[libc] Added isValidState to CharacterConverter class to ensure a provided mbstate is valid (#145564)

Added isValidState to CharacterConverter class to ensure a provided
mbstate is valid

libc/src/__support/wchar/character_converter.cpp

@@ -25,6 +25,9 @@ constexpr size_t ENCODED_BITS_PER_UTF8 = 6;
 // Information not metadata (# of bits excluding the byte headers)
 constexpr uint32_t MASK_ENCODED_BITS =
     mask_trailing_ones<uint32_t, ENCODED_BITS_PER_UTF8>();
+// Maximum value for utf-32 for a utf-8 sequence of a given length
+constexpr char32_t MAX_VALUE_PER_UTF8_LEN[] = {0x7f, 0x7ff, 0xffff, 0x10ffff};
+constexpr int MAX_UTF8_LENGTH = 4;
 
 CharacterConverter::CharacterConverter(mbstate *mbstate) { state = mbstate; }
 
@@ -40,6 +43,17 @@ bool CharacterConverter::isFull() {
 
 bool CharacterConverter::isEmpty() { return state->bytes_stored == 0; }
 
+bool CharacterConverter::isValidState() {
+  if (state->total_bytes > MAX_UTF8_LENGTH)
+    return false;
+
+  const char32_t max_utf32_value =
+      state->total_bytes == 0 ? 0
+                              : MAX_VALUE_PER_UTF8_LEN[state->total_bytes - 1];
+  return state->bytes_stored <= state->total_bytes &&
+         state->partial <= max_utf32_value;
+}
+
 int CharacterConverter::push(char8_t utf8_byte) {
   uint8_t num_ones = static_cast<uint8_t>(cpp::countl_one(utf8_byte));
   // Checking the first byte if first push
@@ -90,9 +104,7 @@ int CharacterConverter::push(char32_t utf32) {
   state->partial = utf32;
 
   // determine number of utf-8 bytes needed to represent this utf32 value
-  constexpr char32_t MAX_VALUE_PER_UTF8_LEN[] = {0x7f, 0x7ff, 0xffff, 0x10ffff};
-  constexpr int NUM_RANGES = 4;
-  for (uint8_t i = 0; i < NUM_RANGES; i++) {
+  for (uint8_t i = 0; i < MAX_UTF8_LENGTH; i++) {
     if (state->partial <= MAX_VALUE_PER_UTF8_LEN[i]) {
       state->total_bytes = i + 1;
       state->bytes_stored = i + 1;

libc/src/__support/wchar/character_converter.h

@@ -28,6 +28,7 @@ public:
   void clear();
   bool isFull();
   bool isEmpty();
+  bool isValidState();
 
   int push(char8_t utf8_byte);
   int push(char32_t utf32);

libc/test/src/__support/wchar/utf32_to_8_test.cpp

@@ -186,3 +186,45 @@ TEST(LlvmLibcCharacterConverterUTF32To8Test, CantPushMidConversion) {
   int err = cr.push(utf32);
   ASSERT_EQ(err, -1);
 }
+
+TEST(LlvmLibcCharacterConverterUTF32To8Test, InvalidState) {
+  LIBC_NAMESPACE::internal::mbstate s1;
+  LIBC_NAMESPACE::internal::CharacterConverter c1(&s1);
+  ASSERT_TRUE(c1.isValidState());
+
+  LIBC_NAMESPACE::internal::mbstate s2{0, 2, 0};
+  LIBC_NAMESPACE::internal::CharacterConverter c2(&s2);
+  ASSERT_FALSE(c2.isValidState());
+
+  LIBC_NAMESPACE::internal::mbstate s3{0x7f, 1, 1};
+  LIBC_NAMESPACE::internal::CharacterConverter c3(&s3);
+  ASSERT_TRUE(c3.isValidState());
+  LIBC_NAMESPACE::internal::mbstate s4{0x80, 1, 1};
+  LIBC_NAMESPACE::internal::CharacterConverter c4(&s4);
+  ASSERT_FALSE(c4.isValidState());
+
+  LIBC_NAMESPACE::internal::mbstate s5{0x7ff, 1, 2};
+  LIBC_NAMESPACE::internal::CharacterConverter c5(&s5);
+  ASSERT_TRUE(c5.isValidState());
+  LIBC_NAMESPACE::internal::mbstate s6{0x800, 1, 2};
+  LIBC_NAMESPACE::internal::CharacterConverter c6(&s6);
+  ASSERT_FALSE(c6.isValidState());
+
+  LIBC_NAMESPACE::internal::mbstate s7{0xffff, 1, 3};
+  LIBC_NAMESPACE::internal::CharacterConverter c7(&s7);
+  ASSERT_TRUE(c7.isValidState());
+  LIBC_NAMESPACE::internal::mbstate s8{0x10000, 1, 3};
+  LIBC_NAMESPACE::internal::CharacterConverter c8(&s8);
+  ASSERT_FALSE(c8.isValidState());
+
+  LIBC_NAMESPACE::internal::mbstate s9{0x10ffff, 1, 4};
+  LIBC_NAMESPACE::internal::CharacterConverter c9(&s9);
+  ASSERT_TRUE(c9.isValidState());
+  LIBC_NAMESPACE::internal::mbstate s10{0x110000, 1, 2};
+  LIBC_NAMESPACE::internal::CharacterConverter c10(&s10);
+  ASSERT_FALSE(c10.isValidState());
+
+  LIBC_NAMESPACE::internal::mbstate s11{0, 0, 5};
+  LIBC_NAMESPACE::internal::CharacterConverter c11(&s11);
+  ASSERT_FALSE(c11.isValidState());
+}