[GWP-ASan] Add public-facing documentation [6].

[hctim]

Summary

Note: Do not submit this documentation until Scudo support is reviewed and submitted (should be #[5]).

See D60593 for further information.

This patch introduces the public-facing documentation for GWP-ASan, as well as updating the definition of one of the options, which wasn't properly merged. The document describes the design and features of GWP-ASan, as well as how to use GWP-ASan from both a user's standpoint, and development documentation for supporting allocators.

[kcc]

Maybe add an example?
something like my favorite

#include <iostream>
#include <string>
#include <string_view>

int main() {
  std::string s = "Hellooooooooooooooo ";
  std::string_view sv = s + "World\n";
  std::cout << sv;
}


for((i=0;i<10000; i++)); do GWP_ASAN_OPTIONS=Enabled=1:SampleRate=500  ./a.out > /dev/null; done

Segmentation fault
*** GWP-ASan detected a memory error ***
Use after free at 0x7f43300df000 (0 bytes into a 41-byte allocation at 0x7f43300df000) by thread 26228 here:
#0 ./a.out(_ZN8gwp_asan20GuardedPoolAllocator19reportErrorInternalEmNS0_5ErrorE+0x25a) [0x56454249ecea]
#1 ./a.out(+0x1bede) [0x56454249dede]
#2 /lib/x86_64-linux-gnu/libpthread.so.0(+0x123a0) [0x7ffbb526f3a0]
#3 /lib/x86_64-linux-gnu/libc.so.6(+0x15c27e) [0x7ffbb49d627e]
#4 /lib/x86_64-linux-gnu/libc.so.6(_IO_default_xsputn+0xa8) [0x7ffbb48f7058]
#5 /lib/x86_64-linux-gnu/libc.so.6(_IO_file_xsputn+0x151) [0x7ffbb48f52b1]
#6 /lib/x86_64-linux-gnu/libc.so.6(fwrite+0xd8) [0x7ffbb48ea1f8]
#7 /lib/x86_64-linux-gnu/libstdc++.so.6(_ZSt16__ostream_insertIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_PKS3_l+0x124) [0x7ffbb590d3a4]
#8 ./a.out(_ZStlsIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_St17basic_string_viewIS3_S4_E+0x45) [0x5645424b3bc5]
#9 ./a.out(main+0x9f) [0x5645424b393f]

Also, please provide some guidance about symbolizing the reports.

[kcc]

With a build command line, of course

clang++ -g -std=c++17 string-view-use-after-free.cc  -fsanitize=scudo

[hctim]

• Added GWP-ASan symbolization example and script.

[morehouse]

compiler-rt/lib/gwp_asan/scripts/symbolize.sh

15

Can reduce indentation by putting a continue here and removing else below.

llvm/docs/GwpAsan.rst

89

I find this explanation confusing. Maybe we don't need to mention that it's a contiguous buffer, since that detail isn't key to the approach.

Example wording: The collection of all guarded slots make up the *guarded allocation pool*.

95

insert "detection"

100

"helpful" is redundant.

103

Increase versus what? Maybe this would be clearer if we say something like "allocations are randomly selected to be either left- or right-aligned to provide equal detection of both underflows and overflows".

108

provides

109

s/the guard page/its guarded slot

117

Above two sentences could be combined and made more succinct.

e.g,, "To keep memory overhead fixed while still detecting bugs, deallocated slots are randomly reused to guard future allocations".

122

It's a small difference, but I'd prefer a more positive (think advertising) wording here.

e.g.,

"GWP-ASan already ships by default in Scudo, so building with -fsanitize=scudo is the quickest and easiest way to try out GWP-ASan. For other allocators, using GWP-ASan is as easy as..."

134

I'm confused. Is this per-allocator or per-process?

137

Is this a CMake variable, a preprocessor macro, or something else?

197

ephemeral + temporary seems redundant

197

"inappropriately assigned" - Can we be more precise?

[hctim]

llvm/docs/GwpAsan.rst

122

I think the "for other allocators..." part is covered above in the "Allocator Support" section, but otherwise SGTM+done.

134

Hopefully the rewording adds some clarity.

[hctim]

• Updated from Matt's comments. Thanks Matt for taking a look on short notice :)

[morehouse]

llvm/docs/GwpAsan.rst

137

This helps, but it's still not clear whether this is a cmake variable or a preprocessor variable.

