RFR: 8325002: Exceptions::fthrow needs to ensure it truncates to a valid utf8 string [v2]

[Daniel Jeliński]

On Mon, 29 Jul 2024 09:54:09 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.
>
> David Holmes has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - Fix logic for 4th byte of 6.
>  - Fix logic error and typo
>  - Ensure the buffer is > 5 bytes

src/hotspot/share/utilities/exceptions.cpp line 275:

> 273:   // we may also have a truncated UTF-8 sequence. In such cases we need to fix the buffer so the UTF-8
> 274:   // sequence is valid.
> 275:   if ((ret == -1 || ret >= max_msg_size) && strlen(msg) > 0) {

Do we need to check if `strlen(msg) == max_msg_size - 1`? If strlen is shorter, the bytes between the null terminator and max_msg_size are undefined, which might trigger an assertion while truncating.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20345#discussion_r1695241148