[fuzzer] Add basic support for emscripten.

[metzman]

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.

[metzman]

• fix comment

[metzman]

Demo https://jonathanmetzman.github.io/wasm-fuzzing-demo/brotli/decode_fuzzer.html

Note that with this CL one needs to open the console to see libFuzzer's output, in the demo it appears on the page.
I may not land that change because it hurts performance and requires bigger changes to libFuzzer than I want to deal with now.

[hctim]

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

287

Can we hoist this into the caller - having:

if (!LIBFUZZER_EMSCRIPTEN)
  StartRssThread(F, A);

... seems clearer to me than silently not doing the thing the function says it does some of the time.

[metzman]

• hoist as requested

[metzman]

• fmt

[metzman]

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

287

Done.
Do you like the way I did it now (with an #if macro?) or do you want the if to be code? (I guess the macro way might result in a complaint about unused function and is IMO uglier but clearer.

[hctim]

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

287

I believe this is the canonical way to guard features around macros, so LGTM. If we have any -Wunused-function problems then we can guard against it later.

[metzman]

• fix comment

compiler-rt/lib/fuzzer/FuzzerDefs.h

 #ifdef __linux__
 #define LIBFUZZER_APPLE 0
 #define LIBFUZZER_FUCHSIA 0
 #define LIBFUZZER_LINUX 1
 #define LIBFUZZER_NETBSD 0
 #define LIBFUZZER_FREEBSD 0
 #define LIBFUZZER_OPENBSD 0
 #define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
 #elif __APPLE__
 #define LIBFUZZER_APPLE 1
 #define LIBFUZZER_FUCHSIA 0
 #define LIBFUZZER_LINUX 0
 #define LIBFUZZER_NETBSD 0
 #define LIBFUZZER_FREEBSD 0
 #define LIBFUZZER_OPENBSD 0
 #define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
 #elif __NetBSD__
 #define LIBFUZZER_APPLE 0
 #define LIBFUZZER_FUCHSIA 0
 #define LIBFUZZER_LINUX 0
 #define LIBFUZZER_NETBSD 1
 #define LIBFUZZER_FREEBSD 0
 #define LIBFUZZER_OPENBSD 0
 #define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
 #elif __FreeBSD__
 #define LIBFUZZER_APPLE 0
 #define LIBFUZZER_FUCHSIA 0
 #define LIBFUZZER_LINUX 0
 #define LIBFUZZER_NETBSD 0
 #define LIBFUZZER_FREEBSD 1
 #define LIBFUZZER_OPENBSD 0
 #define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
 #elif __OpenBSD__
 #define LIBFUZZER_APPLE 0
 #define LIBFUZZER_FUCHSIA 0
 #define LIBFUZZER_LINUX 0
 #define LIBFUZZER_NETBSD 0
 #define LIBFUZZER_FREEBSD 0
 #define LIBFUZZER_OPENBSD 1
 #define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
 #elif _WIN32
 #define LIBFUZZER_APPLE 0
 #define LIBFUZZER_FUCHSIA 0
 #define LIBFUZZER_LINUX 0
 #define LIBFUZZER_NETBSD 0
 #define LIBFUZZER_FREEBSD 0
 #define LIBFUZZER_OPENBSD 0
 #define LIBFUZZER_WINDOWS 1
+#define LIBFUZZER_EMSCRIPTEN 0
 #elif __Fuchsia__
 #define LIBFUZZER_APPLE 0
 #define LIBFUZZER_FUCHSIA 1
 #define LIBFUZZER_LINUX 0
 #define LIBFUZZER_NETBSD 0
 #define LIBFUZZER_FREEBSD 0
 #define LIBFUZZER_OPENBSD 0
 #define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 0
+#elif __EMSCRIPTEN__
+#define LIBFUZZER_APPLE 0
+#define LIBFUZZER_FUCHSIA 0
+#define LIBFUZZER_LINUX 0
+#define LIBFUZZER_NETBSD 0
+#define LIBFUZZER_FREEBSD 0
+#define LIBFUZZER_OPENBSD 0
+#define LIBFUZZER_WINDOWS 0
+#define LIBFUZZER_EMSCRIPTEN 1
 #else
 #error "Support for your platform has not been implemented"
 #endif
 
 #if defined(_MSC_VER) && !defined(__clang__)
 // MSVC compiler is being used.
 #define LIBFUZZER_MSVC 1
 #else
 #define LIBFUZZER_MSVC 0
 #endif
 
 #ifndef __has_attribute
 #  define __has_attribute(x) 0
 #endif
 
 #define LIBFUZZER_POSIX                                                        \
   (LIBFUZZER_APPLE || LIBFUZZER_LINUX || LIBFUZZER_NETBSD ||                   \
-   LIBFUZZER_FREEBSD || LIBFUZZER_OPENBSD)
+   LIBFUZZER_FREEBSD || LIBFUZZER_OPENBSD || LIBFUZZER_EMSCRIPTEN)
 
 #ifdef __x86_64
 #  if __has_attribute(target)
 #    define ATTRIBUTE_TARGET_POPCNT __attribute__((target("popcnt")))
 #  else
 #    define ATTRIBUTE_TARGET_POPCNT
 #  endif
 #else

