Fix put_time() crash on invalid format specifier

stl/inc/xloctime

@@ -651,6 +651,21 @@ protected:
     __CLR_OR_THIS_CALL ~time_get_byname() noexcept override {}
 };
 
+// C23 7.29.3.5 "The strftime function"/3
+_INLINE_VAR constexpr char _Valid_strftime_specifiers[] = {'a', 'A', 'b', 'B', 'c', 'C', 'd', 'D', 'e', 'F', 'g', 'G',
+    'h', 'H', 'I', 'j', 'm', 'M', 'n', 'p', 'r', 'R', 'S', 't', 'T', 'u', 'U', 'V', 'w', 'W', 'x', 'X', 'y', 'Y', 'z',
+    'Z'};
+
+_NODISCARD constexpr bool _Is_valid_strftime_specifier(const char _Specifier) {
+    for (const auto& _Valid_specifier : _Valid_strftime_specifiers) {
+        if (_Specifier == _Valid_specifier) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 _EXPORT_STD extern "C++" template <class _Elem, class _OutIt = ostreambuf_iterator<_Elem, char_traits<_Elem>>>
 class time_put : public locale::facet { // facet for converting encoded times to text
 public:
@@ -687,7 +702,19 @@ public:
                     _Specifier = _Ctype_fac.narrow(*_Fmtfirst);
                 }
 
-                _Dest = do_put(_Dest, _Iosbase, _Fill, _Pt, _Specifier, _Modifier); // convert a single field
+                if (_Specifier == '%' && _Modifier == '\0') {
+                    // if the specifier is percent and no modifier is set, just append it
+                    *_Dest++ = _Percent;
+                } else if (!_Is_valid_strftime_specifier(_Specifier)) {
+                    // no valid specifier, directly copy as literal elements
+                    *_Dest++ = _Percent;
+                    if (_Modifier != '\0') {
+                        *_Dest++ = _Modifier;
+                    }
+                    *_Dest++ = _Specifier;
+                } else {
+                    _Dest = do_put(_Dest, _Iosbase, _Fill, _Pt, _Specifier, _Modifier); // convert a single field
+                }
             }
         }
 
@@ -811,7 +838,19 @@ public:
                     _Specifier = _Ctype_fac.narrow(*_Fmtfirst);
                 }
 
-                _Dest = do_put(_Dest, _Iosbase, _Fill, _Pt, _Specifier, _Modifier); // convert a single field
+                if (_Specifier == '%' && _Modifier == '\0') {
+                    // if the specifier is percent and no modifier is set, just append it
+                    *_Dest++ = _Percent;
+                } else if (!_Is_valid_strftime_specifier(_Specifier)) {
+                    // no valid specifier, directly copy as literal elements
+                    *_Dest++ = _Percent;
+                    if (_Modifier != '\0') {
+                        *_Dest++ = _Raw;
+                    }
+                    *_Dest++ = *_Fmtfirst;
+                } else {
+                    _Dest = do_put(_Dest, _Iosbase, _Fill, _Pt, _Specifier, _Modifier); // convert a single field
+                }
             }
         }
 
@@ -927,14 +966,15 @@ public:
                 *_Dest++ = _Fmtfirst[-1];
                 break;
             } else { // get specifier after %
-                char _Specifier = _Ctype_fac.narrow(*_Fmtfirst);
+                _Elem _Raw      = *_Fmtfirst;
+                char _Specifier = _Ctype_fac.narrow(_Raw);
                 char _Modifier  = '\0';
                 _Elem _Percent  = _Fmtfirst[-1];
 
                 if (_Specifier == 'E' || _Specifier == 'O' || _Specifier == 'Q' || _Specifier == '#') {
                     if (++_Fmtfirst == _Fmtlast) { // no specifier, copy %[E0Q#] as literal elements
                         *_Dest++ = _Percent;
-                        *_Dest++ = _Specifier;
+                        *_Dest++ = _Raw;
                         break;
                     }
 
@@ -943,7 +983,19 @@ public:
                     _Specifier = _Ctype_fac.narrow(*_Fmtfirst);
                 }
 
