.. _Sanitizers:

=========================
Debugging with Sanitizers
=========================

.. _sanitizer_ubsan:

Undefined Behaviour Sanitizer
=============================

Clang's `undefined behavior sanitizer`__ (UBSan) is available for use with
Emscripten. This makes it much easier to catch bugs in your code.

__ https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html

To use UBSan, simply pass ``-fsanitize=undefined`` to ``emcc`` or ``em++``.

Catching Null Dereference
-------------------------

By default, with Emscripten, deferencing a null pointer does not immediately
cause a segmentation fault, unlike traditional platforms. Instead, it checks
a magic cookie stored at address 0 at the end of the program execution.

This only detects null pointer writes, and not reads, and it's rather difficult
to find where the null pointer write occurred.

Emscripten provides a ``SAFE_HEAP`` mode, which can be activated by running
``emcc`` with ``-s SAFE_HEAP=1``. This will catch both null pointer reads and
writes, and causes an exception, but it is slower, and does not tell you the
exact line numbers unless you compile with ``-g``.

UBSan will tell you exactly where the null deference happened, and works for
both reads and writes, with much less performance penalty than ``SAFE_HEAP``.

Consider the following program, ``null-assign.c``:

.. code-block:: c

  int main(void) {
      int *a = 0;
      *a = 0;
  }

Without UBSan or ``SAFE_HEAP``, you get an error when the program exits:

.. code-block:: console

  $ emcc null-assign.c
  $ node a.out.js
  Runtime error: The application has corrupted its heap memory area (address zero)!

With UBSan, you get the exact line number where this happened:

.. code-block:: console

  $ emcc -fsanitize=undefined null-assign.c
  $ node a.out.js
  null-assign.c:3:5: runtime error: store to null pointer of type 'int'
  Runtime error: The application has corrupted its heap memory area (address zero)!

Consider the following program, ``null-read.c``:

.. code-block:: c

  int main(void) {
      int *a = 0, b;
      b = *a;
  }

Without UBSan or ``SAFE_HEAP``, there is no feedback:

.. code-block:: console

  $ emcc null-read.c
  $ node a.out.js
  $

With UBSan, you get the exact line number where this happened:

.. code-block:: console

  $ emcc -fsanitize=undefined null-assign.c
  $ node a.out.js
  null-read.c:3:9: runtime error: load of null pointer of type 'int'

Minimal Runtime
---------------

UBSan's runtime is non-trivial, and its use can unnecessarily increase the
attack surface. For this reason, there is a minimal UBSan runtime that is
designed for production uses.

The minimal runtime is supported by Emscripten. To use it, pass the flag
``-fsanitize-minimal-runtime`` in addition to your ``-fsanitize`` flag.

.. code-block:: console

  $ emcc -fsanitize=null -fsanitize-minimal-runtime null-read.c
  $ node a.out.js
  ubsan: type-mismatch
  $ emcc -fsanitize=null -fsanitize-minimal-runtime null-assign.c
  $ node a.out.js
  ubsan: type-mismatch
  Runtime error: The application has corrupted its heap memory area (address zero)!

.. _sanitizer_asan:

Address Sanitizer
=================

Clang's `address sanitizer`__ (ASan) is also available for use with Emscripten.
This makes it much easier to catch buffer overflows, memory leaks, and other
related bugs in your code.

__ https://clang.llvm.org/docs/AddressSanitizer.html

To use ASan, simply pass ``-fsanitize=address`` to ``emcc`` or ``em++``.

You probably need to increase ``TOTAL_MEMORY`` to at least 64 MB or pass
``-s ALLOW_MEMORY_GROWTH`` so that ASan has enough memory to start. Otherwise,
you will receive an error message that looks something like:

  Cannot enlarge memory arrays to size 55152640 bytes (OOM). Either (1) compile
  with  -s TOTAL_MEMORY=X  with X higher than the current value 50331648, (2)
  compile with  -s ALLOW_MEMORY_GROWTH=1  which allows increasing the size at
  runtime, or (3) if you want malloc to return NULL (0) instead of this abort,
  compile with  -s ABORTING_MALLOC=0

Examples
--------

Here are some examples of how AddressSanitizer can be used to help find bugs.

