MemorySanitizer build fixes.

These teach MSan about syscalls, asm behavior, or otherwise address
clang memory sanitizer flagged issues.

Modules/_ctypes/callproc.c

@@ -75,6 +75,10 @@
 #include <alloca.h>
 #endif
 
+#ifdef MEMORY_SANITIZER
+#include <sanitizer/msan_interface.h>
+#endif
+
 #if defined(_DEBUG) || defined(__MINGW32__)
 /* Don't use structured exception handling on Windows if this is defined.
    MingW, AFAIK, doesn't support it.
@@ -1125,6 +1129,13 @@ PyObject *_ctypes_callproc(PPROC pProc,
     rtype = _ctypes_get_ffi_type(restype);
     resbuf = alloca(max(rtype->size, sizeof(ffi_arg)));
 
+#ifdef MEMORY_SANITIZER
+    /* ffi_call actually initializes resbuf, but from asm, which
+     * MemorySanitizer can't detect. Avoid false positives from MSan. */
+    if (resbuf != NULL) {
+        __msan_unpoison(resbuf, max(rtype->size, sizeof(ffi_arg)));
+    }
+#endif
     avalues = (void **)alloca(sizeof(void *) * argcount);
     atypes = (ffi_type **)alloca(sizeof(ffi_type *) * argcount);
     if (!resbuf || !avalues || !atypes) {

Modules/_posixsubprocess.c

@@ -21,6 +21,10 @@
 #include <dirent.h>
 #endif
 
+#ifdef MEMORY_SANITIZER
+# include <sanitizer/msan_interface.h>
+#endif
+
 #if defined(__ANDROID__) && __ANDROID_API__ < 21 && !defined(SYS_getdents64)
 # include <sys/linux-syscalls.h>
 # define SYS_getdents64  __NR_getdents64
@@ -287,6 +291,9 @@ _close_open_fds_safe(int start_fd, PyObject* py_fds_to_keep)
                                 sizeof(buffer))) > 0) {
             struct linux_dirent64 *entry;
             int offset;
+#ifdef MEMORY_SANITIZER
+            __msan_unpoison(buffer, bytes);
+#endif
             for (offset = 0; offset < bytes; offset += entry->d_reclen) {
                 int fd;
                 entry = (struct linux_dirent64 *)(buffer + offset);

Modules/faulthandler.c

@@ -1370,7 +1370,7 @@ void _PyFaulthandler_Fini(void)
 #ifdef HAVE_SIGALTSTACK
     if (stack.ss_sp != NULL) {
         /* Fetch the current alt stack */
-        stack_t current_stack;
+        stack_t current_stack = {};
         if (sigaltstack(NULL, &current_stack) == 0) {
             if (current_stack.ss_sp == stack.ss_sp) {
                 /* The current alt stack is the one that we installed.

Python/bootstrap_hash.c

@@ -20,6 +20,10 @@
 #  endif
 #endif
 
+#ifdef MEMORY_SANITIZER
+#  include <sanitizer/msan_interface.h>
+#endif
+
 #ifdef Py_DEBUG
 int _Py_HashSecret_Initialized = 0;
 #else
@@ -143,6 +147,11 @@ py_getrandom(void *buffer, Py_ssize_t size, int blocking, int raise)
         else {
             n = syscall(SYS_getrandom, dest, n, flags);
         }
+#  ifdef MEMORY_SANITIZER
+        if (n > 0) {
+             __msan_unpoison(dest, n);
+        }
+#  endif
 #endif
 
         if (n < 0) {

Python/pymath.c

@@ -17,7 +17,9 @@ double _Py_force_double(double x)
 
 /* inline assembly for getting and setting the 387 FPU control word on
    gcc/x86 */
-
+#ifdef MEMORY_SANITIZER
+__attribute__((no_sanitize_memory))
+#endif
 unsigned short _Py_get_387controlword(void) {
     unsigned short cw;
     __asm__ __volatile__ ("fnstcw %0" : "=m" (cw));