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281 lines
13 KiB
ReStructuredText
========
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GWP-ASan
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========
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.. contents::
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:local:
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:depth: 2
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Introduction
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============
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GWP-ASan is a sampled allocator framework that assists in finding use-after-free
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and heap-buffer-overflow bugs in production environments. It informally is a
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recursive acronym, "**G**\WP-ASan **W**\ill **P**\rovide **A**\llocation
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**SAN**\ity".
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GWP-ASan is based on the classic
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`Electric Fence Malloc Debugger <https://linux.die.net/man/3/efence>`_, with a
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key adaptation. Notably, we only choose a very small percentage of allocations
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to sample, and apply guard pages to these sampled allocations only. The sampling
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is small enough to allow us to have very low performance overhead.
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There is a small, tunable memory overhead that is fixed for the lifetime of the
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process. This is approximately ~40KiB per process using the default settings,
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depending on the average size of your allocations.
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GWP-ASan vs. ASan
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=================
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Unlike `AddressSanitizer <https://clang.llvm.org/docs/AddressSanitizer.html>`_,
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GWP-ASan does not induce a significant performance overhead. ASan often requires
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the use of dedicated canaries to be viable in production environments, and as
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such is often impractical.
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GWP-ASan is only capable of finding a subset of the memory issues detected by
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ASan. Furthermore, GWP-ASan's bug detection capabilities are only probabilistic.
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As such, we recommend using ASan over GWP-ASan in testing, as well as anywhere
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else that guaranteed error detection is more valuable than the 2x execution
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slowdown/binary size bloat. For the majority of production environments, this
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impact is too high, and GWP-ASan proves extremely useful.
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Design
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======
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**Please note:** The implementation of GWP-ASan is largely in-flux, and these
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details are subject to change. There are currently other implementations of
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GWP-ASan, such as the implementation featured in
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`Chromium <https://cs.chromium.org/chromium/src/components/gwp_asan/>`_. The
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long-term support goal is to ensure feature-parity where reasonable, and to
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support compiler-rt as the reference implementation.
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Allocator Support
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-----------------
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GWP-ASan is not a replacement for a traditional allocator. Instead, it works by
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inserting stubs into a supporting allocator to redirect allocations to GWP-ASan
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when they're chosen to be sampled. These stubs are generally implemented in the
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implementaion of ``malloc()``, ``free()`` and ``realloc()``. The stubs are
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extremely small, which makes using GWP-ASan in most allocators fairly trivial.
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The stubs follow the same general pattern (example ``malloc()`` pseudocode
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below):
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.. code:: cpp
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#ifdef INSTALL_GWP_ASAN_STUBS
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gwp_asan::GuardedPoolAllocator GWPASanAllocator;
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#endif
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void* YourAllocator::malloc(..) {
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#ifdef INSTALL_GWP_ASAN_STUBS
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if (GWPASanAllocator.shouldSample(..))
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return GWPASanAllocator.allocate(..);
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#endif
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// ... the rest of your allocator code here.
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}
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Then, all the supporting allocator needs to do is compile with
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``-DINSTALL_GWP_ASAN_STUBS`` and link against the GWP-ASan library! For
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performance reasons, we strongly recommend static linkage of the GWP-ASan
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library.
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Guarded Allocation Pool
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-----------------------
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The core of GWP-ASan is the guarded allocation pool. Each sampled allocation is
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backed using its own *guarded* slot, which may consist of one or more accessible
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pages. Each guarded slot is surrounded by two *guard* pages, which are mapped as
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inaccessible. The collection of all guarded slots makes up the *guarded
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allocation pool*.
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Buffer Underflow/Overflow Detection
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-----------------------------------
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We gain buffer-overflow and buffer-underflow detection through these guard
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pages. When a memory access overruns the allocated buffer, it will touch the
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inaccessible guard page, causing memory exception. This exception is caught and
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handled by the internal crash handler. Because each allocation is recorded with
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metadata about where (and by what thread) it was allocated and deallocated, we
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can provide information that will help identify the root cause of the bug.
