Dynamic analysis with Clang

This document describes how to use Clang to perform analysis on Python and its libraries. In addition to performing the analysis, the document will cover downloading, building and installing the latest Clang/LLVM combination (which is currently 3.4).

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++.

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.


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 compliment other methods, including Code Reviews, Valgrind, Coverity, etc.

Clang/LLVM setup

This portion of the document covers downloading, building and installing Clang and LLVM. There are three components to download and build. They are the LLVM compiler, the compiler front end and the compiler runtime library.

In preparation you should create a scratch directory. Also ensure you are using Python 2 and not Python 3. Python 3 will cause the build to fail.

Download, build and install

Perform the following to download, build and install the Clang/LLVM 3.4.

# Download
wget https://llvm.org/releases/3.4/llvm-3.4.src.tar.gz
wget https://llvm.org/releases/3.4/clang-3.4.src.tar.gz
wget https://llvm.org/releases/3.4/compiler-rt-3.4.src.tar.gz

tar xvf llvm-3.4.src.tar.gz
cd llvm-3.4/tools

# Clang Front End
tar xvf ../../clang-3.4.src.tar.gz
mv clang-3.4 clang

# Compiler RT
cd ../projects
tar xvf ../../compiler-rt-3.4.src.tar.gz
mv compiler-rt-3.4/ compiler-rt

# Build
cd ..
./configure --enable-optimized --prefix=/usr/local
make -j4
sudo make install


If you receive an error 'LibraryDependencies.inc' file not found, then ensure you are utilizing Python 2 and not Python 3. If you encounter the error after switching to Python 2, then delete everything and start over.

After make install executes, the compilers will be installed in /usr/local/bin and the various libraries will be installed in /usr/local/lib/clang/3.4/lib/linux/:

$ ls /usr/local/lib/clang/3.4/lib/linux/
libclang_rt.asan-x86_64.a   libclang_rt.profile-x86_64.a
libclang_rt.dfsan-x86_64.a  libclang_rt.san-x86_64.a
libclang_rt.full-x86_64.a   libclang_rt.tsan-x86_64.a
libclang_rt.lsan-x86_64.a   libclang_rt.ubsan_cxx-x86_64.a
libclang_rt.msan-x86_64.a   libclang_rt.ubsan-x86_64.a

On macOS, the libraries are installed in /usr/local/lib/clang/3.3/lib/darwin/:

$ ls /usr/local/lib/clang/3.3/lib/darwin/
libclang_rt.10.4.a                    libclang_rt.ios.a
libclang_rt.asan_osx.a                libclang_rt.osx.a
libclang_rt.asan_osx_dynamic.dylib    libclang_rt.profile_ios.a
libclang_rt.cc_kext.a                 libclang_rt.profile_osx.a
libclang_rt.cc_kext_ios5.a            libclang_rt.ubsan_osx.a


You should never have to add the libraries to a project. Clang will handle it for you. If you find you cannot pass the -fsanitize=XXX flag through make’s implicit variables (CFLAGS, CXXFLAGS, CC, CXXFLAGS, LDFLAGS) during configure, then you should modify the makefile after configuring to ensure the flag is passed through the compiler.

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. You can copy the components by hand with:

sudo mkdir /usr/local/bin/scan-build
sudo cp -r llvm-3.4/tools/clang/tools/scan-build /usr/local/bin
sudo mkdir /usr/local/bin/scan-view
sudo cp -r llvm-3.4/tools/clang/tools/scan-view /usr/local/bin


Because the installer does not install all the components needed on occasion, you should not delete the scratch directory until you are sure things work as expected. If a library is missing, then you should search for it in the Clang/LLVM build directory.

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. Two checkers are used - ASan and UBSan.

Because the sanitizers are runtime checkers, its best to have as many positive and negative self tests as possible. You can never have enough self tests.

The general idea is to compile and link with the sanitizer flags. At link time, Clang will include the needed runtime libraries. However, you can’t use CFLAGS and CXXFLAGS to pass the options through the compiler to the linker because the makefile rules for BUILDPYTHON, _testembed and _freeze_importlib don’t use the implicit variables.

As a workaround to the absence of flags to the linker, you can pass the sanitizer options by way of the compilers - CC and CXX. Passing the flags though the compiler is used below, but passing them through LDFLAGS is also reported to work.

Building Python

To begin, export the variables of interest with the desired sanitizers. Its OK to specify both sanitizers:

# ASan
export CC="/usr/local/bin/clang -fsanitize=address"
export CXX="/usr/local/bin/clang++ -fsanitize=address -fno-sanitize=vptr"


# UBSan
export CC="/usr/local/bin/clang -fsanitize=undefined"
export CXX="/usr/local/bin/clang++ -fsanitize=undefined -fno-sanitize=vptr"

The -fno-sanitize=vptr removes vtable checks that are part of UBSan from C++ projects due to noise. Its not needed with Python, but you will likely need it for other C++ projects.

After exporting CC and CXX, configure as normal:

$ ./configure
checking build system type... x86_64-unknown-linux-gnu
checking host system type... x86_64-unknown-linux-gnu
checking for --enable-universalsdk... no
checking for --with-universal-archs... 32-bit
checking MACHDEP... linux
checking for --without-gcc... no
checking for gcc... /usr/local/bin/clang -fsanitize=undefined
checking whether the C compiler works... yes

Next is a standard make (formatting added for clarity):

$ make
/usr/local/bin/clang -fsanitize=undefined -c -Wno-unused-result
    -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I.
    -IInclude -I./Include -DPy_BUILD_CORE -o Modules/python.o
/usr/local/bin/clang -fsanitize=undefined -c -Wno-unused-result
    -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I.
    -IInclude -I./Include -DPy_BUILD_CORE -o Parser/acceler.o

Finally is make test (formatting added for clarity):

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, its important to pipe the output through asan_symbolize.py to get a good trace. 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
  #3 0x620d5c in PyImport_ImportFrozenModule
  #4 0x63fd07 in import_init Python-3.3.5/./Python/pythonrun.c:206
  #5 0x63f636 in _Py_InitializeEx_Private
  #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
  #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
make: *** [pybuilddir.txt] Error 1


asan_symbolize.py is supposed to be installed during make install. If its not installed, then look in the Clang/LLVM build directory for it and copy it to /usr/local/bin.

Blacklisting (ignoring) findings

Clang allows you to alter the behavior of sanitizer tools for certain source-level by providing a special blacklist file at compile-time. The blacklist 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 blacklist with -fsanitize-blacklist=XXX. For example:


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


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 blacklist entry would include:


Or, you could ignore the entire file with:


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

The documentation is available at Sanitizer special case list.