Buffer Overflow
^^^^^^^^^^^^^^^

Consider ``buffer_overflow.c``:

.. code-block:: c

  #include <string.h>

  int main(void) {
    char x[10];
    memset(x, 0, 11);
  }

.. code-block:: console

  $ emcc -g4 -fsanitize=address -s ALLOW_MEMORY_GROWTH buffer_overflow.c
  $ node a.out.js
  =================================================================
  ==42==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x02965e5a at pc 0x000015f0 bp 0x02965a30 sp 0x02965a30
  WRITE of size 11 at 0x02965e5a thread T0
      #0 0x15f0 in __asan_memset+0x15f0 (a.out.wasm+0x15f0)
      #1 0xc46 in __original_main stack_buffer_overflow.c:5:3
      #2 0xcbc in main+0xcbc (a.out.wasm+0xcbc)
      #3 0x800019bc in Object.Module._main a.out.js:6588:32
      #4 0x80001aeb in Object.callMain a.out.js:6891:30
      #5 0x80001b25 in doRun a.out.js:6949:60
      #6 0x80001b33 in run a.out.js:6963:5
      #7 0x80001ad6 in runCaller a.out.js:6870:29

  Address 0x02965e5a is located in stack of thread T0 at offset 26 in frame
      #0 0x11  (a.out.wasm+0x11)

    This frame has 1 object(s):
      [16, 26) 'x' (line 4) <== Memory access at offset 26 overflows this variable
  HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
        (longjmp and C++ exceptions *are* supported)
  SUMMARY: AddressSanitizer: stack-buffer-overflow (a.out.wasm+0x15ef)
  ...

Use After Free
^^^^^^^^^^^^^^

Consider ``use_after_free.cpp``:

.. code-block:: cpp

  int main() {
    int *array = new int[100];
    delete [] array;
    return array[0];
  }

.. code-block:: console

  $ em++ -g4 -fsanitize=address -s ALLOW_MEMORY_GROWTH use_after_free.cpp
  $ node a.out.js
  =================================================================
  ==42==ERROR: AddressSanitizer: heap-use-after-free on address 0x03203e40 at pc 0x00000c1b bp 0x02965e70 sp 0x02965e7c
  READ of size 4 at 0x03203e40 thread T0
      #0 0xc1b in __original_main use_after_free.cpp:4:10
      #1 0xc48 in main+0xc48 (a.out.wasm+0xc48)

  0x03203e40 is located 0 bytes inside of 400-byte region [0x03203e40,0x03203fd0)
  freed by thread T0 here:
      #0 0x5fe8 in operator delete[](void*)+0x5fe8 (a.out.wasm+0x5fe8)
      #1 0xb76 in __original_main use_after_free.cpp:3:3
      #2 0xc48 in main+0xc48 (a.out.wasm+0xc48)
      #3 0x800019b5 in Object.Module._main a.out.js:6581:32
      #4 0x80001ade in Object.callMain a.out.js:6878:30
      #5 0x80001b18 in doRun a.out.js:6936:60
      #6 0x80001b26 in run a.out.js:6950:5
      #7 0x80001ac9 in runCaller a.out.js:6857:29

  previously allocated by thread T0 here:
      #0 0x5db4 in operator new[](unsigned long)+0x5db4 (a.out.wasm+0x5db4)
      #1 0xb41 in __original_main use_after_free.cpp:2:16
      #2 0xc48 in main+0xc48 (a.out.wasm+0xc48)
      #3 0x800019b5 in Object.Module._main a.out.js:6581:32
      #4 0x80001ade in Object.callMain a.out.js:6878:30
      #5 0x80001b18 in doRun a.out.js:6936:60
      #6 0x80001b26 in run a.out.js:6950:5
      #7 0x80001ac9 in runCaller a.out.js:6857:29

  SUMMARY: AddressSanitizer: heap-use-after-free (a.out.wasm+0xc1a)
  ...

