Skip to content

Commit b9e542b

Browse files
gbMattNgbMattN
committed
[TySan] Add initial documentation
1 parent d54d8d7 commit b9e542b

File tree

3 files changed

+209
-0
lines changed

3 files changed

+209
-0
lines changed

‎clang/docs/TypeSanitizer.rst

Lines changed: 205 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,205 @@
1+
=============
2+
TypeSanitizer
3+
=============
4+
5+
.. contents::
6+
:local:
7+
8+
Introduction
9+
============
10+
11+
The TypeSanitizer is a detector for strict type aliasing violations. It consists of a compiler
12+
instrumentation module and a run-time library. C/C++ has type-based aliasing rules, and LLVM
13+
can exploit these for optimizations given the TBAA metadata Clang emits. In general, a pointer
14+
of a given type cannot access an object of a different type, with only a few exceptions.
15+
16+
These rules aren't always apparent to users, which leads to code that violates these rules
17+
(e.g. for type punning). This can lead to optimization passes introducing bugs unless the
18+
code is build with ``-fno-strict-aliasing``, sacrificing performance.
19+
20+
TypeSanitizer is built to catch when these strict aliasing rules have been violated, helping
21+
users find where such bugs originate in their code despite the code looking valid at first glance.
22+
23+
As TypeSanitizer is still experimental, it can currently have a large impact on runtime speed,
24+
memory use, and code size. It also has a large compile-time overhead. Work is being done to
25+
reduce these impacts.
26+
27+
The TypeSanitizer Algorithm
28+
===========================
29+
For each TBAA type-access descriptor, encoded in LLVM IR using TBAA Metadata, the instrumentation
30+
pass generates descriptor tales. Thus there is a unique pointer to each type (and access descriptor).
31+
These tables are comdat (except for anonymous-namespace types), so the pointer values are unique
32+
across the program.
33+
34+
The descriptors refer to other descriptors to form a type aliasing tree, like how LLVM's TBAA data
35+
does.
36+
37+
The runtime uses 8 bytes of shadow memory, the size of the pointer to the type descriptor, for
38+
every byte of accessed data in the program. The first byte of a type will have its shadow memory
39+
be set to the pointer to its type descriptor. Aside from that, there are some other values it may be.
40+
41+
* 0 is used to represent an unknown type
42+
* Negative numbers represent an interior byte: A byte inside a type that is not the first one. As an
43+
example, a value of -2 means you are in the third byte of a type.
44+
45+
The Instrumentation first checks for an exact match between the type of the current access and the
46+
type for that address in the shadow memory. This can quickly be done by checking pointer values. If
47+
it matches, it checks the remaining shadow memory of the type to ensure they are the correct negative
48+
numbers. If this fails, it calls the "slow path" check. If the exact match fails, we check to see if
49+
the value, and the remainder of the shadow bytes, is 0. If they are, we can set the shadow memory to
50+
the correct type descriptor pointer for the first byte, and the correct negative numbers for the rest
51+
of the type's shadow.
52+
53+
If the type in shadow memory is neither an exact match nor 0, we call the slower runtime check. It
54+
uses the full TBAA algorithm, just as the compiler does, to determine when two types are permitted to
55+
alias.
56+
57+
The instrumentation pass inserts calls to the memset intrinsic to set the memory updated by memset,
58+
memcpy, and memmove, as well as allocas/byval (and for lifetime.start/end) to reset the shadow memory
59+
to reflect that the type is now unknown. The runtime intercepts memset, memcpy, etc. to perform the
60+
same function for the library calls.
61+
62+
How to build
63+
============
64+
65+
