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gh-127081: lock non-re-entrant *pwent calls #132748

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gh-127081: lock non-re-entrant *pwent calls #132748

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78c9cde
gh-127081: lock non-re-entrant *pwent calls
duaneg Apr 16, 2025
d30a139
Defer dec-refs on created entries until outside mutex to avoid possib…
duaneg Apr 20, 2025
6b78b29
Address reviewer feedback
duaneg Apr 26, 2025
33f7fea
Switch back to using functions available in the limited API.
duaneg Apr 26, 2025
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2025-04-21-00-58-04.gh-issue-127081.3DCl92.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix libc thread safety issues with :mod:`pwd` by locking access to
``getpwall``.
35 changes: 30 additions & 5 deletions Modules/pwdmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@

#include "Python.h"
#include "posixmodule.h"
#include "listobject.h" // PyList_Size/PyList_GetItem

#include <errno.h> // ERANGE
#include <pwd.h> // getpwuid()
Expand Down Expand Up @@ -301,18 +302,42 @@ pwd_getpwall_impl(PyObject *module)
struct passwd *p;
if ((d = PyList_New(0)) == NULL)
return NULL;

#ifdef Py_GIL_DISABLED
static PyMutex getpwall_mutex = {0};
PyMutex_Lock(&getpwall_mutex);
#endif
int failure = 0;
PyObject *orphan = NULL;
setpwent();
while ((p = getpwent()) != NULL) {
/* NOTE: Ref counts are not decremented here, as we cannot allow
* re-entrancy while holding the mutex. */
PyObject *v = mkpwent(module, p);
if (v == NULL || PyList_Append(d, v) != 0) {
Py_XDECREF(v);
Py_DECREF(d);
endpwent();
return NULL;
orphan = v;
failure = 1;
goto done;
}
Py_DECREF(v);
}

done:
endpwent();
#ifdef Py_GIL_DISABLED
PyMutex_Unlock(&getpwall_mutex);
#endif
/* Deferred decref on entries created above and added to the list */
Py_ssize_t n = PyList_Size(d);
for (Py_ssize_t i = 0; i < n; ++i) {
PyObject *entry = PyList_GetItem(d, i);
Py_DECREF(entry);
}
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Suggested change
Py_ssize_t n = PyList_Size(d);
for (Py_ssize_t i = 0; i < n; ++i) {
PyObject *entry = PyList_GetItem(d, i);
Py_DECREF(entry);
}
Py_ssize_t n = PyList_GET_SIZE(d);
while (--n >= 0) {
Py_DECREF(PyList_GET_ITEM(d, n));
}

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This change broke GIL builds, as they use the limited API only, which does not include those functions. Unfortunately I was using a free-threading build, so didn't notice initially. I'll switch back to using the ones available in the limited API, but keep the better loop construct.

if (failure) {
/* If there was a failure we might have created an entry but not added
* it: dec-ref that, if it exists, before clearing the list. */
Py_XDECREF(orphan);
Py_CLEAR(d);
}
return d;
}
#endif
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