[GWP-ASan] enable/disable and fork support.

[eugenis]

Summary

• Implement enable() and disable() in GWP-ASan.
• Setup atfork handler.
• Improve test harness sanity and re-enable GWP-ASan in Scudo.

Scudo_standalone disables embedded GWP-ASan as necessary around fork().
Standalone GWP-ASan sets the atfork handler in init() if asked to. This
requires a working malloc(), therefore GWP-ASan initialization in Scudo
is delayed to the post-init callback.

Test harness changes are about setting up a single global instance of
the GWP-ASan allocator so that pthread_atfork() does not create
dangling pointers.

Test case shamelessly stolen from D72470.

[eugenis]

I've merged two earlier changes (gwp-asan re-enablement and atfork/enable/disable support) in one because they are interconnected.

[hctim]

compiler-rt/lib/gwp_asan/guarded_pool_allocator.h

112

// AdjustedSampleRatePlusOne is designed to intentionally underflow. This class must be valid when zero-initialised, and we wish to sample as infrequently as possible when this is the case, hence we underflow to UINT32_MAX.

121

Now that GuardedPagePool is zero-initialised, the fast path has an additional branch. Can be fixed via:
return P < GuardedPagePoolEnd && GuardedPagePool <= P;

compiler-rt/lib/gwp_asan/platform_specific/guarded_pool_allocator_posix.cpp

95

Why does this need to disable the singleton? Why not just disable this class (which should be equivalent).

compiler-rt/lib/gwp_asan/tests/enable_disable.cpp

45

Nit: prefer c++-style casting

46

What do mean by "initialize" here? GPA should already be inited by the parent.

If you're trying to init the sample counters to some nonzero value here, calling shouldSample() is the only way, not allocate().

compiler-rt/lib/gwp_asan/tests/harness.cpp

7

OnlyOnce?

compiler-rt/lib/gwp_asan/tests/harness.h

28

Why a single global instance?

[eugenis]

address comments

compiler-rt/lib/gwp_asan/guarded_pool_allocator.h

121

Sure.

compiler-rt/lib/gwp_asan/tests/enable_disable.cpp

46

That's left over from the scudo tests. Removed the initialization and cleaned up synchronization a bit. We can not get rid of the sleep, unfortunately, because the thread needs to act while the other one is blocked inside fork(), and there is no clear way to detect that condition.

compiler-rt/lib/gwp_asan/tests/harness.h

28

Yeah, I don't think it is required. We do need SetUp/TearDown logic though to properly set and remove the signal handlers.

[hctim]

LGTM with installAtFork() and cond_wait() comments.

compiler-rt/lib/gwp_asan/tests/enable_disable.cpp

52

I don't see why this value is needed - we don't expect someone else to send the cond_signal for this conditional, right?

[merge_guards_bot]

Unit tests: fail. 62144 tests passed, 5 failed and 811 were skipped.

failed: libc++.std/language_support/cmp/cmp_partialord/partialord.pass.cpp
failed: libc++.std/language_support/cmp/cmp_strongeq/cmp.strongeq.pass.cpp
failed: libc++.std/language_support/cmp/cmp_strongord/strongord.pass.cpp
failed: libc++.std/language_support/cmp/cmp_weakeq/cmp.weakeq.pass.cpp
failed: libc++.std/language_support/cmp/cmp_weakord/weakord.pass.cpp

clang-tidy: fail. clang-tidy found 32 errors and 19 warnings. 0 of them are added as review comments below (why?).

clang-format: fail. Please format your changes with clang-format by running git-clang-format HEAD^ or applying this patch.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

Pre-merge checks is in beta. Report issue. Please join beta or enable it for your project.

[eugenis]

compiler-rt/lib/gwp_asan/tests/enable_disable.cpp

52

pthread_cond_wait is allowed spurious wake ups.
Using it without a loop with an external condition is almost always wrong.

[eugenis]

fix lint warnings

[merge_guards_bot]

Unit tests: pass. 62190 tests passed, 0 failed and 815 were skipped.

clang-tidy: fail. clang-tidy found 31 errors and 11 warnings. 0 of them are added as review comments below (why?).

