RFR (S) 8204055: SIGSEGV in java -XX:

David Holmes david.holmes at oracle.com
Thu May 31 07:11:29 UTC 2018

```On 31/05/2018 4:16 PM, Thomas Stüfe wrote:
> Looks good.

Thanks Thomas!

> It raises the philosophical question though whether two zero length
> strings are completely alike or unlike each other :)

Hmm maybe I should return 1 in that case ;-)

David

> ..Thomas
>
> On Thu, May 31, 2018 at 7:55 AM, David Holmes <david.holmes at oracle.com> wrote:
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8204055
>> webrev: http://cr.openjdk.java.net/~dholmes/8204055/webrev/
>>
>>
>> The SEGV was introduced with the fuzzy matching flag logic refactoring in
>> JDK-8198554. In:
>>
>> double StringUtils::similarity(const char* str1, size_t len1, const
>>                                 char* str2, size_t len2) {
>>    size_t total = len1 + len2;
>>
>>    size_t hit = 0;
>>    for (size_t i = 0; i < len1 - 1; i++) {
>>      for (size_t j = 0; j < len2 - 1; j++) {
>>        if ((str1[i] == str2[j]) && (str1[i+1] == str2[j+1])) {
>>          ++hit;
>>          break;
>>        }
>>      }
>>    }
>>
>> If len2 is zero (which it is in this case) we have passed it as an unsigned
>> size_t, so len2-1 gives a massive positive value and so we enter the loop
>> and try to access str2[n] for some n>0 and we get a SEGV.
>>
>> The original code had:
>>
>> - for (int j = 0; j < (int) len2 -1; ++j) {
>>
>> so the huge positive value reverted to a small negative value and we don't
>> enter the loop.
>>
>> The fix applied is to check explicitly for lengths of zero.
>>
>> Added missing testcases to:
>>
>> test/hotspot/gtest/logging/test_logConfiguration.cpp
>> test/hotspot/jtreg/runtime/CommandLine/UnrecognizedVMOption.java
>>
>> verified they both crash before the fix.
>>
>> Testing (in progress): tier1,2,3 per mach5 CI
>>
>> Thanks,
>> David
```