[Python-Dev] Real segmentation fault handler

[Victor Stinner]

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 "<stdin>", line 1, in <module>
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>