same architecture, will generate a different <em>nexe</em> for
defense-in-depth purposes, but may cause code that reads invalid
stack values or code sections on the heap to observe these
randomizations.</li>
</ul>
</li>
</ul>
<h2 id="specification">Specification</h2>
<p>PNaCl&#8217;s goal is that a single <em>pexe</em> should work reliably in the same
manner on all architectures, irrespective of runtime parameters and
through Chrome updates. This goal is unfortunately not attainable; PNaCl
therefore specifies as much as it can and outlines areas for
improvement.</p>
<p>One interesting solution is to offer good support for LLVM&#8217;s sanitizer
tools (including <a class="reference external" href="http://clang.llvm.org/docs/UsersManual.html#controlling-code-generation">UBSan</a>)
at development time, so that developers can test their code against
undefined behavior. Shipping code would then still get good performance,
and diverging behavior would be rare.</p>
<p>Note that none of these issues are vulnerabilities in PNaCl and Chrome:
the NaCl sandboxing still constrains the code through Software Fault
Isolation.</p>
<h2 id="behavior-in-pnacl-bitcode">Behavior in PNaCl Bitcode</h2>
<h3 id="well-defined">Well-Defined</h3>
<p>The following are traditionally undefined behavior in C/C++ but are well
defined at the <em>pexe</em> level:</p>
<ul class="small-gap">
<li>Dynamic initialization order dependencies: the order is deterministic
in the <em>pexe</em>.</li>
<li>Bool which isn&#8217;t <code>0</code>/<code>1</code>: the bitcode instruction sequence is
deterministic in the <em>pexe</em>.</li>
<li>Out-of-range <code>enum</code> value: the backing integer type and bitcode
instruction sequence is deterministic in the <em>pexe</em>.</li>
<li>Aggressive optimizations based on type-based alias analysis: TBAA
optimizations are done before stable bitcode is generated and their
metadata is stripped from the <em>pexe</em>; behavior is therefore
deterministic in the <em>pexe</em>.</li>
<li>Operator and subexpression evaluation order in the same expression
(e.g. function parameter passing, or pre-increment): the order is
defined in the <em>pexe</em>.</li>
<li>Signed integer overflow: two&#8217;s complement integer arithmetic is
assumed.</li>
<li>Atomic access to a non-atomic memory location (not declared as
<code>std::atomic</code>): atomics and <code>volatile</code> variables all lower to the
same compatible intrinsics or external functions; the behavior is
therefore deterministic in the <em>pexe</em> (see <a class="reference internal" href="/native-client/reference/pnacl-c-cpp-language-support.html#memory-model-and-atomics"><em>Memory Model and
Atomics</em></a>).</li>
<li>Integer divide by zero: always raises a fault (through hardware on
x86, and through integer divide emulation routine or explicit checks
on ARM).</li>
</ul>
<h3 id="not-well-defined">Not Well-Defined</h3>
<p>The following are traditionally undefined behavior in C/C++ which also
exhibit undefined behavior at the <em>pexe</em> level. Some are easier to fix
than others.</p>
<h4 id="potentially-fixable">Potentially Fixable</h4>
<ul class="small-gap">
<li><p class="first">Shift by greater-than-or-equal to left-hand-side&#8217;s bit-width or
negative (see <a class="reference external" href="https://code.google.com/p/nativeclient/issues/detail?id=3604">bug 3604</a>).</p>
<ul class="small-gap">
<li>Some of the behavior will be specified in the <em>pexe</em> depending on
constant propagation and integer type of variables.</li>
<li>There is still some architecture-specific behavior.</li>
<li>PNaCl could force-mask the right-hand-side to <cite>bitwidth-1</cite>, which
could become a no-op on some architectures while ensuring all
architectures behave similarly. Regular optimizations could also be
applied, removing redundant masks.</li>
</ul>
</li>
<li><p class="first">Using a virtual pointer of the wrong type, or of an unallocated
object.</p>
<ul class="small-gap">
<li>Will produce wrong results which will depend on what data is treated
as a <cite>vftable</cite>.</li>
<li>PNaCl could add runtime checks for this, and elide them when types
are provably correct (see this CFI <a class="reference external" href="https://code.google.com/p/nativeclient/issues/detail?id=3786">bug 3786</a>).</li>
</ul>
</li>
<li><p class="first">Some unaligned load/store (see <a class="reference external" href="https://code.google.com/p/nativeclient/issues/detail?id=3445">bug 3445</a>).</p>
<ul class="small-gap">
<li>Could force everything to <cite>align 1</cite>; performance cost should be
measured.</li>
<li>The frontend could also be more pessimistic when it sees dubious
casts.</li>
</ul>
</li>
<li>Some values can be marked as <code>undef</code> (see <a class="reference external" href="https://code.google.com/p/nativeclient/issues/detail?id=3796">bug 3796</a>).</li>
<li>Reaching end-of-value-returning-function without returning a value:
reduces to <code>ret i32 undef</code> in bitcode. This is mostly-defined, but
