[Python-Dev] Real segmentation fault handler (original) (raw)

Ralf W. Grosse-Kunstleve rwgk at yahoo.com
Tue Sep 30 04:06:07 CEST 2008

FWIW: I didn't have much luck translating segfaults into exceptions. It (seemed) to work on some platforms, but not others; this was in the context of C++. In my experience, it is more useful to generate Python and C stack traces and bail out. I also do this for floating-point exceptions. The handlers are installed at runtime from a low-level extension module:

http://cctbx.svn.sourceforge.net/viewvc/cctbx/trunk/boost_adaptbx/meta_ext.cpp?view=markup

Example output is below. It works under Linux and partially under Mac OS X.

Ralf

% boost_adaptbx.segmentation_fault Now dereferencing null-pointer ... show_stack(1): /net/chevy/raid1/rwgk/dist/boost_adaptbx/command_line/segmentation_fault.py(10) run show_stack(2): /net/chevy/raid1/rwgk/dist/boost_adaptbx/command_line/segmentation_fault.py(14) libc backtrace (18 frames, most recent call last): /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python [0x4118e9] /lib64/libc.so.6(__libc_start_main+0xf4) [0x363241e074] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(Py_Main+0x935) [0x4123c5] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyRun_SimpleFileExFlags+0x1a0) [0x4a8860] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyRun_FileExFlags+0x10e) [0x4a85ce] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyEval_EvalCode+0x32) [0x487402] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyEval_EvalCodeEx+0x81f) [0x4873bf] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyEval_EvalFrameEx+0x6bc1) [0x486541] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyEval_EvalFrameEx+0x2bb9) [0x482539] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyObject_Call+0x13) [0x415ae3] /net/chevy/raid1/rwgk/bintbx_py252/lib/libboost_python.so [0x2aaaaba7c6f7] /net/chevy/raid1/rwgk/bintbx_py252/lib/libboost_python.so(boost::python::handle_exception_impl(boost::function0)+0x28) [0x2aaaaba87148] /net/chevy/raid1/rwgk/bintbx_py252/lib/libboost_python.so(boost::function0::operator()() const+0x19e) [0x2aaaaba8816e] /net/chevy/raid1/rwgk/bintbx_py252/lib/libboost_python.so [0x2aaaaba7fef8] /net/chevy/raid1/rwgk/bintbx_py252/lib/libboost_python.so(boost::python::objects::function::call(_object*, _object*) const+0x7d) [0x2aaaaba7fb5d] /net/chevy/raid1/rwgk/bintbx_py252/lib/boost_python_meta_ext.so(boost::python::objects::caller_py_function_impl<boost::python::detail::caller<char ()(char const*), boost::python::default_call_policies, boost::mpl::vector2<char, char const*> > >::operator()(_object, _object*)+0x29) [0x2aaaab8470a9] /net/chevy/raid1/rwgk/bintbx_py252/lib/boost_python_meta_ext.so [0x2aaaab843790] /lib64/libc.so.6 [0x3632430f30] Segmentation fault (Python and libc call stacks above)