Memory Leaks
^^^^^^^^^^^^

Consider ``leak.cpp``:

.. code-block:: cpp

  int main() {
    new int[10];
  }

.. code-block:: console

  $ em++ -g4 -fsanitize=address -s ALLOW_MEMORY_GROWTH -s EXIT_RUNTIME leak.cpp
  $ node a.out.js

  =================================================================
  ==42==ERROR: LeakSanitizer: detected memory leaks

  Direct leak of 40 byte(s) in 1 object(s) allocated from:
      #0 0x5ce5 in operator new[](unsigned long)+0x5ce5 (a.out.wasm+0x5ce5)
      #1 0xb24 in __original_main leak.cpp:2:3
      #2 0xb3a in main+0xb3a (a.out.wasm+0xb3a)
      #3 0x800019b8 in Object.Module._main a.out.js:6584:32
      #4 0x80001ae1 in Object.callMain a.out.js:6881:30
      #5 0x80001b1b in doRun a.out.js:6939:60
      #6 0x80001b29 in run a.out.js:6953:5
      #7 0x80001acc in runCaller a.out.js:6860:29

  SUMMARY: AddressSanitizer: 40 byte(s) leaked in 1 allocation(s).

Note that since leak checks take place at program exit, you must use
``-s EXIT_RUNTIME``, or invoke ``__lsan::DoLeakCheck`` manually.

You can detect that AddressSanitizer is enabled and run ``__lsan::DoLeakCheck``
by doing:

.. code-block:: c

  #if defined(__has_feature)
  #if __has_feature(address_sanitizer)
    // code for ASan-enabled builds
    __lsan::DoLeakCheck();
  #endif
  #endif

Also, if you only want to check for memory leaks, you may use
``-fsanitize=leak`` instead of ``-fsanitize=address``. ``-fsanitize=leak``
does not instrument all memory accesses, and as a result is much faster than
``-fsanitize=address``.

Use After Return
^^^^^^^^^^^^^^^^

Consider ``use_after_return.c``:

.. code-block:: c

  #include <stdio.h>

  const char *__asan_default_options() {
    return "detect_stack_use_after_return=1";
  }

  int *f() {
    int buf[10];
    return buf;
  }

  int main() {
    *f() = 1;
  }

Note that to do this check, you have to use the ASan option
``detect_stack_use_after_return``. You may enable this option by declaring
a function called ``__asan_default_options`` like the example, or you can
define ``Module['ASAN_OPTIONS'] = 'detect_stack_use_after_return=1'`` in the
generated JavaScript. ``--pre-js`` is helpful here.

This option is fairly expensive because it converts stack allocations into
heap allocations, and these allocations are not reused so that future accesses
can cause traps. Hence, it is not enabled by default.

.. code-block:: console

  $ emcc -g4 -fsanitize=address -s ALLOW_MEMORY_GROWTH=1 use_after_return.c
  $ node a.out.js
  =================================================================
  ==42==ERROR: AddressSanitizer: stack-use-after-return on address 0x02a95010 at pc 0x00000d90 bp 0x02965f70 sp 0x02965f7c
  WRITE of size 4 at 0x02a95010 thread T0
      #0 0xd90 in __original_main use_after_return.c:13:10
      #1 0xe0a in main+0xe0a (a.out.wasm+0xe0a)

  Address 0x02a95010 is located in stack of thread T0 at offset 16 in frame
      #0 0x11  (a.out.wasm+0x11)

    This frame has 1 object(s):
      [16, 56) 'buf' (line 8) <== Memory access at offset 16 is inside this variable
  HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
        (longjmp and C++ exceptions *are* supported)
  SUMMARY: AddressSanitizer: stack-use-after-return (a.out.wasm+0xd8f)
  ...