Do I do this with:

cmake ... -DGWP_ASAN_DEFAULT_OPTIONS="..."

or

clang++ -fsanitize=scudo ... -DGWP_ASAN_DEFAULT_OPTIONS="..."

[vlad.tsyrklevich]

compiler-rt/lib/gwp_asan/scripts/symbolize.sh

4

s/looks/look/

llvm/docs/GwpAsan.rst

51

s/reasonble/reasonable/

[hctim]

• Addressed Matt's and Vlad's comments.
• Updated memory overhead description in the docs.

Updated memory overhead description in the docs.

Assumes 8KiB for allocation/deallocation stack traces (256B compressed *
2 [alloc + dealloc] * 16 slots).
Assumes a full 4KiB of wasted memory (if we have 1-byte allocations) *
8 [half of the 16 possible slots].

Note that freed guarded allocations take no space, as the pages are
released back to the OS.

llvm/docs/GwpAsan.rst

137

Added some clarifying comments, let me know if they LGTY.

[morehouse]

llvm/docs/GwpAsan.rst

134

Shouldn't this be -DGWP_ASAN_DEFAULT_OPTIONS?

135

What manual?

138

Do we expect/want-to-support building outside of compiler-rt? If we encourage this, we may end up with many forks of GWP-ASan.

[hctim]

llvm/docs/GwpAsan.rst

135

Was supposed to be the manual for the supporting allocator, but I've removed some words here to try and clarify.

138

I think that supporting out-of-RT builds is necessary. We've got an Android repository slice up and going that will definitely be out-of-tree builds.

My goal is basically to have the core **/*.h and **/*.cpp. If you want to use GWP-ASan, simply pull in the slice of the repository and add it to your build.

Maybe I should copybara slice this out of CRT and keep a makefile-independent (read: only cpp and header files) version so that it can be mirrored everywhere... WDYT?

[hctim]

• Update from Matt's comments.

[morehouse]

llvm/docs/GwpAsan.rst

138

Not sure if we have a need/demand for this. I'm inclined to not do this unless we have a good reason.

[hctim]

llvm/docs/GwpAsan.rst

138

I forsee Chrome on Android using this configuration method to start-time disable platform GWP-ASan, as they'll use their own variant.

@vlad.tsyrklevich, do you have any thoughts?

[vlad.tsyrklevich]

llvm/docs/GwpAsan.rst

138

We probably do want to disable GWP-ASan for Android Chrome, though it doesn't have to be this method specifically.

[hctim]

llvm/docs/GwpAsan.rst

138

Ways to configure:

• Compile time (for GWP-ASan) configuration options: Use -DGWP_ASAN_OPTIONS as described above.
• Compile time (for application) configuration options: Use __gwp_asan_default_options here.
• Program start time: Use GWP_ASAN_OPTIONS environment as per below.

We could possibly do detection of Chrome in the initialisation of platform GWP-ASan on Android - but I don't think this is generally suitable.

@morehouse are you okay with providing this config option? I think that #2 is probably the best way for applications to "always off" GWP-ASan, unless we have a better solution in mind.

[morehouse]

llvm/docs/GwpAsan.rst

138

These three options are fine with me.

[rupprecht]

The docs don't build and rL369554 doesn't fully fix it

$ ninja docs-llvm-html
[1/1] Generating html Sphinx documentation
FAILED: docs/CMakeFiles/docs-llvm-html

Warning, treated as error:
llvm/docs/GwpAsan.rst:document isn't included in any toctree
ninja: build stopped: subcommand failed.

Submitted rL369556 to temporarily make docs build again. However that basically suppresses the warning; you likely want to put it in a toctree instead of making it an orphan doc, e.g. https://reviews.llvm.org/D66183#change-D4KqPHFormwF

[hctim]

In D62875#1639703, @rupprecht wrote:

The docs don't build and rL369554 doesn't fully fix it

$ ninja docs-llvm-html
[1/1] Generating html Sphinx documentation
FAILED: docs/CMakeFiles/docs-llvm-html

Warning, treated as error:
llvm/docs/GwpAsan.rst:document isn't included in any toctree
ninja: build stopped: subcommand failed.

