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

[Daniel Jeliński]

On Fri, 26 Jul 2024 04:03:10 GMT, David Holmes <dholmes 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 398:

> 396: // byte sequence.
> 397: static bool is_starting_byte(unsigned char b) {
> 398:   return b >= 0xC0 && b <= 0xEF;;

Do you plan to use this method only for modified UTF-8 or for standard Utf-8 as well? Standard UTF-8 also uses F0-F7 as starting bytes.

Also, remove the double semicolon.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20345#discussion_r1693919203