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bpo-29753: fix merging packed bitfields in ctypes struct/union (GH-19850)

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From the commit message:

> When the structure is packed we should always expand when needed,
> otherwise we will add some padding between the fields. This patch makes
> sure we always merge bitfields together. It also changes the field merging
> algorithm so that it handles bitfields correctly.

Automerge-Triggered-By: GH:jaraco
This commit is contained in:
Filipe Laíns 2021-02-28 22:43:19 +00:00 • committed by GitHub
parent af5fa13ef6
commit 0d7ad9fb38
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GPG key ID: 4AEE18F83AFDEB23
3 changed files with 91 additions and 8 deletions
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35
Modules/_ctypes/cfield.c
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@ -71,6 +71,18 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index,
Py_DECREF(self);
return NULL;
}
#ifndef MS_WIN32
/* if we have a packed bitfield, calculate the minimum number of bytes we
need to fit it. otherwise use the specified size. */
if (pack && bitsize) {
size = (bitsize - 1) / 8 + 1;
} else
#endif
size = dict->size;
proto = desc;
if (bitsize /* this is a bitfield request */
&& *pfield_size /* we have a bitfield open */
#ifdef MS_WIN32
@ -87,7 +99,9 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index,
} else if (bitsize /* this is a bitfield request */
&& *pfield_size /* we have a bitfield open */
&& dict->size * 8 >= *pfield_size
&& (*pbitofs + bitsize) <= dict->size * 8) {
/* if this is a packed bitfield, always expand it.
otherwise calculate if we need to expand it. */
&& (((*pbitofs + bitsize) <= dict->size * 8) || pack)) {
/* expand bit field */
fieldtype = EXPAND_BITFIELD;
#endif
@ -95,7 +109,9 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index,
/* start new bitfield */
fieldtype = NEW_BITFIELD;
*pbitofs = 0;
*pfield_size = dict->size * 8;
/* use our calculated size (size) instead of type size (dict->size),
which can be different for packed bitfields */
*pfield_size = size * 8;
} else {
/* not a bit field */
fieldtype = NO_BITFIELD;
@ -103,9 +119,6 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index,
*pfield_size = 0;
}
size = dict->size;
proto = desc;
/* Field descriptors for 'c_char * n' are be scpecial cased to
return a Python string instead of an Array object instance...
*/
@ -170,10 +183,16 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index,
break;
case EXPAND_BITFIELD:
*poffset += dict->size - *pfield_size/8;
*psize += dict->size - *pfield_size/8;
/* increase the size if it is a packed bitfield.
EXPAND_BITFIELD should not be selected for non-packed fields if the
current size isn't already enough. */
if (pack)
size = (*pbitofs + bitsize - 1) / 8 + 1;
*pfield_size = dict->size * 8;
*poffset += size - *pfield_size/8;
*psize += size - *pfield_size/8;
*pfield_size = size * 8;
if (big_endian)
self->size = (bitsize << 16) + *pfield_size - *pbitofs - bitsize;