% boost_adaptbx.divide_by_zero Now dividing by zero (in C++) ... show_stack(1): /net/chevy/raid1/rwgk/dist/boost_adaptbx/command_line/divide_by_zero.py(10) run show_stack(2): /net/chevy/raid1/rwgk/dist/boost_adaptbx/command_line/divide_by_zero.py(14) libc backtrace (18 frames, most recent call last): /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python [0x4118e9] /lib64/libc.so.6(__libc_start_main+0xf4) [0x363241e074] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(Py_Main+0x935) [0x4123c5] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyRun_SimpleFileExFlags+0x1a0) [0x4a8860] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyRun_FileExFlags+0x10e) [0x4a85ce] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyEval_EvalCode+0x32) [0x487402] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyEval_EvalCodeEx+0x81f) [0x4873bf] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyEval_EvalFrameEx+0x6bc1) [0x486541] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyEval_EvalFrameEx+0x2bb9) [0x482539] /net/chevy/raid1/rwgk/bintbx_py252/base/bin/python(PyObject_Call+0x13) [0x415ae3] /net/chevy/raid1/rwgk/bintbx_py252/lib/libboost_python.so [0x2aaaaba7c6f7] /net/chevy/raid1/rwgk/bintbx_py252/lib/libboost_python.so(boost::python::handle_exception_impl(boost::function0)+0x28) [0x2aaaaba87148] /net/chevy/raid1/rwgk/bintbx_py252/lib/libboost_python.so(boost::function0::operator()() const+0x19e) [0x2aaaaba8816e] /net/chevy/raid1/rwgk/bintbx_py252/lib/libboost_python.so [0x2aaaaba7fef8] /net/chevy/raid1/rwgk/bintbx_py252/lib/libboost_python.so(boost::python::objects::function::call(_object*, _object*) const+0x7d) [0x2aaaaba7fb5d] /net/chevy/raid1/rwgk/bintbx_py252/lib/boost_python_meta_ext.so(boost::python::objects::caller_py_function_impl<boost::python::detail::caller<double ()(double const&, double const&), boost::python::default_call_policies, boost::mpl::vector3<double, double const&, double const&> > >::operator()(_object, _object*)+0x12a) [0x2aaaab84759a] /net/chevy/raid1/rwgk/bintbx_py252/lib/boost_python_meta_ext.so [0x2aaaab8437a4] /lib64/libc.so.6 [0x3632430f30] Floating-point error (Python and libc call stacks above)

----- Original Message ---- From: Victor Stinner <victor.stinner at haypocalc.com> To: python-dev at python.org Sent: Monday, September 29, 2008 4:05:53 PM Subject: [Python-Dev] Real segmentation fault handler

Hi,

I would like to be able to catch SIGSEGV in my Python code! So I started to hack Python trunk to support this feature. The idea is to use a signal handler which call longjmp(), and add setjmp() at Py_EvalFrameEx() enter.

See attached ("small") patch: segfault.patch

Example read.py with the evil ctypes module of invalid memory read: ------------------- 8< -------------- from ctypes import string_at

def fault(): text = string_at(1, 10) print("text = {0!r}".format(text))

def test(): print("test: 1") try: fault() except MemoryError, err: print "ooops!" print err

print("test: 2")
try:
    fault()
except MemoryError, err:
    print "ooops!"
    print err

print("test: end")

def main(): test()

if name == "main": main() ------------------- 8< --------------

Result: ------------------- 8< -------------- $ python read.py test: 1 sizeof()=160 ooops! segmentation fault test: 2 sizeof()=160 ooops! segmentation fault test: end ------------------- 8< --------------

Example bug1.py of a stack overflow:

loop = None, for i in xrange(10**5): loop = loop, None

Result:

$ python -i bug1.py

print loop (((((((((...Traceback (most recent call last): File "", line 1, in MemoryError: segmentation fault

Python is able to restore a valid state (stack/heap) after a segmentation fault and raise a classical Python exception (I choosed MemoryError, but it could be a specific exception).

On my computer (Ubuntu Gutsy/i386), each segfault_frame takes sizeof(sigjmpbuf) + sizeof(void*) = 160 bytes, allocated on the stack. I don't know if it's huge or not, but that will limit the number of recursive calls. The feature can be optional if we add a configure option and some #ifdef/#endif. A dedicated stack is needed to be call the signal handler on stack overflow error. I choosed 4 KB, but since I only call longjmp(), smaller stack might also works.

Does other VM support such feature? JVM, Mono, .NET, etc. ?

I had the idea of catching SIGSEGV after reading the issue 1069092 (stack overflow because of too many recursive calls).

-- Victor Stinner aka haypo http://www.haypocalc.com/blog/

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