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

[David Holmes]

On Fri, 26 Jul 2024 05:23:28 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/exceptions.cpp line 276:
> 
>> 274:   // sequence is valid.
>> 275:   if ((ret == -1 || ret >= max_msg_size) && strlen(msg) > 0) {
>> 276:     assert(msg[max_msg_size - 1] == '\0', "should be null terminated");
> 
> Would this always be true? For a formatting error, too?
> Maybe just to be sure, instead of asserting set the last byte to zero.

vsnprintf is supposed to guarantee it, and os::vsnprint does IIRC, so this is just a sanity check.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20345#discussion_r1693624938