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bpo-46606: Reduce stack usage of getgroups and setgroups #31073

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Feb 22, 2022
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bpo-46606: Reduce stack usage of getgroups and setgroups #31073

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79 changes: 23 additions & 56 deletions Modules/posixmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -7574,21 +7574,15 @@ static PyObject *
os_getgroups_impl(PyObject *module)
/*[clinic end generated code: output=42b0c17758561b56 input=d3f109412e6a155c]*/
{
PyObject *result = NULL;
gid_t grouplist[MAX_GROUPS];

/* On MacOSX getgroups(2) can return more than MAX_GROUPS results
* This is a helper variable to store the intermediate result when
* that happens.
*
* To keep the code readable the OSX behaviour is unconditional,
* according to the POSIX spec this should be safe on all unix-y
* systems.
* See bpo-7900.
*/
gid_t* alt_grouplist = grouplist;
gid_t *grouplist = NULL;
int n;

#ifdef __APPLE__
/* Issue #17557: As of OS X 10.8, getgroups(2) no longer raises EINVAL if
* there are more groups than can fit in grouplist. Therefore, on OS X
* always first call getgroups with length 0 to get the actual number
Expand All @@ -7597,56 +7591,25 @@ os_getgroups_impl(PyObject *module)
n = getgroups(0, NULL);
if (n < 0) {
return posix_error();
} else if (n <= MAX_GROUPS) {
/* groups will fit in existing array */
alt_grouplist = grouplist;
} else {
alt_grouplist = PyMem_New(gid_t, n);
if (alt_grouplist == NULL) {
n++; // Avoid malloc(0)
grouplist = PyMem_New(gid_t, n+1);
if (grouplist == NULL) {
return PyErr_NoMemory();
}
}

n = getgroups(n, alt_grouplist);
n = getgroups(n, grouplist);
if (n == -1) {
if (alt_grouplist != grouplist) {
PyMem_Free(alt_grouplist);
}
PyMem_Free(grouplist);
return posix_error();
}
#else
n = getgroups(MAX_GROUPS, grouplist);
if (n < 0) {
if (errno == EINVAL) {
n = getgroups(0, NULL);
if (n == -1) {
return posix_error();
}
if (n == 0) {
/* Avoid malloc(0) */
alt_grouplist = grouplist;
} else {
alt_grouplist = PyMem_New(gid_t, n);
if (alt_grouplist == NULL) {
return PyErr_NoMemory();
}
n = getgroups(n, alt_grouplist);
if (n == -1) {
PyMem_Free(alt_grouplist);
return posix_error();
}
}
} else {
return posix_error();
}
}
#endif

result = PyList_New(n);
PyObject *result = PyList_New(n);
if (result != NULL) {
int i;
for (i = 0; i < n; ++i) {
PyObject *o = _PyLong_FromGid(alt_grouplist[i]);
PyObject *o = _PyLong_FromGid(grouplist[i]);
if (o == NULL) {
Py_DECREF(result);
result = NULL;
Expand All @@ -7656,9 +7619,7 @@ os_getgroups_impl(PyObject *module)
}
}

if (alt_grouplist != grouplist) {
PyMem_Free(alt_grouplist);
}
PyMem_Free(grouplist);

return result;
}
Expand Down Expand Up @@ -8163,42 +8124,48 @@ static PyObject *
os_setgroups(PyObject *module, PyObject *groups)
/*[clinic end generated code: output=3fcb32aad58c5ecd input=fa742ca3daf85a7e]*/
{
Py_ssize_t i, len;
gid_t grouplist[MAX_GROUPS];

if (!PySequence_Check(groups)) {
PyErr_SetString(PyExc_TypeError, "setgroups argument must be a sequence");
return NULL;
}
len = PySequence_Size(groups);
Py_ssize_t len = PySequence_Size(groups);
if (len < 0) {
return NULL;
}
if (len > MAX_GROUPS) {
PyErr_SetString(PyExc_ValueError, "too many groups");
return NULL;
}
for(i = 0; i < len; i++) {

gid_t *grouplist = PyMem_New(gid_t, len+1); // Avoid malloc(0)
for (Py_ssize_t i = 0; i < len; i++) {
PyObject *elem;
elem = PySequence_GetItem(groups, i);
if (!elem)
if (!elem) {
PyMem_Free(grouplist);
return NULL;
}
if (!PyLong_Check(elem)) {
PyErr_SetString(PyExc_TypeError,
"groups must be integers");
Py_DECREF(elem);
PyMem_Free(grouplist);
return NULL;
} else {
if (!_Py_Gid_Converter(elem, &grouplist[i])) {
Py_DECREF(elem);
PyMem_Free(grouplist);
return NULL;
}
}
Py_DECREF(elem);
}

if (setgroups(len, grouplist) < 0)
if (setgroups(len, grouplist) < 0) {
PyMem_Free(grouplist);
return posix_error();
}
PyMem_Free(grouplist);
Py_RETURN_NONE;
}
#endif /* HAVE_SETGROUPS */
Expand Down