Build LLVM/Clang with `CMake <https://llvm.org/docs/CMake.html>`_ and enable
66+
the ``compiler-rt`` runtime. An example CMake configuration that will allow
67+
for the use/testing of TypeSanitizer:
68+
69+
.. code-block:: console
70+
71+
$ cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_ENABLE_RUNTIMES="compiler-rt" <path to source>/llvm
72+
73+
Usage
74+
=====
75+
76+
Compile and link your program with ``-fsanitize=type`` flag. The
77+
TypeSanitizer run-time library should be linked to the final executable, so
78+
make sure to use ``clang`` (not ``ld``) for the final link step. To
79+
get a reasonable performance add ``-O1`` or higher.
80+
TypeSanitizer by default doesn't print the full stack trace in error messages. Use ``TYSAN_OPTIONS=print_stacktrace=1``
81+
to print the full trace. To get nicer stack traces in error messages add ``-fno-omit-frame-pointer`` and
82+
``-g``. To get perfect stack traces you may need to disable inlining (just use ``-O1``) and tail call elimination
83+
(``-fno-optimize-sibling-calls``).
84+
85+
.. code-block:: console
86+
87+
% cat example_AliasViolation.c
88+
int main(int argc, char **argv) {
89+
int x = 100;
90+
float *y = (float*)&x;
91+
*y += 2.0f; // Strict aliasing violation
92+
return 0;
93+
}
94+
95+
# Compile and link
96+
% clang++ -g -fsanitize=type example_AliasViolation.cc
97+
98+
The program will print an error message to ``stderr`` each time a strict aliasing violation is detected.
99+
The program won't terminate, which will allow you to detect many strict aliasing violations in one
100+
run.
101+
102+
.. code-block:: console
103+
104+
% ./a.out
105+
==1375532==ERROR: TypeSanitizer: type-aliasing-violation on address 0x7ffeebf1a72c (pc 0x5b3b1145ff41 bp 0x7ffeebf1a660 sp 0x7ffeebf19e08 tid 1375532)
106+
READ of size 4 at 0x7ffeebf1a72c with type float accesses an existing object of type int
107+
#0 0x5b3b1145ff40 in main example_AliasViolation.c:4:10
108+
109+
==1375532==ERROR: TypeSanitizer: type-aliasing-violation on address 0x7ffeebf1a72c (pc 0x5b3b1146008a bp 0x7ffeebf1a660 sp 0x7ffeebf19e08 tid 1375532)
110+
WRITE of size 4 at 0x7ffeebf1a72c with type float accesses an existing object of type int
111+
#0 0x5b3b11460089 in main example_AliasViolation.c:4:10
112+
113+
Error terminology
114+
------------------
115+
116+
There are some terms that may appear in TypeSanitizer errors that are derived from
117+
`TBAA Metadata <https://llvm.org/docs/LangRef.html#tbaa-metadata>`. This section hopes to provide a
118+
brief dictionary of these terms.
119+
120+
* ``omnipotent char``: This is a special type which can alias with anything. Its name comes from the C/C++
121+
type ``char``.
122+
* ``type p[x]``: This signifies pointers to the type. ``x`` is the number of indirections to reach the final value.
123+
As an example, a pointer to a pointer to an integer would be ``type p2 int``.
124+
125+
TypeSanitizer is still experimental. User-facing error messages should be improved in the future to remove
126+
references to LLVM IR specific terms.
127+
128+
Sanitizer features
129+
==================
130+
131+
``__has_feature(type_sanitizer)``
132+
------------------------------------
133+
134+
In some cases one may need to execute different code depending on whether
135+
TypeSanitizer is enabled.
136+
:ref:`\_\_has\_feature <langext-__has_feature-__has_extension>` can be used for
137+
this purpose.
138+
139+
.. code-block:: c
140+
141+
#if defined(__has_feature)
142+
# if __has_feature(type_sanitizer)
143+
// code that builds only under TypeSanitizer
144+
# endif
145+
#endif
146+
147+
``__attribute__((no_sanitize("type")))``
148+
-----------------------------------------------
149+
150+
Some code you may not want to be instrumented by TypeSanitizer. One may use the
151+
function attribute ``no_sanitize("type")`` to disable instrumenting type aliasing.
152+
It is possible, depending on what happens in non-instrumented code, that instrumented code
