Do not link in libFuzzer with -fsanitize=fuzzer when producing a shared object

[george.karpenkov]

Summary

Previous behavior was incorrect and resulted in multiple copies of libfuzzer being loaded when fuzzing shared objects.

lib/Driver/ToolChains/CommonArgs.cpp

 bool tools::addSanitizerRuntimes(const ToolChain &TC, const ArgList &Args,
                                  ArgStringList &CmdArgs) {
   SmallVector<StringRef, 4> SharedRuntimes, StaticRuntimes,
       NonWholeStaticRuntimes, HelperStaticRuntimes, RequiredSymbols;
   collectSanitizerRuntimes(TC, Args, SharedRuntimes, StaticRuntimes,
                            NonWholeStaticRuntimes, HelperStaticRuntimes,
                            RequiredSymbols);
   // Inject libfuzzer dependencies.
-  if (TC.getSanitizerArgs().needsFuzzer()) {
+  if (TC.getSanitizerArgs().needsFuzzer()
+      && !Args.hasArg(options::OPT_shared)) {
     addLibFuzzerRuntime(TC, Args, CmdArgs);
   }
 
   for (auto RT : SharedRuntimes)
     addSanitizerRuntime(TC, Args, CmdArgs, RT, true, false);
   for (auto RT : HelperStaticRuntimes)
     addSanitizerRuntime(TC, Args, CmdArgs, RT, false, true);
   bool AddExportDynamic = false;

lib/Driver/ToolChains/Darwin.cpp

   if (Sanitize.needsAsanRt())
     AddLinkSanitizerLibArgs(Args, CmdArgs, "asan");
   if (Sanitize.needsLsanRt())
     AddLinkSanitizerLibArgs(Args, CmdArgs, "lsan");
   if (Sanitize.needsUbsanRt())
     AddLinkSanitizerLibArgs(Args, CmdArgs, "ubsan");
   if (Sanitize.needsTsanRt())
     AddLinkSanitizerLibArgs(Args, CmdArgs, "tsan");
-  if (Sanitize.needsFuzzer())
+  if (Sanitize.needsFuzzer() && !Args.hasArg(options::OPT_dynamiclib))
     AddFuzzerLinkArgs(Args, CmdArgs);
   if (Sanitize.needsStatsRt()) {
     StringRef OS = isTargetMacOS() ? "osx" : "iossim";
     AddLinkRuntimeLib(Args, CmdArgs,
                       (Twine("libclang_rt.stats_client_") + OS + ".a").str(),
                       /*AlwaysLink=*/true);
     AddLinkSanitizerLibArgs(Args, CmdArgs, "stats");
   }

test/Driver/fuzzer.c

 // RUN: %clang -fsanitize=fuzzer -target i386-unknown-linux -stdlib=platform %s -### 2>&1 | FileCheck --check-prefixes=CHECK-LIBCXX-LINUX %s
 //
 // CHECK-LIBCXX-LINUX: -lstdc++
 
 // RUN: %clang -target x86_64-apple-darwin14 -fsanitize=fuzzer %s -### 2>&1 | FileCheck --check-prefixes=CHECK-LIBCXX-DARWIN %s
 //
 // CHECK-LIBCXX-DARWIN: -lc++
 
+// Check that we don't link in libFuzzer.a when producing a shared object.
+// RUN: %clang -fsanitize=fuzzer %s -shared -o %t.so -### 2>&1 | FileCheck --check-prefixes=CHECK-NOLIB-SO %s
+// CHECK-NOLIB-SO-NOT: libLLVMFuzzer.a
+
 int LLVMFuzzerTestOneInput(const char *Data, long Size) {
   return 0;
 }