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 440:

> 438:         // Could be first or fourth byte. If fourth
> 439:         // then 2 bytes before will have second byte pattern (0b1010xxxx)
> 440:         if ((index - 3) >= 0 && ((buffer[index - 2] & 0xA0) == 0xA0)) {

Suggestion:

        if ((index - 3) >= 0 && ((buffer[index - 2] & 0xF0) == 0xA0)) {

src/hotspot/share/utilities/utf8.cpp line 442:

> 440:         if ((index - 3) >= 0 && ((buffer[index - 2] & 0xA0) == 0xA0)) {
> 441:           // it was fourth byte so truncate 3 bytes earlier
> 442:           assert(buffer[index - 3] == 0xED, "malformed sequence");

This needs to be an if, not an assert: ec-a0-80 is a [legitimate 3-byte UTF-8](https://www.compart.com/en/unicode/U+C800)

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PR Review Comment: https://git.openjdk.org/jdk/pull/20345#discussion_r1692684932
PR Review Comment: https://git.openjdk.org/jdk/pull/20345#discussion_r1692684622