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

Victor Stinner victor.stinner at haypocalc.com
Tue Sep 30 01:05:53 CEST 2008

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/ -------------- next part -------------- A non-text attachment was scrubbed... Name: segfault.patch Type: text/x-diff Size: 4195 bytes Desc: not available URL: <http://mail.python.org/pipermail/python-dev/attachments/20080930/d2f489cd/attachment-0001.patch>

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