153+
emits false-positives/ false-negatives. This attribute may not be supported by other
154+
compilers, so we suggest to use it together with ``__has_feature(type_sanitizer)``.
155+
156+
``__attribute__((disable_sanitizer_instrumentation))``
157+
--------------------------------------------------------
158+
159+
The ``disable_sanitizer_instrumentation`` attribute can be applied to functions
160+
to prevent all kinds of instrumentation. As a result, it may introduce false
161+
positives and incorrect stack traces. Therefore, it should be used with care,
162+
and only if absolutely required; for example for certain code that cannot
163+
tolerate any instrumentation and resulting side-effects. This attribute
164+
overrides ``no_sanitize("type")``.
165+
166+
Ignorelist
167+
----------
168+
169+
TypeSanitizer supports ``src`` and ``fun`` entity types in
170+
:doc:`SanitizerSpecialCaseList`, that can be used to suppress aliasing
171+
violation reports in the specified source files or functions. Like
172+
with other methods of ignoring instrumentation, this can result in false
173+
positives/ false-negatives.
174+
175+
Limitations
176+
-----------
177+
178+
* TypeSanitizer uses more real memory than a native run. It uses 8 bytes of
179+
shadow memory for each byte of user memory.
180+
* There are transformation passes which run before TypeSanitizer. If these
181+
passes optimize out an aliasing violation, TypeSanitizer cannot catch it.
182+
* Currently, all instrumentation is inlined. This can result in a **15x**
183+
(on average) increase in generated file size, and **3x** to **7x** increase
184+
in compile time. In some documented cases this can cause the compiler to hang.
185+
There are plans to improve this in the future.
186+
* Codebases that use unions and struct-initialized variables can see incorrect
187+
results, as TypeSanitizer doesn't yet instrument these reliably.
188+
* Since Clang & LLVM's TBAA system is used to generate the checks used by the
189+
instrumentation, TypeSanitizer follows Clang & LLVM's rules for type aliasing.
190+
There may be situations where that disagrees with the standard. However this
191+
does at least mean that TypeSanitizer will catch any aliasing violations that
192+
would cause bugs when compiling with Clang & LLVM.
193+
* TypeSanitizer cannot currently be run alongside other sanitizers such as
194+
AddressSanitizer, ThreadSanitizer or UndefinedBehaviourSanitizer.
195+
196+
Current Status
197+
--------------
198+
199+
TypeSanitizer is brand new, and still in development. There are some known
200+
issues, especially in areas where Clang's emitted TBAA data isn't extensive
201+
enough for TypeSanitizer's runtime.
202+
203+
We are actively working on enhancing the tool --- stay tuned. Any help,
204+
issues, pull requests, ideas, is more than welcome. You can find the
205+
`issue tracker here.<https://github.com/llvm/llvm-project/issues?q=is%3Aissue%20state%3Aopen%20TySan%20label%3Acompiler-rt%3Atysan>`

‎clang/docs/UsersManual.rst

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2103,7 +2103,10 @@ are listed below.
21032103

21042104
``-fsanitize=undefined``: :doc:`UndefinedBehaviorSanitizer`,
21052105
a fast and compatible undefined behavior checker.
2106+
- .. _opt_fsanitize_type:
21062107

2108+
``-fsanitize=type``: :doc:`TypeSanitizer`, a detector for strict
2109+
aliasing violations.
21072110
- ``-fsanitize=dataflow``: :doc:`DataFlowSanitizer`, a general data
21082111
flow analysis.
21092112
- ``-fsanitize=cfi``: :doc:`control flow integrity <ControlFlowIntegrity>`

‎clang/docs/index.rst

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -35,6 +35,7 @@ Using Clang as a Compiler
3535
UndefinedBehaviorSanitizer
3636
DataFlowSanitizer
3737
LeakSanitizer
38+
TypeSanitizer
3839
RealtimeSanitizer
3940
SanitizerCoverage
4041
SanitizerStats

0 commit comments

Comments
 (0)