[Libguestfs] [libnbd PATCH 3/2] python: Slice pread_structured buffer from original

[Eric Blake]

On Tue, May 31, 2022 at 10:49:03AM -0500, Eric Blake wrote:
> This patch fixes the corner-case regression introduced by the previous
> patch where the pread_structured callback buffer lifetime ends as soon
> as the callback (that is, where accessing a stashed callback parameter
> can result in ValueError instead of modifying a harmless copy).  With
> careful effort, we can create a memoryview of the Python object that
> we read into, then slice that memoryview as part of the callback; now
> the slice will be valid as long as the original object is also valid.
> 

> | @@ -76,8 +77,24 @@ chunk_wrapper (void *user_data, const vo
> |    PyObject *py_subbuf = NULL;
> |    PyObject *py_error = NULL;
> |
> | -  /* Cast away const */
> | -  py_subbuf = PyMemoryView_FromMemory ((char *) subbuf, count, PyBUF_READ);
> | +  if (data->buf) {
> | +    PyObject *start, *end, *slice;
> | +    assert (PyMemoryView_Check (data->buf));
> | +    ptrdiff_t offs = subbuf - PyMemoryView_GET_BUFFER (data->buf)->buf;
> | +    start = PyLong_FromLong (offs);
> | +    if (!start) { PyErr_PrintEx (0); goto out; }
> | +    end = PyLong_FromLong (offs + count);
> | +    if (!end) { Py_DECREF (start); PyErr_PrintEx (0); goto out; }
> | +    slice = PySlice_New (start, end, NULL);
> | +    Py_DECREF (start);
> | +    Py_DECREF (end);
> | +    if (!slice) { PyErr_PrintEx (0); goto out; }
> | +    py_subbuf = PyObject_GetItem (data->buf, slice);

Missing a Py_DECREF (slice) here.  Easy enough to add...

> +++ b/generator/Python.ml
> @@ -187,8 +187,24 @@ let
>         pr "    PyList_SET_ITEM (py_%s, i_%s, py_e_%s);\n" n n n;
>         pr "  }\n"
>      | CBBytesIn (n, len) ->
> -       pr "  /* Cast away const */\n";
> -       pr "  py_%s = PyMemoryView_FromMemory ((char *) %s, %s, PyBUF_READ);\n" n n len;
> +       pr "  if (data->buf) {\n";
> +       pr "    PyObject *start, *end, *slice;\n";
> +       pr "    assert (PyMemoryView_Check (data->buf));\n";
> +       pr "    ptrdiff_t offs = %s - PyMemoryView_GET_BUFFER (data->buf)->buf;\n" n;
> +       pr "    start = PyLong_FromLong (offs);\n";
> +       pr "    if (!start) { PyErr_PrintEx (0); goto out; }\n";
> +       pr "    end = PyLong_FromLong (offs + %s);\n" len;
> +       pr "    if (!end) { Py_DECREF (start); PyErr_PrintEx (0); goto out; }\n";
> +       pr "    slice = PySlice_New (start, end, NULL);\n";
> +       pr "    Py_DECREF (start);\n";
> +       pr "    Py_DECREF (end);\n";
> +       pr "    if (!slice) { PyErr_PrintEx (0); goto out; }\n";
> +       pr "    py_%s = PyObject_GetItem (data->buf, slice);\n" n;

...here

And I really wish python didn't make it so hard to grab a slice of
another object using C code.  Having to create 3 temporary PyObjects
instead of having a utility C function that takes normal integers was
annoying.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org