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Allocations are randomly selected to be either left- or right-aligned to provide
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equal detection of both underflows and overflows.
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Use after Free Detection
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------------------------
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The guarded allocation pool also provides use-after-free detection. Whenever a
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sampled allocation is deallocated, we map its guarded slot as inaccessible. Any
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memory accesses after deallocation will thus trigger the crash handler, and we
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can provide useful information about the source of the error.
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Please note that the use-after-free detection for a sampled allocation is
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transient. To keep memory overhead fixed while still detecting bugs, deallocated
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slots are randomly reused to guard future allocations.
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Usage
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=====
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GWP-ASan already ships by default in the
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`Scudo Hardened Allocator <https://llvm.org/docs/ScudoHardenedAllocator.html>`_,
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so building with ``-fsanitize=scudo`` is the quickest and easiest way to try out
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GWP-ASan.
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Options
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-------
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GWP-ASan's configuration is managed by the supporting allocator. We provide a
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generic configuration management library that is used by Scudo. It allows
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several aspects of GWP-ASan to be configured through the following methods:
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- When the GWP-ASan library is compiled, by setting
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``-DGWP_ASAN_DEFAULT_OPTIONS`` to the options string you want set by default.
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If you're building GWP-ASan as part of a compiler-rt/LLVM build, add it during
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cmake configure time (e.g. ``cmake ... -DGWP_ASAN_DEFAULT_OPTIONS="..."``). If
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you're building GWP-ASan outside of compiler-rt, simply ensure that you
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specify ``-DGWP_ASAN_DEFAULT_OPTIONS="..."`` when building
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``optional/options_parser.cpp``).
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- By defining a ``__gwp_asan_default_options`` function in one's program that
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returns the options string to be parsed. Said function must have the following
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prototype: ``extern "C" const char* __gwp_asan_default_options(void)``, with a
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default visibility. This will override the compile time define;
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- Depending on allocator support (Scudo has support for this mechanism): Through
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the environment variable ``GWP_ASAN_OPTIONS``, containing the options string
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to be parsed. Options defined this way will override any definition made
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through ``__gwp_asan_default_options``.
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The options string follows a syntax similar to ASan, where distinct options
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can be assigned in the same string, separated by colons.
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For example, using the environment variable:
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.. code:: console
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GWP_ASAN_OPTIONS="MaxSimultaneousAllocations=16:SampleRate=5000" ./a.out
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Or using the function:
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.. code:: cpp
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extern "C" const char *__gwp_asan_default_options() {
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return "MaxSimultaneousAllocations=16:SampleRate=5000";
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}
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The following options are available:
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+----------------------------+---------+--------------------------------------------------------------------------------+
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| Option | Default | Description |
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+----------------------------+---------+--------------------------------------------------------------------------------+
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| Enabled | true | Is GWP-ASan enabled? |
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+----------------------------+---------+--------------------------------------------------------------------------------+
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| PerfectlyRightAlign | false | When allocations are right-aligned, should we perfectly align them up to the |
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| | | page boundary? By default (false), we round up allocation size to the nearest |
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| | | power of two (2, 4, 8, 16) up to a maximum of 16-byte alignment for |
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| | | performance reasons. Setting this to true can find single byte |
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| | | buffer-overflows at the cost of performance, and may be incompatible with |
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| | | some architectures. |
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+----------------------------+---------+--------------------------------------------------------------------------------+
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| MaxSimultaneousAllocations | 16 | Number of simultaneously-guarded allocations available in the pool. |
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+----------------------------+---------+--------------------------------------------------------------------------------+
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| SampleRate | 5000 | The probability (1 / SampleRate) that a page is selected for GWP-ASan |
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| | | sampling. Sample rates up to (2^31 - 1) are supported. |
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+----------------------------+---------+--------------------------------------------------------------------------------+
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| InstallSignalHandlers | true | Install GWP-ASan signal handlers for SIGSEGV during dynamic loading. This |
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| | | allows better error reports by providing stack traces for allocation and |
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| | | deallocation when reporting a memory error. GWP-ASan's signal handler will |
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| | | forward the signal to any previously-installed handler, and user programs |
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| | | that install further signal handlers should make sure they do the same. Note, |
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| | | if the previously installed SIGSEGV handler is SIG_IGN, we terminate the |
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| | | process after dumping the error report. |
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+----------------------------+---------+--------------------------------------------------------------------------------+
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Example
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-------
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The below code has a use-after-free bug, where the ``string_view`` is created as
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a reference to the temporary result of the ``string+`` operator. The
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use-after-free occurs when ``sv`` is dereferenced on line 8.