could be improved (see <a class="reference external" href="https://code.google.com/p/nativeclient/issues/detail?id=3796">bug 3796</a>).</li>
<li><p class="first">Reaching “unreachable” code.</p>
<ul class="small-gap">
<li>LLVM provides an IR instruction called “unreachable” whose effect
will be undefined.  PNaCl could change this to always trap, as the
<code>llvm.trap</code> intrinsic does.</li>
</ul>
</li>
<li>Zero or negative-sized variable-length array (and <code>alloca</code>) aren&#8217;t
defined behavior. PNaCl&#8217;s frontend or the translator could insert
checks with <code>-fsanitize=vla-bound</code>.</li>
</ul>
<h4 id="floating-point"><span id="undefined-behavior-fp"></span>Floating-Point</h4>
<p>PNaCl offers a IEEE-754 implementation which is as correct as the
underlying hardware allows, with a few limitations. These are a few
sources of undefined behavior which are believed to be fixable:</p>
<ul class="small-gap">
<li>Float cast overflow is currently undefined.</li>
<li>Float divide by zero is currently undefined.</li>
<li>The default denormal behavior is currently unspecified, which isn&#8217;t
IEEE-754 compliant (denormals must be supported in IEEE-754). PNaCl
could mandate flush-to-zero, and may give an API to enable denormals
in a future release. The latter is problematic for SIMD and
vectorization support, where some platforms do not support denormal
SIMD operations.</li>
<li><code>NaN</code> values are currently not guaranteed to be canonical; see <a class="reference external" href="https://code.google.com/p/nativeclient/issues/detail?id=3536">bug
3536</a>.</li>
<li>Passing <code>NaN</code> to STL functions (the math is defined, but the
function implementation isn&#8217;t, e.g. <code>std::min</code> and <code>std::max</code>), is
well-defined in the <em>pexe</em>.</li>
</ul>
<h4 id="simd-vectors">SIMD Vectors</h4>
<p>SIMD vector instructions aren&#8217;t part of the C/C++ standards and as such
their behavior isn&#8217;t specified at all in C/C++; it is usually left up to
the target architecture to specify behavior. Portable Native Client
instead exposed <a class="reference internal" href="/native-client/reference/pnacl-c-cpp-language-support.html#portable-simd-vectors"><em>Portable SIMD Vectors</em></a> and
offers the same guarantees on these vectors as the guarantees offered by
the contained elements. Of notable interest amongst these guarantees are
those of alignment for load/store instructions on vectors: they have the
same alignment restriction as the contained elements.</p>
<h4 id="hard-to-fix">Hard to Fix</h4>
<ul class="small-gap">
<li><p class="first">Null pointer/reference has behavior determined by the NaCl sandbox:</p>
<ul class="small-gap">
<li>Raises a segmentation fault in the bottom <code>64KiB</code> bytes on all
platforms, and on some sandboxes there are further non-writable
pages after the initial <code>64KiB</code>.</li>
<li>Negative offsets aren&#8217;t handled consistently on all platforms:
x86-64 and ARM will wrap around to the stack (because they mask the
address), whereas x86-32 will fault (because of segmentation).</li>
</ul>
</li>
<li><p class="first">Accessing uninitialized/free&#8217;d memory (including out-of-bounds array
access):</p>
<ul class="small-gap">
<li>Might cause a segmentation fault or not, depending on where memory
is allocated and how it gets reclaimed.</li>
<li>Added complexity because of the NaCl sandboxing: some of the
load/stores might be forced back into sandbox range, or eliminated
entirely if they fall out of the sandbox.</li>
</ul>
</li>
<li><p class="first">Executing non-program data (jumping to an address obtained from a
non-function pointer is undefined, can only do <code>void(*)()</code> to
<code>intptr_t</code> to <code>void(*)()</code>).</p>
<ul class="small-gap">
<li>Just-In-Time code generation is supported by NaCl, but is not
currently supported by PNaCl. It is currently not possible to mark
code as executable.</li>
<li>Offering full JIT capabilities would reduce PNaCl&#8217;s ability to
change the sandboxing model. It would also require a &#8220;jump to JIT
code&#8221; syscall (to guarantee a calling convention), and means that
JITs aren&#8217;t portable.</li>
<li>PNaCl could offer &#8220;portable&#8221; JIT capabilities where the code hands
PNaCl some form of LLVM IR, which PNaCl then JIT-compiles.</li>
</ul>
</li>
<li>Out-of-scope variable usage: will produce unknown data, mostly
dependent on stack and memory allocation.</li>
<li>Data races: any two operations that conflict (target overlapping
memory), at least one of which is a store or atomic read-modify-write,
and at least one of which is not atomic: this will be very dependent
on processor and execution sequence, see <a class="reference internal" href="/native-client/reference/pnacl-c-cpp-language-support.html#memory-model-and-atomics"><em>Memory Model and
Atomics</em></a>.</li>
</ul>
</section>

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