Instrumentation of c-ares fuzzers

[Dor1s]

The build is failing (https://oss-fuzz-build-logs.storage.googleapis.com/index.html) since ares_create_query_fuzzer has only 42 coverage edges... It might be true, but another fuzz target (c-ares_ares_parse_reply_fuzzer) has 697 of them. Need to take a closer look.

[Dor1s]

Looks legit. https://github.com/c-ares/c-ares/blob/master/test/ares-test-fuzz-name.c target is simply too small.

[kcc]

But: https://github.com/google/fuzzer-test-suite/tree/master/c-ares-CVE-2016-5180

[Dor1s]

Yeah :( Should we revisit sancov -print-coverage-pcs ./fuzzer | wc -l threshold (currently 90) or find another workaround for tiny but legit fuzz targets?

[kcc]

Maybe require a smaller number of PCs but at least two source files? (one for fuzz target, one for the library itself)

[morehouse]

@kcc @Dor1s Can this be closed? c-ares build has been green for a while, but do we still want to change our instrumentation check?

[Dor1s]

Let me check that and consider re-enabling #838.

[Dor1s]

Yes, we need to change our instrumentation check now. sancov -print-coverage-pcs ./fuzzer | wc -l doesn't seem to make sense anymore.

[Dor1s]

root@f19d4d77eabd:/out# objdump -t ares_create_query_fuzzer | egrep -c '__sancov'
21
root@f19d4d77eabd:/out# objdump -t ares_parse_reply_fuzzer | egrep -c '__sancov'
23