-                _Dest = do_put(_Dest, _Iosbase, _Fill, _Pt, _Specifier, _Modifier); // convert a single field
+                if (_Specifier == '%' && _Modifier == '\0') {
+                    // if the specifier is percent and no modifier is set, just append it
+                    *_Dest++ = _Percent;
+                } else if (!_Is_valid_strftime_specifier(_Specifier)) {
+                    // no valid specifier, directly copy as literal elements
+                    *_Dest++ = _Percent;
+                    if (_Modifier != '\0') {
+                        *_Dest++ = _Raw;
+                    }
+                    *_Dest++ = *_Fmtfirst;
+                } else {
+                    _Dest = do_put(_Dest, _Iosbase, _Fill, _Pt, _Specifier, _Modifier); // convert a single field
+                }
             }
         }
 

tests/std/tests/Dev11_0836436_get_time/test.cpp

@@ -110,6 +110,7 @@ void test_invalid_argument();
 void test_buffer_resizing();
 void test_gh_2618();
 void test_gh_2848();
+void test_gh_4820();
 
 int main() {
     assert(read_hour("12 AM") == 0);
@@ -157,6 +158,7 @@ int main() {
     test_buffer_resizing();
     test_gh_2618();
     test_gh_2848();
+    test_gh_4820();
 }
 
 typedef istreambuf_iterator<char> Iter;
@@ -792,16 +794,17 @@ void test_invalid_argument() {
     time_t t = time(nullptr);
     tm currentTime;
     localtime_s(&currentTime, &t);
+    currentTime.tm_hour = 25; // set invalid hour
 
     {
         wstringstream wss;
-        wss << put_time(&currentTime, L"%Y-%m-%d-%H-%M-%s");
+        wss << put_time(&currentTime, L"%Y-%m-%d-%H-%M");
         assert(wss.rdstate() == ios_base::badbit);
     }
 
     {
         stringstream ss;
-        ss << put_time(&currentTime, "%Y-%m-%d-%H-%M-%s");
+        ss << put_time(&currentTime, "%Y-%m-%d-%H-%M");
         assert(ss.rdstate() == ios_base::badbit);
     }
 #endif // _M_CEE_PURE
@@ -905,3 +908,39 @@ void test_gh_2848() {
         assert(err == (ios_base::eofbit | ios_base::failbit));
     }
 }
+
+void test_gh_4820() {
+    // GH-4820 <iomanip>: std::put_time should copy unknown conversion specifiers instead of crash
+    time_t t = time(nullptr);
+    tm currentTime;
+    localtime_s(&currentTime, &t);
+
+    // Case 1: Test various unknown conversion specifiers.
+    // Case 2: "%%" is a known escape sequence with a dedicated fast path.
+    // Case 3: "% " is percent followed by space, which is an unknown conversion specifier.
+    // Case 4: "%E%Z" is parsed as "%E%" followed by "Z", so it should be copied unchanged,
+    //         even though "%Z" by itself would be a known conversion specifier (time zone name).
+    //         (In case 1, "%E%J" is parsed the same way; the difference is that "%J" would be unknown.)
+    {
+        wstringstream wss;
+        wss << put_time(&currentTime, L"1:%Ei%!%E%J%P 2:%% 3:% 4:%E%Z");
+        assert(wss.rdstate() == ios_base::goodbit);
+        assert(wss.str() == L"1:%Ei%!%E%J%P 2:% 3:% 4:%E%Z");
+    }
+
+    {
+        stringstream ss;
+        ss << put_time(&currentTime, "1:%Ei%!%E%J%P 2:%% 3:% 4:%E%Z");
+        assert(ss.rdstate() == ios_base::goodbit);
+        assert(ss.str() == "1:%Ei%!%E%J%P 2:% 3:% 4:%E%Z");
+    }
+
+    // Also verify that wide characters aren't truncated.
+    // This tests a character appearing by itself, two as specifiers, and two as modified specifiers.
+    {
+        wstringstream wss;
+        wss << put_time(&currentTime, L"\x043a%\x043e%\x0448%E\x043a%O\x0430");
+        assert(wss.rdstate() == ios_base::goodbit);
+        assert(wss.str() == L"\x043a%\x043e%\x0448%E\x043a%O\x0430");
+    }
+}