Dynamic analysis with Clang

This document describes how to use Clang to perform analysis on Python and its libraries.

This document does not cover interpreting the findings. For a discussion of interpreting results, see Marshall Clow’s Testing libc++ with -fsanitize=undefined. The blog posting is a detailed examinations of issues uncovered by Clang in libc++.

The document focuses on Clang, although most techniques should generally apply to GCC’s sanitizers as well.

The instructions were tested on Linux, but they should work on macOS as well. Instructions for Windows are incomplete.

What is Clang?

Clang is the C, C++ and Objective C front-end for the LLVM compiler. The front-end provides access to LLVM’s optimizer and code generator. The sanitizers - or checkers - are hooks into the code generation phase to instrument compiled code so suspicious behavior is flagged.

What are sanitizers?

Clang sanitizers are runtime checkers used to identify suspicious and undefined behavior. The checking occurs at runtime with actual runtime parameters so false positives are kept to a minimum.

There are a number of sanitizers available, but two that should be used on a regular basis are the Address Sanitizer (or ASan) and the Undefined Behavior Sanitizer (or UBSan). ASan is invoked with the compiler option -fsanitize=address, and UBSan is invoked with -fsanitize=undefined. The flags are passed through CFLAGS and CXXFLAGS, and sometimes through CC and CXX (in addition to the compiler).

A complete list of sanitizers can be found at Controlling Code Generation.

Note

Because sanitizers operate at runtime on real program parameters, its important to provide a complete set of positive and negative self tests.

Clang and its sanitizers have strengths (and weaknesses). Its just one tool in the war chest to uncovering bugs and improving code quality. Clang should be used to complement other methods, including Code Reviews, Valgrind, etc.

Clang/LLVM setup

Pre-built Clang builds are available for most platforms:

  • On macOS, Clang is the default compiler.

  • For mainstream Linux distros, you can install a clang package. In some cases, you also need to install llvm separately, otherwise some tools are not available.

  • On Windows, the installer for Visual Studio (not Code) includes the “C++ clang tools for windows” feature.

You can also build clang from source; refer to the clang documentation for details.

The installer does not install all the components needed on occasion. For example, you might want to run a scan-build or examine the results with scan-view. If this is your case, you can build Clang from source and copy tools from tools/clang/tools to a directory on your PATH.

Another reason to build from source is to get the latest version of Clang/LLVM, if your platform’s channels don’t provide it yet. Newer versions of Clang/LLVM introduce new sanitizer checks.

Python build setup

This portion of the document covers invoking Clang and LLVM with the options required so the sanitizers analyze Python with under its test suite.

Set the compiler to Clang, in case it’s not the default:

export CC="clang"

If you want to use additional sanitizer options (found in Clang documentation), add them to the CFLAGS variable. For example, you may want the checked process to exit after the first failure:

export CFLAGS="-fno-sanitize-recover"

Then, run ./configure with the relevant flags:

  • ASan: --with-address-sanitizer --without-pymalloc

  • UBsan: --with-undefined-behavior-sanitizer

It is OK to specify both sanitizers.

After that, run make and make test as usual. Note that make itself may fail with a sanitizer failure, since the just-compiled Python runs during later stages of the build.

Build setup for enabling sanitizers for all code

Some parts of Python (for example, _testembed, _freeze_importlib, test_cppext) may not use the variables set by configure, and with the above settings they’ll be compiled without sanitization.

As a workaround, you can pass the sanitizer options by way of the compilers, CC (for C) and CXX (for C++). This is used below. Passing the options through LDFLAGS is also reported to work.

For ASan, use:

# ASan
export CC="clang -fsanitize=address"
export CXX="clang++ -fsanitize=address -fno-sanitize=vptr"

And for UBSan:

# UBSan
export CC="clang -fsanitize=undefined"
export CXX="clang++ -fsanitize=undefined -fno-sanitize=vptr"

It’s OK to specify both sanitizers.

After this, run ./configure, make and make test as usual.

Analyzing the output

Sanitizer failures will make the process fail and output a diagnostic, for example:

Objects/longobject.c:39:42: runtime error: index -1 out of bounds
    for type 'PyLongObject [262]'
Objects/tupleobject.c:188:13: runtime error: member access within
    misaligned address 0x2b76be018078 for type 'PyGC_Head' (aka
    'union _gc_head'), which requires 16 byte alignment
    0x2b76be018078: note: pointer points here
    00 00 00 00  40 53 5a b6 76 2b 00 00  60 52 5a b6 ...
                 ^
...

If you are using the address sanitizer, an additional tool is needed to get good traces. Usually, this happens automatically through the llvm-symbolizer tool. If this tool is not installed on your PATH, you can set ASAN_SYMBOLIZER_PATH to the location of the tool, or pipe test output through asan_symbolize.py script from the Clang distribution. For example, from Issue 20953 during compile (formatting added for clarity):

$ make test 2>&1 | asan_symbolize.py
...

