RFR: 8328874: Class::forName0 should validate the class name length early [v10]

[David Holmes]

On Thu, 28 Aug 2025 15:32:58 GMT, Guanqiang Han <ghan at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/Class.java line 4170:
>> 
>>> 4168:         // The check utfLen >= nameLen ensures we don't incorrectly return true in case of int overflow.
>>> 4169:         int utfLen = ModifiedUtf.utfLen(name, 0);
>>> 4170:         return utfLen <= JAVA_CLASSNAME_MAX_LEN && utfLen >= nameLen;
>> 
>> A typical overflow-conscious idiom is to subtract the unknown value from the known positive number and compare with 0.
>> Suggestion:
>> 
>>         int utfLen = ModifiedUtf.utfLen(name, 0);
>>         return JAVA_CLASSNAME_MAX_LEN - utfLen >= 0;
>
> @RogerRiggs Good catch! Fixed.

That doesn't seem right to me. If we massively overflow to get a value > -JAVA_CLASSNAME_MAX_LEN but < 0  then the subtraction becomes addition and we get a small positive value.

Really ModifiedUtf.utflen should be defined to return long so the caller can more easily deal with overflow. Giant strings are a PITA.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26802#discussion_r2309215932