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.. code:: cpp
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1: #include <iostream>
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2: #include <string>
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3: #include <string_view>
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4:
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5: int main() {
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6: std::string s = "Hellooooooooooooooo ";
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7: std::string_view sv = s + "World\n";
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8: std::cout << sv;
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9: }
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Compiling this code with Scudo+GWP-ASan will probabilistically catch this bug
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and provide us a detailed error report:
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.. code:: console
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$ clang++ -fsanitize=scudo -std=c++17 -g buggy_code.cpp
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$ for i in `seq 1 200`; do
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GWP_ASAN_OPTIONS="SampleRate=100" ./a.out > /dev/null;
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done
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| *** GWP-ASan detected a memory error ***
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| Use after free at 0x7feccab26000 (0 bytes into a 41-byte allocation at 0x7feccab26000) by thread 31027 here:
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| ...
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| #9 ./a.out(_ZStlsIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_St17basic_string_viewIS3_S4_E+0x45) [0x55585c0afa55]
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| #10 ./a.out(main+0x9f) [0x55585c0af7cf]
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| #11 /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xeb) [0x7fecc966952b]
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| #12 ./a.out(_start+0x2a) [0x55585c0867ba]
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| 0x7feccab26000 was deallocated by thread 31027 here:
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| ...
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| #7 ./a.out(main+0x83) [0x55585c0af7b3]
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| #8 /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xeb) [0x7fecc966952b]
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| #9 ./a.out(_start+0x2a) [0x55585c0867ba]
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| 0x7feccab26000 was allocated by thread 31027 here:
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| ...
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| #12 ./a.out(main+0x57) [0x55585c0af787]
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| #13 /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xeb) [0x7fecc966952b]
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| #14 ./a.out(_start+0x2a) [0x55585c0867ba]
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| *** End GWP-ASan report ***
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| Segmentation fault
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To symbolize these stack traces, some care has to be taken. Scudo currently uses
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GNU's ``backtrace_symbols()`` from ``<execinfo.h>`` to unwind. The unwinder
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provides human-readable stack traces in ``function+offset`` form, rather than
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the normal ``binary+offset`` form. In order to use addr2line or similar tools to
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recover the exact line number, we must convert the ``function+offset`` to
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``binary+offset``. A helper script is available at
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``compiler-rt/lib/gwp_asan/scripts/symbolize.sh``. Using this script will
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attempt to symbolize each possible line, falling back to the previous output if
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anything fails. This results in the following output:
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.. code:: console
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$ cat my_gwp_asan_error.txt | symbolize.sh
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| *** GWP-ASan detected a memory error ***
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| Use after free at 0x7feccab26000 (0 bytes into a 41-byte allocation at 0x7feccab26000) by thread 31027 here:
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| ...
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| #9 /usr/lib/gcc/x86_64-linux-gnu/8.0.1/../../../../include/c++/8.0.1/string_view:547
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| #10 /tmp/buggy_code.cpp:8
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| 0x7feccab26000 was deallocated by thread 31027 here:
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| ...
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| #7 /tmp/buggy_code.cpp:8
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| #8 /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xeb) [0x7fecc966952b]
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| #9 ./a.out(_start+0x2a) [0x55585c0867ba]
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| 0x7feccab26000 was allocated by thread 31027 here:
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| ...
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| #12 /tmp/buggy_code.cpp:7
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| #13 /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xeb) [0x7fecc966952b]
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| #14 ./a.out(_start+0x2a) [0x55585c0867ba]
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| *** End GWP-ASan report ***
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| Segmentation fault
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