[fuzzer] Add basic support for emscripten.

Summary:
Add basic support for emscripten.

This enables libFuzzer to build (using build.sh) for emscripten and fuzz
a target compiled with
-fsanitize-coverage=inline-8bit-counters.

Basic fuzzing and bug finding work with this commit.
RSS limit and timeouts will not work because they depend on system
functions that are not implemented/widely supported in emscripten.

Reviewers: kcc, vitalybuka, hctim

Reviewed By: hctim

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D71285

1 files changed, 6 insertions, 1 deletions

diff --git a/compiler-rt/lib/fuzzer/FuzzerDriver.cpp b/compiler-rt/lib/fuzzer/FuzzerDriver.cpp
index 44c90655b93..dd3cab0ee8d 100644
--- a/compiler-rt/lib/fuzzer/FuzzerDriver.cpp
+++ b/compiler-rt/lib/fuzzer/FuzzerDriver.cpp
@@ -280,7 +280,8 @@ static void RssThread(Fuzzer *F, size_t RssLimitMb) {
 }
 
 static void StartRssThread(Fuzzer *F, size_t RssLimitMb) {
-  if (!RssLimitMb) return;
+  if (!RssLimitMb)
+    return;
   std::thread T(RssThread, F, RssLimitMb);
   T.detach();
 }
@@ -737,7 +738,11 @@ int FuzzerDriver(int *argc, char ***argv, UserCallback Callback) {
     if (U.size() <= Word::GetMaxSize())
       MD->AddWordToManualDictionary(Word(U.data(), U.size()));
 
+      // Threads are only supported by Chrome. Don't use them with emscripten
+      // for now.
+#if !LIBFUZZER_EMSCRIPTEN
   StartRssThread(F, Flags.rss_limit_mb);
+#endif // LIBFUZZER_EMSCRIPTEN
 
   Options.HandleAbrt = Flags.handle_abrt;
   Options.HandleBus = Flags.handle_bus;