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

while (true) {
     SleepSeconds(1);
     size_t Peak = GetPeakRSSMb();
     if (Peak > RssLimitMb)
       F->RssLimitCallback();
   }
 }
 
 static void StartRssThread(Fuzzer *F, size_t RssLimitMb) {
-  if (!RssLimitMb) return;
+  if (!RssLimitMb)
+    return;
   std::thread T(RssThread, F, RssLimitMb);
   T.detach();
 }
 
 int RunOneTest(Fuzzer *F, const char *InputFilePath, size_t MaxLen) {
   Unit U = FileToVector(InputFilePath);
   if (MaxLen && MaxLen < U.size())
     U.resize(MaxLen);
   F->ExecuteCallback(U.data(), U.size());
   F->TryDetectingAMemoryLeak(U.data(), U.size(), true);
   return 0;
int FuzzerDriver(int *argc, char ***argv, UserCallback Callback) {
   auto *MD = new MutationDispatcher(Rand, Options);
   auto *Corpus = new InputCorpus(Options.OutputCorpus);
   auto *F = new Fuzzer(Callback, *Corpus, *MD, Options);
 
   for (auto &U: Dictionary)
     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;
   Options.HandleFpe = Flags.handle_fpe;
   Options.HandleIll = Flags.handle_ill;
   Options.HandleInt = Flags.handle_int;
   Options.HandleSegv = Flags.handle_segv;
   Options.HandleTerm = Flags.handle_term;

compiler-rt/lib/fuzzer/FuzzerExtFunctionsWeak.cpp

 //===- FuzzerExtFunctionsWeak.cpp - Interface to external functions -------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // Implementation for Linux. This relies on the linker's support for weak
 // symbols. We don't use this approach on Apple platforms because it requires
 // clients of LibFuzzer to pass ``-U _<symbol_name>`` to the linker to allow
 // weak symbols to be undefined. That is a complication we don't want to expose
 // to clients right now.
 //===----------------------------------------------------------------------===//
 #include "FuzzerDefs.h"
 #if LIBFUZZER_LINUX || LIBFUZZER_NETBSD || LIBFUZZER_FUCHSIA ||                \
-    LIBFUZZER_FREEBSD || LIBFUZZER_OPENBSD
+    LIBFUZZER_FREEBSD || LIBFUZZER_OPENBSD || LIBFUZZER_EMSCRIPTEN
 
 #include "FuzzerExtFunctions.h"
 #include "FuzzerIO.h"
 
 extern "C" {
 // Declare these symbols as weak to allow them to be optionally defined.
 #define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
   __attribute__((weak, visibility("default"))) RETURN_TYPE NAME FUNC_SIG

compiler-rt/lib/fuzzer/FuzzerExtraCounters.cpp

 //===- FuzzerExtraCounters.cpp - Extra coverage counters ------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // Extra coverage counters defined by user code.
 //===----------------------------------------------------------------------===//
 
 #include "FuzzerDefs.h"
 
 #if LIBFUZZER_LINUX || LIBFUZZER_NETBSD || LIBFUZZER_FREEBSD ||                \
-    LIBFUZZER_OPENBSD || LIBFUZZER_FUCHSIA
+    LIBFUZZER_OPENBSD || LIBFUZZER_FUCHSIA || LIBFUZZER_EMSCRIPTEN
 __attribute__((weak)) extern uint8_t __start___libfuzzer_extra_counters;
 __attribute__((weak)) extern uint8_t __stop___libfuzzer_extra_counters;
 
 namespace fuzzer {
 uint8_t *ExtraCountersBegin() { return &__start___libfuzzer_extra_counters; }
 uint8_t *ExtraCountersEnd() { return &__stop___libfuzzer_extra_counters; }
 ATTRIBUTE_NO_SANITIZE_ALL
 void ClearExtraCounters() {  // hand-written memset, don't asan-ify.

compiler-rt/lib/fuzzer/FuzzerUtilLinux.cpp

 //===- FuzzerUtilLinux.cpp - Misc utils for Linux. ------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // Misc utils for Linux.
 //===----------------------------------------------------------------------===//
 #include "FuzzerDefs.h"
 #if LIBFUZZER_LINUX || LIBFUZZER_NETBSD || LIBFUZZER_FREEBSD ||                \
-    LIBFUZZER_OPENBSD
+    LIBFUZZER_OPENBSD || LIBFUZZER_EMSCRIPTEN
 #include "FuzzerCommand.h"
 
 #include <stdlib.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
 
 

compiler-rt/lib/fuzzer/FuzzerUtilPosix.cpp

void SetTimer(int Seconds) {
   if (setitimer(ITIMER_REAL, &T, nullptr)) {
     Printf("libFuzzer: setitimer failed with %d\n", errno);
     exit(1);
   }
   SetSigaction(SIGALRM, AlarmHandler);
 }
 
 void SetSignalHandler(const FuzzingOptions& Options) {
-  if (Options.UnitTimeoutSec > 0)
+  // setitimer is not implemented in emscripten.
+  if (Options.UnitTimeoutSec > 0 && !LIBFUZZER_EMSCRIPTEN)
     SetTimer(Options.UnitTimeoutSec / 2 + 1);
   if (Options.HandleInt)
     SetSigaction(SIGINT, InterruptHandler);
   if (Options.HandleTerm)
     SetSigaction(SIGTERM, InterruptHandler);
   if (Options.HandleSegv)
     SetSigaction(SIGSEGV, SegvHandler);
   if (Options.HandleBus)

 
 unsigned long GetPid() { return (unsigned long)getpid(); }
 
 size_t GetPeakRSSMb() {
   struct rusage usage;
   if (getrusage(RUSAGE_SELF, &usage))
     return 0;
   if (LIBFUZZER_LINUX || LIBFUZZER_FREEBSD || LIBFUZZER_NETBSD ||
-      LIBFUZZER_OPENBSD) {
+      LIBFUZZER_OPENBSD || LIBFUZZER_EMSCRIPTEN) {
     // ru_maxrss is in KiB
     return usage.ru_maxrss >> 10;
   } else if (LIBFUZZER_APPLE) {
     // ru_maxrss is in bytes
     return usage.ru_maxrss >> 20;
   }
   assert(0 && "GetPeakRSSMb() is not implemented for your platform");
   return 0;