[BUG] Segfault in py::buffer::request on a ctypes array

[fritzr]

Issue description

py::buffer::request() causes a segmentation fault when attempting to get the py::buffer_info for a ctypes array in Python 2 because it has a NULL stride.

Reproducible example code

test_ctypes_buffer.cpp

#include <pybind11/pybind11.h>
namespace py = pybind11; 

PYBIND11_MODULE(test_ctypes_buffer, m) {
  m.def("get_ctypes_buffer_size", [](py::buffer buffer) {  
    py::buffer_info info = buffer.request();
    return info.shape[0];    
  });    
}   

test_ctypes_buffer.py

from test_ctypes_buffer import get_ctypes_buffer_size
import ctypes
assert get_ctypes_buffer_size((ctypes.c_char * 10)()) == 10

Segmentation fault:

$ g++ -dumpversion
7.4.0
$ g++ -g -O0 -Wall -shared -std=c++11 -fPIC `/usr/bin/python -m pybind11 --includes` test_ctypes_buffer.cpp -o test_ctypes_buffer.so
$ /usr/bin/python -V
Python 2.7.5
$ /usr/bin/python test_ctypes_buffer.py
Segmentation fault (core dumped)

Post-mortem analysis:

$ gdb /usr/bin/python core.*
[...]
(gdb) bt
[...]
#0  0x00007f5647fda7d0 in __memmove_ssse3_back ()  from /lib64/libc.so.6
#1  0x00007f564172673a in std::__copy_move<false, true, std::random_access_iterator_tag>::__copy_m<long> (__first=0x0, __last=0x8, __result=0x1023660)  at /opt/gcc-7.4.0/include/c++/7.4.0/bits/stl_algobase.h:368
[...]
#11 0x00007f5641715a40 in pybind11::detail::any_container<long>::any_container<long*, void> (this=0x7ffdd557a7e0, first=0x0, last=0x8)  at /home/fritzr/.local/lib/python2.7/site-packages/pybind11/include/pybind11/detail/common.h:811
#12 0x00007f564170b71a in pybind11::buffer_info::buffer_info (this=0x7ffdd557a8d0, view=0x10235f0, ownview=true)  at /home/fritzr/.local/lib/python2.7/site-packages/pybind11/include/pybind11/buffer_info.h:56
#13 0x00007f564170d25f in pybind11::buffer::request (this=0x7ffdd557a9a8, writable=false)  at /home/foreese/.local/lib/python2.7/site-packages/pybind11/include/pybind11/pytypes.h:1333
[...]
(gdb) up 12
#12 0x00007f564170b71a in pybind11::buffer_info::buffer_info (this=0x7ffdd557a8d0, view=0x10235f0, ownview=true) at /home/fritzr/.local/lib/python2.7/site-packages/pybind11/include/pybind11/buffer_info.h:56
56	            {view->shape, view->shape + view->ndim}, {view->strides, view->strides + view->ndim}, view->readonly) {
(gdb) list 54,+5
    54	    explicit buffer_info(Py_buffer *view, bool ownview = true)
    55	    : buffer_info(view->buf, view->itemsize, view->format, view->ndim,
==> 56	            {view->shape, view->shape + view->ndim}, {view->strides, view->strides + view->ndim}, view->readonly) {
    57	        this->view = view;
    58	        this->ownview = ownview;
    59	    }
(gdb) p *view
$1 = {
  buf = 0x7f5648fcc910, 
  obj = 0x7f5648fcc8c0, 
  len = 10, 
  itemsize = 1, 
  readonly = 0, 
  ndim = 1, 
  format = 0x1035b50 "(10)<c", 
  shape = 0x1035f60, 
  strides = 0x0, 
  suboffsets = 0x0, 
  smalltable =     {0,
    0}, 
  internal = 0x0
}
(gdb) p *view->shape
$2 = 10

We see that the ctypes array's buffer has NULL stride. Therefore the construction of buffer_info::strides{NULL, NULL + view->ndim} causes the segmentation fault.

[YannickJadoul]

Fixed in #2503