Issue 30484: Garbage Collector can cause Segfault whilst iterating dictionary items (original) (raw)

We discovered this issue whilst using h5py (HDF5 python library) under python 3.5.2 on Ubuntu 16.04.2 x86_64. The construct used is very dubious, and I will separately be raising an issue with the h5py team. However I thought you might like to know there is a possible way to cause a segmentation violation using pure python.

There appears to be a new implementation of dictionaries in 3.6 vs 3.5, so I compiled up 3.6.1 and it also segfaults, but not so obviously related to the dictionary iteration (both stack traces are included below).

Regards, Jim.

$ cat segfault.py #!/usr/bin/python3

class CyclicObject: def init(self, key, register): self.key = key self.self = self self.register = register self.register[self.key] = None

def __del__(self):
    del self.register[self.key]

while True: register = { } objs = set([CyclicObject(i, register) for i in range(10000)])

while len(objs) > 0:
    objs.remove(next(iter(objs)))

    try:
        list(register.items())
    except RuntimeError as err:
        print(err)

$ python3 --version Python 3.5.2

$ gdb --args python3 segfault.py GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1

(gdb) r Starting program: /usr/bin/python3 segfault.py [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". dictionary changed size during iteration dictionary changed size during iteration dictionary changed size during iteration

Program received signal SIGSEGV, Segmentation fault. dictiter_iternextitem.lto_priv () at ../Objects/dictobject.c:3158 3158 ../Objects/dictobject.c: No such file or directory.

(gdb) bt #0 dictiter_iternextitem.lto_priv () at ../Objects/dictobject.c:3158 #1 0x000000000059f024 in listextend.lto_priv () at ../Objects/listobject.c:855 #2 0x000000000058fd1c in list_init.lto_priv () at ../Objects/listobject.c:2314 #3 0x000000000055d17c in type_call.lto_priv () at ../Objects/typeobject.c:905 #4 0x00000000005b7167 in PyObject_Call () at ../Objects/abstract.c:2165 #5 0x0000000000528d06 in do_call (nk=, na=, pp_stack=0x7fffffffd9b0, func=) at ../Python/ceval.c:4936 #6 call_function (oparg=, pp_stack=0x7fffffffd9b0) at ../Python/ceval.c:4732 #7 PyEval_EvalFrameEx () at ../Python/ceval.c:3236 #8 0x000000000052d2e3 in _PyEval_EvalCodeWithName () at ../Python/ceval.c:4018 #9 0x000000000052dfdf in PyEval_EvalCodeEx () at ../Python/ceval.c:4039 #10 PyEval_EvalCode (co=, globals=, locals=) at ../Python/ceval.c:777 #11 0x00000000005fd2c2 in run_mod () at ../Python/pythonrun.c:976 #12 0x00000000005ff76a in PyRun_FileExFlags () at ../Python/pythonrun.c:929 #13 0x00000000005ff95c in PyRun_SimpleFileExFlags () at ../Python/pythonrun.c:396 #14 0x000000000063e7d6 in run_file (p_cf=0x7fffffffdc20, filename=0xa73280 L"segfault.py", fp=0xad19a0) at ../Modules/main.c:318 #15 Py_Main () at ../Modules/main.c:768 #16 0x00000000004cfe41 in main () at ../Programs/python.c:65 #17 0x00007ffff7811830 in __libc_start_main (main=0x4cfd60

$ ./python --version Python 3.6.1

$ gdb --args ./python ./segfault.py GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1

(gdb) r Starting program: ./python ./segfault.py [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". dictionary changed size during iteration dictionary changed size during iteration dictionary changed size during iteration dictionary changed size during iteration dictionary changed size during iteration

Program received signal SIGSEGV, Segmentation fault. _PyObject_Alloc (ctx=0x0, elsize=28, nelem=1, use_calloc=0) at Objects/obmalloc.c:1258 1258 if ((pool->freeblock = *(block **)bp) != NULL) {

(gdb) bt #0 _PyObject_Alloc (ctx=0x0, elsize=28, nelem=1, use_calloc=0) at Objects/obmalloc.c:1258 #1 _PyObject_Malloc (ctx=0x0, nbytes=28) at Objects/obmalloc.c:1437 #2 0x0000000000490732 in _PyLong_New (size=1) at Objects/longobject.c:196 #3 PyLong_FromLong (ival=) at Objects/longobject.c:254 #4 0x0000000000542fda in _PyEval_EvalFrameDefault (f=, throwflag=) at Python/ceval.c:3060 #5 0x000000000053d671 in PyEval_EvalFrameEx (throwflag=0, f=0x7ffff7fa1648) at Python/ceval.c:718 #6 _PyFunction_FastCall (co=, args=, nargs=1, globals=globals@entry=0x7ffff7f452d0) at Python/ceval.c:4880 #7 0x000000000053e521 in fast_function (kwnames=0x0, nargs=, stack=, func=0x7ffff7eae510) at Python/ceval.c:4915 #8 call_function (pp_stack=pp_stack@entry=0x7fffffffd920, oparg=oparg@entry=1, kwnames=kwnames@entry=0x0) at Python/ceval.c:4819 #9 0x0000000000542c17 in _PyEval_EvalFrameDefault (f=, throwflag=) at Python/ceval.c:3284 #10 0x000000000053e015 in PyEval_EvalFrameEx (throwflag=0, f=0x93a368) at Python/ceval.c:718 #11 _PyEval_EvalCodeWithName (_co=_co@entry=0x7ffff7ec26f0, globals=globals@entry=0x7ffff7f5e078, locals=locals@entry=0x7ffff7ec26f0, args=args@entry=0x0, argcount=argcount@entry=0, kwnames=kwnames@entry=0x0, kwargs=0x8, kwcount=0, kwstep=2, defs=0x0, defcount=0, kwdefs=0x0, closure=0x0, name=0x0, qualname=0x0) at Python/ceval.c:4128 #12 0x000000000053ee43 in PyEval_EvalCodeEx (closure=0x0, kwdefs=0x0, defcount=0, defs=0x0, kwcount=0, kws=0x0, argcount=0, args=0x0, locals=locals@entry=0x7ffff7ec26f0, globals=globals@entry=0x7ffff7f5e078, _co=_co@entry=0x7ffff7ec26f0) at Python/ceval.c:4149 #13 PyEval_EvalCode (co=co@entry=0x7ffff7ec26f0, globals=globals@entry=0x7ffff7f452d0, locals=locals@entry=0x7ffff7f452d0) at Python/ceval.c:695 #14 0x000000000042740f in run_mod (arena=0x7ffff7f5e078, flags=0x7fffffffdc00, locals=0x7ffff7f452d0, globals=0x7ffff7f452d0, filename=0x7ffff7e844f8, mod=0x933c28) at Python/pythonrun.c:980 #15 PyRun_FileExFlags (fp=0x93a350, filename_str=, start=, globals=0x7ffff7f452d0, locals=0x7ffff7f452d0, closeit=1, flags=0x7fffffffdc00) at Python/pythonrun.c:933 #16 0x000000000042763c in PyRun_SimpleFileExFlags (fp=0x93a350, filename=, closeit=1, flags=0x7fffffffdc00) at Python/pythonrun.c:396 #17 0x000000000043b975 in run_file (p_cf=0x7fffffffdc00, filename=0x8f82a0 L"./segfault.py", fp=0x93a350) at Modules/main.c:338 #18 Py_Main (argc=argc@entry=2, argv=argv@entry=0x8f7010) at Modules/main.c:809 #19 0x000000000041dc52 in main (argc=2, argv=) at ./Programs/python.c:69