diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-12-30-15-36-23.bpo-35214.GWDQcv.rst b/Misc/NEWS.d/next/Core and Builtins/2018-12-30-15-36-23.bpo-35214.GWDQcv.rst
index 62dee0e3700..fa61f78a932 100644
--- a/Misc/NEWS.d/next/Core and Builtins/2018-12-30-15-36-23.bpo-35214.GWDQcv.rst	
+++ b/Misc/NEWS.d/next/Core and Builtins/2018-12-30-15-36-23.bpo-35214.GWDQcv.rst	
@@ -1,2 +1,2 @@
 clang Memory Sanitizer build instrumentation was added to work around false
-positives from socket, time, test_io, and test_faulthandler.
+positives from posix, socket, time, test_io, and test_faulthandler.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 909b06e5a9d..f58572b2ceb 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -393,6 +393,10 @@ static int win32_can_symlink = 0;
 #define HAVE_STRUCT_STAT_ST_FSTYPE 1
 #endif
 
+#ifdef _Py_MEMORY_SANITIZER
+# include <sanitizer/msan_interface.h>
+#endif
+
 #ifdef HAVE_FORK
 static void
 run_at_forkers(PyObject *lst, int reverse)
@@ -5689,6 +5693,9 @@ os_sched_rr_get_interval_impl(PyObject *module, pid_t pid)
         posix_error();
         return -1.0;
     }
+#ifdef _Py_MEMORY_SANITIZER
+    __msan_unpoison(&interval, sizeof(interval));
+#endif
     return (double)interval.tv_sec + 1e-9*interval.tv_nsec;
 }
 #endif /* HAVE_SCHED_RR_GET_INTERVAL */
@@ -6155,6 +6162,12 @@ posix_getgrouplist(PyObject *self, PyObject *args)
         return posix_error();
     }
 
+#ifdef _Py_MEMORY_SANITIZER
+    /* Clang memory sanitizer libc intercepts don't know getgrouplist. */
+    __msan_unpoison(&ngroups, sizeof(ngroups));
+    __msan_unpoison(groups, ngroups*sizeof(*groups));
+#endif
+
     list = PyList_New(ngroups);
     if (list == NULL) {
         PyMem_Del(groups);