Submitted rL369556 to temporarily make docs build again. However that basically suppresses the warning; you likely want to put it in a toctree instead of making it an orphan doc, e.g. https://reviews.llvm.org/D66183#change-D4KqPHFormwF

Solved in rL369560. Thanks!

compiler-rt/lib/gwp_asan/options.inc

GWP_ASAN_OPTION(
     bool, PerfectlyRightAlign, false,
     "When allocations are right-aligned, should we perfectly align them up to "
     "the page boundary? By default (false), we round up allocation size to the "
     "nearest power of two (1, 2, 4, 8, 16) up to a maximum of 16-byte "
     "alignment for performance reasons. Setting this to true can find single "
     "byte buffer-overflows for multibyte allocations at the cost of "
     "performance, and may be incompatible with some architectures.")
 
-GWP_ASAN_OPTION(
-    int, MaxSimultaneousAllocations, 16,
-    "Number of usable guarded slots in the allocation pool. Defaults to 16.")
+GWP_ASAN_OPTION(int, MaxSimultaneousAllocations, 16,
+                "Number of simultaneously-guarded allocations available in the "
+                "pool. Defaults to 16.")
 
 GWP_ASAN_OPTION(int, SampleRate, 5000,
                 "The probability (1 / SampleRate) that an allocation is "
                 "selected for GWP-ASan sampling. Default is 5000. Sample rates "
                 "up to (2^31 - 1) are supported.")
 
 GWP_ASAN_OPTION(
     bool, InstallSignalHandlers, true,
     "Install GWP-ASan signal handlers for SIGSEGV during dynamic loading. This "
     "allows better error reports by providing stack traces for allocation and "
     "deallocation when reporting a memory error. GWP-ASan's signal handler "
     "will forward the signal to any previously-installed handler, and user "
     "programs that install further signal handlers should make sure they do "
     "the same. Note, if the previously installed SIGSEGV handler is SIG_IGN, "
     "we terminate the process after dumping the error report.")

compiler-rt/lib/gwp_asan/scripts/symbolize.sh

+#!/bin/bash
+
+# The lines that we're looking to symbolize look like this:
+  #0 ./a.out(_foo+0x3e6) [0x55a52e64c696]
+# ... which come from the backtrace_symbols() symbolisation function used by
+# default in Scudo's implementation of GWP-ASan.
+
+while read -r line; do
+  # Check that this line needs symbolization.
+  should_symbolize="$(echo $line |\
+     grep -E '^[ ]*\#.*\(.*\+0x[0-9a-f]+\) \[0x[0-9a-f]+\]$')"
+
+  if [ -z "$should_symbolize" ]; then
+    echo "$line"
+    continue
+  fi
+
+  # Carve up the input line into sections.
+  binary_name="$(echo $line | grep -oE ' .*\(' | rev | cut -c2- | rev |\
+      cut -c2-)"
+  function_name="$(echo $line | grep -oE '\([^+]*' | cut -c2-)"
+  function_offset="$(echo $line | grep -oE '\(.*\)' | grep -oE '\+.*\)' |\
+      cut -c2- | rev | cut -c2- | rev)"
+  frame_number="$(echo $line | grep -oE '\#[0-9]+ ')"
+
+  if [ -z "$function_name" ]; then
+    # If the offset is binary-relative, just resolve that.
+    symbolized="$(echo $function_offset | addr2line -e $binary_name)"
+  else
+    # Otherwise, the offset is function-relative. Get the address of the
+    # function, and add it to the offset, then symbolize.
+    function_addr="0x$(echo $function_offset |\
+       nm --defined-only $binary_name 2> /dev/null |\
+       grep -E " $function_name$" | cut -d' ' -f1)"
+
+    # Check that we could get the function address from nm.
+    if [ -z "$function_addr" ]; then
+      echo "$line"
+      continue
+    fi
+
+    # Add the function address and offset to get the offset into the binary.
+    binary_offset="$(printf "0x%X" "$((function_addr+function_offset))")"
+    symbolized="$(echo $binary_offset | addr2line -e $binary_name)"
+  fi
+
+  # Check that it symbolized properly. If it didn't, output the old line.
+  echo $symbolized | grep -E ".*\?.*:" > /dev/null
+  if [ "$?" -eq "0" ]; then
+    echo "$line"
+    continue
+  else
+    echo "${frame_number}${symbolized}"
+  fi
+done

llvm/docs/GwpAsan.rst

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