[libFuzzer][Windows]Enable building fuzz targets on Windows.

Bug: 888363
Change-Id: Idfb7c644fa3294ad11327609b4b39eacfd3f5dd2
Reviewed-on: https://chromium-review.googlesource.com/1238917Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Jonathan Metzman <metzman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594525}

BUILD.gn

@@ -690,8 +690,8 @@ group("gn_all") {
     }
   }
 
-  if ((is_linux && !is_chromeos && !is_chromecast) || (is_win && use_drfuzz) ||
-      (use_libfuzzer && is_mac)) {
+  if ((is_linux && !is_chromeos && !is_chromecast) ||
+      (is_win && (use_drfuzz || use_libfuzzer)) || (use_libfuzzer && is_mac)) {
     deps += [
       "//testing/libfuzzer/fuzzers",
       "//testing/libfuzzer/tests:libfuzzer_tests",

build/config/sanitizers/BUILD.gn

@@ -227,29 +227,38 @@ config("default_sanitizer_ldflags") {
         }
       }
     }
-  } else if (is_win && is_asan) {
+  } else if (is_win) {
     # Windows directly calls link.exe instead of the compiler driver when
-    # linking.  Hence, pass the runtime libraries instead of -fsanitize=address.
-    # In the static-library build, libraries are different for executables
-    # and dlls, see link_executable and link_shared_library below.
-    # This here handles only the component build.
-    if (target_cpu == "x64") {
-      # Windows 64-bit.
-      if (is_component_build) {
+    # linking.  Hence, pass the runtime libraries instead of -fsanitize=address
+    # or -fsanitize=fuzzer.
+    if (is_asan && is_component_build) {
+      # In the static-library build, ASan libraries are different for
+      # executables and dlls, see link_executable and link_shared_library below.
+      # This here handles only the component build.
+      if (target_cpu == "x64") {
+        # Windows 64-bit.
         libs = [
           "clang_rt.asan_dynamic-x86_64.lib",
           "clang_rt.asan_dynamic_runtime_thunk-x86_64.lib",
         ]
-      }
-    } else {
-      assert(target_cpu == "x86", "WinASan unsupported architecture")
-      if (is_component_build) {
+      } else {
+        assert(target_cpu == "x86", "WinASan unsupported architecture")
         libs = [
           "clang_rt.asan_dynamic-i386.lib",
           "clang_rt.asan_dynamic_runtime_thunk-i386.lib",
         ]
       }
     }
+    if (use_libfuzzer) {
+      assert(target_cpu == "x64", "LibFuzzer unsupported architecture")
+      assert(!is_component_build,
+             "LibFuzzer only supports non-component builds on Windows")
+
+      # Incremental linking causes padding that messes up SanitizerCoverage.
+      # Don't do it.
+      ldflags = [ "/INCREMENTAL:NO" ]
+      libs = [ "clang_rt.fuzzer_no_main-x86_64.lib" ]
+    }
   }
 }