/usr/local/bin/clang -fsanitize=address -Xlinker -export-dynamic
    -o python Modules/python.o libpython3.3m.a -ldl -lutil
    /usr/local/ssl/lib/libssl.a /usr/local/ssl/lib/libcrypto.a -lm
./python -E -S -m sysconfig --generate-posix-vars
=================================================================
==24064==ERROR: AddressSanitizer: heap-buffer-overflow on address
0x619000004020 at pc 0x4ed4b2 bp 0x7fff80fff010 sp 0x7fff80fff008
READ of size 4 at 0x619000004020 thread T0
  #0 0x4ed4b1 in PyObject_Free Python-3.3.5/./Objects/obmalloc.c:987
  #1 0x7a2141 in code_dealloc Python-3.3.5/./Objects/codeobject.c:359
  #2 0x620c00 in PyImport_ImportFrozenModuleObject
       Python-3.3.5/./Python/import.c:1098
  #3 0x620d5c in PyImport_ImportFrozenModule
       Python-3.3.5/./Python/import.c:1114
  #4 0x63fd07 in import_init Python-3.3.5/./Python/pythonrun.c:206
  #5 0x63f636 in _Py_InitializeEx_Private
       Python-3.3.5/./Python/pythonrun.c:369
  #6 0x681d77 in Py_Main Python-3.3.5/./Modules/main.c:648
  #7 0x4e6894 in main Python-3.3.5/././Modules/python.c:62
  #8 0x2abf9a525eac in __libc_start_main
       /home/aurel32/eglibc/eglibc-2.13/csu/libc-start.c:244
  #9 0x4e664c in _start (Python-3.3.5/./python+0x4e664c)

AddressSanitizer can not describe address in more detail (wild
memory access suspected).
SUMMARY: AddressSanitizer: heap-buffer-overflow
  Python-3.3.5/./Objects/obmalloc.c:987 PyObject_Free
Shadow bytes around the buggy address:
  0x0c327fff87b0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff87c0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff87d0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff87e0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff87f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c327fff8800: fa fa fa fa[fa]fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff8810: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff8820: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff8830: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff8840: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff8850: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:     fa
  Heap right redzone:    fb
  Freed heap region:     fd
  Stack left redzone:    f1
  Stack mid redzone:     f2
  Stack right redzone:   f3
  Stack partial redzone: f4
  Stack after return:    f5
  Stack use after scope: f8
  Global redzone:        f9
  Global init order:     f6
  Poisoned by user:      f7
  ASan internal:         fe
==24064==ABORTING
make: *** [pybuilddir.txt] Error 1

Note

If asan_symbolize.py is not installed, build Clang from source, then look in the Clang/LLVM build directory for it and use it directly or copy it to a directory on PATH.

Ignoring findings

Clang allows you to alter the behavior of sanitizer tools for certain source-level by providing a special ignorelist file at compile-time. The ignorelist is needed because it reports every instance of an issue, even if the issue is reported 10’s of thousands of time in un-managed library code.

You specify the ignorelist with -fsanitize-ignorelist=XXX. For example:

-fsanitize-ignorelist=my_ignorelist.txt

my_ignorelist.txt would then contain entries such as the following. The entry will ignore a bug in libc++’s ios formatting functions:

fun:_Ios_Fmtflags

As an example with Python 3.4.0, audioop.c will produce a number of findings:

./Modules/audioop.c:422:11: runtime error: left shift of negative value -1
./Modules/audioop.c:446:19: runtime error: left shift of negative value -1
./Modules/audioop.c:476:19: runtime error: left shift of negative value -1
./Modules/audioop.c:504:16: runtime error: left shift of negative value -1
./Modules/audioop.c:533:22: runtime error: left shift of negative value -128
./Modules/audioop.c:775:19: runtime error: left shift of negative value -70
./Modules/audioop.c:831:19: runtime error: left shift of negative value -70
./Modules/audioop.c:881:19: runtime error: left shift of negative value -1
./Modules/audioop.c:920:22: runtime error: left shift of negative value -70
./Modules/audioop.c:967:23: runtime error: left shift of negative value -70
./Modules/audioop.c:968:23: runtime error: left shift of negative value -70
...

One of the function of interest is audioop_getsample_impl (flagged at line 422), and the ignorelist entry would include:

fun:audioop_getsample_imp

Or, you could ignore the entire file with:

src:Modules/audioop.c

Unfortunately, you won’t know what to ignorelist until you run the sanitizer.

The documentation is available at Sanitizer special case list.