RFR: 8325002: Exceptions::fthrow needs to ensure it truncates to a valid utf8 string

[David Holmes]

On Fri, 26 Jul 2024 05:19:32 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Exceptions::fthrow uses a 1024 byte buffer to format the incoming exception message string, but this may not be large enough, leading to truncation. However, we should ensure we truncate to a valid UTF8 sequence.
>> 
>> The process is explained in the code. Thanks to @RogerRiggs and @djelinski for their suggestions on how to tackle this.
>> 
>> Testing:
>>  - new gtest exercises the truncation code with the different possibilities for bad truncation
>>  - tiers 1-3 sanity testing
>> 
>> Thanks.
>
> src/hotspot/share/utilities/utf8.cpp line 407:
> 
>> 405: // To avoid that the caller can choose to check for validity first.
>> 406: // The incoming buffer is still expected to be NUL-terminated.
>> 407: void UTF8::truncate_to_legal_utf8(unsigned char* buffer, int length) {
> 
> Lets make buffer length size_t and avoid awkward casting

No this code uses `int` for length everywhere. Feel free to file a RFE to change it all.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20345#discussion_r1693949553