AK: Ensure short String instances are valid UTF-8

We are currently only validating long strings.

AK/String.cpp

@@ -11,7 +11,6 @@
 #include <AK/MemMem.h>
 #include <AK/Stream.h>
 #include <AK/String.h>
-#include <AK/Utf8View.h>
 #include <AK/Vector.h>
 #include <stdlib.h>
 
@@ -132,10 +131,6 @@ ErrorOr<NonnullRefPtr<StringData>> StringData::from_utf8(char const* utf8_data,
     // Strings of MAX_SHORT_STRING_BYTE_COUNT bytes or less should be handled by the String short string optimization.
     VERIFY(byte_count > String::MAX_SHORT_STRING_BYTE_COUNT);
 
-    Utf8View view(StringView(utf8_data, byte_count));
-    if (!view.validate())
-        return Error::from_string_literal("StringData::from_utf8: Input was not valid UTF-8");
-
     VERIFY(utf8_data);
     u8* buffer = nullptr;
     auto new_string_data = TRY(create_uninitialized(byte_count, buffer));
@@ -143,6 +138,16 @@ ErrorOr<NonnullRefPtr<StringData>> StringData::from_utf8(char const* utf8_data,
     return new_string_data;
 }
 
+static ErrorOr<void> read_stream_into_buffer(Stream& stream, Bytes buffer)
+{
+    TRY(stream.read_entire_buffer(buffer));
+
+    if (!Utf8View { StringView { buffer } }.validate())
+        return Error::from_string_literal("String::from_stream: Input was not valid UTF-8");
+
+    return {};
+}
+
 ErrorOr<NonnullRefPtr<StringData>> StringData::from_stream(Stream& stream, size_t byte_count)
 {
     // Strings of MAX_SHORT_STRING_BYTE_COUNT bytes or less should be handled by the String short string optimization.
@@ -150,12 +155,7 @@ ErrorOr<NonnullRefPtr<StringData>> StringData::from_stream(Stream& stream, size_
 
     u8* buffer = nullptr;
     auto new_string_data = TRY(create_uninitialized(byte_count, buffer));
-    Bytes new_string_bytes = { buffer, byte_count };
+    TRY(read_stream_into_buffer(stream, { buffer, byte_count }));
-    TRY(stream.read_entire_buffer(new_string_bytes));
-
-    Utf8View view(StringView { new_string_bytes });
-    if (!view.validate())
-        return Error::from_string_literal("StringData::from_stream: Input was not valid UTF-8");
 
     return new_string_data;
 }
@@ -230,6 +230,9 @@ void String::destroy_string()
 
 ErrorOr<String> String::from_utf8(StringView view)
 {
+    if (!Utf8View { view }.validate())
+        return Error::from_string_literal("String::from_utf8: Input was not valid UTF-8");
+
     if (view.length() <= MAX_SHORT_STRING_BYTE_COUNT) {
         ShortString short_string;
         if (!view.is_empty())
@@ -246,7 +249,7 @@ ErrorOr<String> String::from_stream(Stream& stream, size_t byte_count)
     if (byte_count <= MAX_SHORT_STRING_BYTE_COUNT) {
         ShortString short_string;
         if (byte_count > 0)
-            TRY(stream.read_entire_buffer({ short_string.storage, byte_count }));
+            TRY(Detail::read_stream_into_buffer(stream, { short_string.storage, byte_count }));
         short_string.byte_count_and_short_string_flag = (byte_count << 1) | SHORT_STRING_FLAG;
         return String { short_string };
     }
@@ -587,9 +590,6 @@ DeprecatedString String::to_deprecated_string() const
 
 ErrorOr<String> String::from_deprecated_string(DeprecatedString const& deprecated_string)
 {
-    Utf8View view(deprecated_string);
-    if (!view.validate())
-        return Error::from_string_literal("String::from_deprecated_string: Input was not valid UTF-8");
     return String::from_utf8(deprecated_string.view());
 }
 

AK/String.h

@@ -20,6 +20,7 @@
 #include <AK/Traits.h>
 #include <AK/Types.h>
 #include <AK/UnicodeUtils.h>
+#include <AK/Utf8View.h>
 #include <AK/Vector.h>
 
 namespace AK {
@@ -72,6 +73,7 @@ public:
     static AK_SHORT_STRING_CONSTEVAL String from_utf8_short_string(StringView string)
     {
         VERIFY(string.length() <= MAX_SHORT_STRING_BYTE_COUNT);
+        VERIFY(Utf8View { string }.validate());
 
         ShortString short_string;
         for (size_t i = 0; i < string.length(); ++i)

Tests/AK/TestString.cpp

@@ -140,6 +140,26 @@ TEST_CASE(long_streams)
     }
 }
 
+TEST_CASE(invalid_utf8)
+{
+    auto string1 = String::from_utf8("long string \xf4\x8f\xbf\xc0"sv); // U+110000
+    EXPECT(string1.is_error());
+    EXPECT(string1.error().string_literal().contains("Input was not valid UTF-8"sv));
+
+    auto string2 = String::from_utf8("\xf4\xa1\xb0\xbd"sv); // U+121C3D
+    EXPECT(string2.is_error());
+    EXPECT(string2.error().string_literal().contains("Input was not valid UTF-8"sv));
+
+    AllocatingMemoryStream stream;
+    MUST(stream.write_value<u8>(0xf4));
+    MUST(stream.write_value<u8>(0xa1));
+    MUST(stream.write_value<u8>(0xb0));
+    MUST(stream.write_value<u8>(0xbd));
+    auto string3 = String::from_stream(stream, stream.used_buffer_size());
+    EXPECT_EQ(string3.is_error(), true);
+    EXPECT(string3.error().string_literal().contains("Input was not valid UTF-8"sv));
+}
+
 TEST_CASE(from_code_points)
 {
     for (u32 code_point = 0; code_point < 0x80; ++code_point) {