libFuzzer support

[guidovranken]

Let's continue here.

This is what I have now: https://github.com/guidovranken/go-fuzz/tree/libfuzzer-mode

clang -fsanitize=fuzzer-no-link -c $GOPATH/src/github.com/dvyukov/go-fuzz/C/main_libFuzzer_extra_counters.c
go build github.com/dvyukov/go-fuzz/go-fuzz-build
./go-fuzz-build -libfuzzer <pkg>
clang++ -fsanitize=fuzzer main_libFuzzer_extra_counters.o <generated archive.a> -o fuzzer

I retained the original functionality in go-fuzz-build/main.go, but removed what wasn't needed in go-fuzz-deps/main.go for now. Maybe we can have a go-fuzz-deps-libFuzzer directory for when -libfuzzer is specified?

[dvyukov]

This is what I should be looking at, right?
https://github.com/guidovranken/go-fuzz/commit/5b859d76c166a50624386b575bdcfb074ac278c4

[dvyukov]

I've created a pull request for this for discussion purposes:
#217

[reijin90]

If you don't mind me asking, what is your rationale behind adding this? Do you expect different behaviour? Better performance? Future CGo compatibility?

[guidovranken]

If you don't mind me asking, what is your rationale behind adding this? Do you expect different behaviour? Better performance? Future CGo compatibility?

Being able to integrate into oss-fuzz which does not support the built-in gofuzz engine.

Also being able to do differential fuzzing against software written in other languages, like https://github.com/guidovranken/bignum-fuzzer (which incorporates Golang code but currently without instrumentation).

[josharian]

-libfuzzer support is in.