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<div class="moz-cite-prefix">On 04/11/2017 03:46 AM, Andrey Bokhanko
wrote:<br>
</div>
<blockquote
cite="mid:CAJHoqadUmQbgyo0aDnTmOc=4Gm7neh=rUFn9c0=8EbBxQJ5mJQ@mail.gmail.com"
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<div>Hal,<br>
<br>
</div>
To clarify, my example meant to illustrate that for
memory references to structures' fields you have to keep
a user-defined type, even for one byte accesses. C++
allows references to "initial member sequence" using
pointers to structures of different types. And yes,
there are programs in the wild that rely on this (don't
remember details... this was from previous life).<br>
<br>
</div>
<div>Another thing to consider -- what about memset /
memcpy / etc that inherently rely on type punning? If
not inlined, they don't present problems for an
optimizer -- probably shouldn't be considered as
aliasing rules violation?<br>
</div>
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<br>
Good point. You (likely) wouldn't want to compile your memcpy /
memset / etc. implementations with the TBAA sanitizer enabled (or
we'd need to provide an attribute to disable it for certain
functions). If they only use char* themselves, then it's fine, but
if not, that's implementation magic. However, memset, etc. does need
to be able to 'unset' the type of memory and memcpy needs to be able
to copy types (at least for PODs). The sanitizer would need to hook
them for that purpose.<br>
<br>
-Hal<br>
<br>
<blockquote
cite="mid:CAJHoqadUmQbgyo0aDnTmOc=4Gm7neh=rUFn9c0=8EbBxQJ5mJQ@mail.gmail.com"
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Yours,<br>
</div>
Andrey<br>
===<br>
</div>
Compiler Architect<br>
</div>
NXP<br>
<br>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Tue, Apr 11, 2017 at 12:05 AM, Hal
Finkel <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:hfinkel@anl.gov" target="_blank">hfinkel@anl.gov</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000"><span class="">
<p><br>
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<div class="m_7989103750356669834moz-cite-prefix">On
04/10/2017 09:55 AM, Andrey Bokhanko wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>
<div>
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<div>
<div>Hi Hal,<br>
<br>
</div>
I wonder how your solution will handle the
following?<br>
<br>
</div>
struct {<br>
</div>
<div> int s1_f1;<br>
</div>
<div> float s1_f2;<br>
</div>
<div> int s1_f3;<br>
</div>
<div> float s1_f4;<br>
</div>
} S1;<br>
</div>
<div><br>
</div>
struct {<br>
</div>
<div> int s2_f1;<br>
</div>
<div> float s2_f2;<br>
</div>
<div> int *s2_f3; // to add some interest, suppose
that sizeof(int) == sizeof(int *)<br>
</div>
<div> float s2_f4;<br>
</div>
} S2;<br>
<div><br>
</div>
<div>S1 *s1; S2 *s2;<br>
</div>
<div>...<br>
s2 = (S1*)s1;<br>
</div>
<div>s2->s2_f1 = 0; // allowed<br>
s2->s2_f2 = 0; // allowed<br>
</div>
<div>s2->s2_f3 = 0; // not allowed<br>
</div>
<div>s2->s2_f4 = 0; // not allowed<br>
<br>
</div>
<div>Also, when you plan to set types for allocated
memory?</div>
</div>
</blockquote>
<br>
</span> The most-general thing seems to be to set the
types along with a store. As a result, the proposed scheme
would not find a fault with the code above, but would
complain if anyone actually later read S1.s1_f3.<br>
<br>
If we want to catch these kinds of problems directly we'd
need to have the compiler insert code when the type is
constructed to mark the types, and then we'd need to check
those types around stores. This also sounds like a useful
enhancement (although somewhat more complicated to
implement).<span class=""><br>
<br>
<blockquote type="cite">
<div dir="ltr">
<div> What types will be set for memory allocated by
a malloc call?<br>
</div>
</div>
</blockquote>
<br>
</span> Memory would be untyped (or of unknown type) when
allocated.<br>
<br>
Thanks again,<br>
Hal
<div>
<div class="h5"><br>
<br>
<blockquote type="cite">
<div dir="ltr">
<div><br>
</div>
<div>Yours,<br>
</div>
<div>Andrey<br>
</div>
<div><br>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Tue, Apr 4, 2017 at
10:13 PM, Hal Finkel via llvm-dev <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:llvm-dev@lists.llvm.org"
target="_blank">llvm-dev@lists.llvm.org</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0
0 0 .8ex;border-left:1px #ccc
solid;padding-left:1ex">Hi everyone,<br>
<br>
At EuroLLVM, Chandler and I chatted about the
design for a potential TBAA sanitizer. Here's
my attempt to summarize:<br>
<br>
C/C++ have type-based aliasing rules, and
LLVM's optimizer can exploit these given TBAA
metadata added by Clang. Roughly, a pointer of
given type cannot be used to access an object
of a different type (with, of course, certain
exceptions). Unfortunately, there's a lot of
code in the wild that violates these rules
(e.g. for type punning), and such code often
must be built with -fno-strict-aliasing.
Performance is often sacrificed as a result.
Part of the problem is the difficulty of
finding TBAA violations. A sanitizer would
help.<br>
<br>
A design goal of a TBAA sanitizer is to limit
the shadow-memory overhead of the
implementation. ASan, for example, uses 1 bit
per byte. Here we're hoping to keep the
overhead down to 2 bits per byte for the TBAA
sanitizing. We might be able to do this, while
handling all common types on the fast path, if
we use both alignment and type information.
When accessing data of B bytes, 2*B bits of
shadow memory can be used. Thus, we'll get 2
bits for a one-byte type, 4 bits for a
two-byte type, etc. Moreover, we need
appropriate holes in the encoding space so
that no type has a shadow encoding that
overlaps with an aligned part of a larger
type's encoding.<br>
For example, we need to detect:<br>
<br>
double f = ...; return *(int*) &f; // We
should catch this.<br>
<br>
We might use the following encoding. The idea
is that the common case, for which we need a
reasonable fast path, is that type types are
exactly equal. For this case, we want a simple
comparison of the shadow type encodings to be
sufficient to validate the access. For cases
where the encodings don't match (and isn't
zero to indicate an unknown type), or for
which there is no direct encoding for the
access type, a slow path must be used. All of
this assumes that we're validating the the
pointer alignment first, and then checking the
type encodings.<br>
<br>
1 Byte:<br>
00 = 0 = unknown type<br>
01 = 1 = hole<br>
10 = 2 = hole<br>
11 = 3 = all one-byte types (slow path, see
note later on this)<br>
<br>
2 Bytes:<br>
0000 = 0 = unknown type<br>
0101 = 5 = short<br>
0110 = 6 = hole (A)<br>
0111 = 7 = wchar_t (under some ABIs)<br>
1001 = 9 = hole (B)<br>
1010 = 10 = hole (C)<br>
1011 = 11 = char16_t<br>
1111 = 15 = all other types (slow path)<br>
<br>
It is important here to have wchar_t have a
direct encoding here because wchar_t is two
bytes on Windows, and moreover, wchar_t is
very commonly used on Windows. The partial
encoding overlap of wchar_t (i.e. 0111) with
the 11 one-byte-type encoding works because 11
always indicates a slow-path check.<br>
<br>
4 Bytes:<br>
0000 0000 = 0 = unknown type<br>
A A = int<br>
A B = float<br>
B A = pointer (under some ABIs)<br>
B B = long (under some ABIs)<br>
A 1111 = wchar_t (under some ABIs)<br>
B 1111 = char32_t<br>
A C = hole (D)<br>
C A = hole (E)<br>
B C = hole (F)<br>
C B = hole (G)<br>
C C = hole (H)<br>
1111 1111 = 255 = all other types (slow path)<br>
<br>
8 Bytes:<br>
0000 0000 0000 0000 = 0 = unknown type<br>
D D = double<br>
D E = long (under some ABIs)<br>
E D = long long (under some ABIs)<br>
E E = long double (under some ABIs)<br>
D F = pointer (under some ABIs)<br>
F D = hole (I)<br>
E F = hole (J)<br>
F E = hole<br>
F F = hole<br>
...<br>
1111 1111 1111 1111 = 65535 = all other types<br>
<br>
16 Bytes:<br>
0 = unknown type<br>
| | = __int128_t<br>
I J = long long (under some ABIs)<br>
J I = long double (under some ABIs)<br>
J J = hole<br>
...<br>
-1 = all other types<br>
<br>
For pointers, this scheme would consider all
pointers to be the same (regardless of pointee
type). Doing otherwise would mostly requiring
putting pointer-type checking on the slow path
(i.e. access via a pointer pointer), and that
could add considerable overhead. We might,
however, split out function pointers from
other pointers. We could provide a
compile-time option to control the granularity
of pointer-type checks.<br>
<br>
Builtin vector types for which the vector
element type has a direct encoding also
naturally have a direct encoding (the
concatenation of the encoding for the element
type).<br>
<br>
Obviously the fact that we have no fast-path
encodings for one-byte types could be
problematic. Note however that:<br>
<br>
1. If a larger type is being used to access a
smaller type (plus more), the encodings won't
match, so we always end up on the slow path.<br>
<br>
2. If the access type is a one-byte type, we
would want to validate quickly. However, most
common one-byte types are universally aliasing
(i.e. not subject to TBAA violations).
Specifically, for C/C++, pointers to char,
unsigned char, signed char (C only), and
std::byte, can be used to access any part of
any type. That leaves signed char (C++ only),
bool/_Bool, and enums with a [signed/unsigned]
char base type (C++ only, std::byte exempted)
as pointee types we might wish to validate.
We'd always need to fall back to the slow path
to validate these. We could provide a
compile-time option to disable such one-byte
access checking if necessary.<br>
<br>
How would the slow path work? First, the code
needs to find the beginning of the allocation.
It can do this by scanning backwards in the
ASan shadow memory. Once located, we'll read a
pointer to a type-description structure from
that "red zone" location. For dynamic
allocations, ASan's allocator will ensure such
a space for the pointer exists. For static
allocations and globals, the compiler will
ensure it exists. The compiler will make sure
that all constructors locate this field and
fill it in. Destructors can clear it. If two
of these type-description-structure pointers
are equal, then we can conclude that the types
are equal. If not, then we need to interpret
the structure. The pointer itself might be to
an interval map (to deal with arrays,
placement new, etc. - we can use the low bit
of the pointer to differentiate between an
actual type-description structure and an
interval map), and the leaves of the interval
map point to actual type-description
structures. The type-description structure is
an array of (offset, type) pairs, where the
type field is also a
type-description-structure pointer. The
type-description structures themselves are
comdat globals emitted in each relevant
translation unit, where the comdat key is
formed using the mangled type name (and size,
etc.), and pointers to these symbols are then
used to identify the types.<br>
<br>
Thoughts?<br>
<br>
Thanks again,<br>
Hal<span class="m_7989103750356669834HOEnZb"><font
color="#888888"><br>
<br>
-- <br>
Hal Finkel<br>
Lead, Compiler Technology and Programming
Languages<br>
Leadership Computing Facility<br>
Argonne National Laboratory<br>
<br>
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<br>
<pre class="m_7989103750356669834moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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