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

Pre-merge checks is in beta. Report issue. Please join beta or enable it for your project.

compiler-rt/lib/gwp_asan/guarded_pool_allocator.h

public:
   // and metadata allocations during destruction. We can't clean up these areas
   // as this may cause a use-after-free on shutdown.
   ~GuardedPoolAllocator() = default;
 
   // Initialise the rest of the members of this class. Create the allocation
   // pool using the provided options. See options.inc for runtime configuration
   // options.
   void init(const options::Options &Opts);
+  void uninitTestOnly();
+
+  void disable();
+  void enable();
 
   // Return whether the allocation should be randomly chosen for sampling.
   GWP_ASAN_ALWAYS_INLINE bool shouldSample() {
     // NextSampleCounter == 0 means we "should regenerate the counter".
     //                   == 1 means we "should sample this allocation".
     if (GWP_ASAN_UNLIKELY(ThreadLocals.NextSampleCounter == 0))
       ThreadLocals.NextSampleCounter =
-          (getRandomUnsigned32() % AdjustedSampleRate) + 1;
+          (getRandomUnsigned32() % (AdjustedSampleRatePlusOne - 1)) + 1;
 
     return GWP_ASAN_UNLIKELY(--ThreadLocals.NextSampleCounter == 0);
   }
 
   // Returns whether the provided pointer is a current sampled allocation that
   // is owned by this pool.
   GWP_ASAN_ALWAYS_INLINE bool pointerIsMine(const void *Ptr) const {
     uintptr_t P = reinterpret_cast<uintptr_t>(Ptr);
     return GuardedPagePool <= P && P < GuardedPagePoolEnd;
   }
 
   // Allocate memory in a guarded slot, and return a pointer to the new
   // allocation. Returns nullptr if the pool is empty, the requested size is too
   // large for this pool to handle, or the requested size is zero.
   void *allocate(size_t Size);
 
   // Deallocate memory in a guarded slot. The provided pointer must have been
private:
   // These functions anonymously map memory or change the permissions of mapped
   // memory into this process in a platform-specific way. Pointer and size
   // arguments are expected to be page-aligned. These functions will never
   // return on error, instead electing to kill the calling process on failure.
   // Note that memory is initially mapped inaccessible. In order for RW
   // mappings, call mapMemory() followed by markReadWrite() on the returned
   // pointer.
   void *mapMemory(size_t Size) const;
+  void unmapMemory(void *Addr, size_t Size) const;
   void markReadWrite(void *Ptr, size_t Size) const;
   void markInaccessible(void *Ptr, size_t Size) const;
 
   // Get the page size from the platform-specific implementation. Only needs to
   // be called once, and the result should be cached in PageSize in this class.
   static size_t getPlatformPageSize();
 
   // Install the SIGSEGV crash handler for printing use-after-free and heap-
   // buffer-{under|over}flow exceptions. This is platform specific as even
   // though POSIX and Windows both support registering handlers through
   // signal(), we have to use platform-specific signal handlers to obtain the
   // address that caused the SIGSEGV exception.
   static void installSignalHandlers();
+  static void uninstallSignalHandlers();
 
   // Returns the index of the slot that this pointer resides in. If the pointer
   // is not owned by this pool, the result is undefined.
   size_t addrToSlot(uintptr_t Ptr) const;
 
   // Returns the address of the N-th guarded slot.
   uintptr_t slotToAddr(size_t N) const;
 
private:
 
   // Returns the diagnosis for an unknown error. If the diagnosis is not
   // Error::INVALID_FREE or Error::UNKNOWN, the metadata for the slot
   // responsible for the error is placed in *Meta.
   Error diagnoseUnknownError(uintptr_t AccessPtr, AllocationMetadata **Meta);
 
   void reportErrorInternal(uintptr_t AccessPtr, Error E);
 
+  static GuardedPoolAllocator *getSingleton();
+
+  // Install a pthread_atfork handler.
+  void installAtFork();
+
   // Cached page size for this system in bytes.
   size_t PageSize = 0;
 
   // A mutex to protect the guarded slot and metadata pool for this class.
   Mutex PoolMutex;
   // The number of guarded slots that this pool holds.
   size_t MaxSimultaneousAllocations = 0;
   // Record the number allocations that we've sampled. We store this amount so
   // that we don't randomly choose to recycle a slot that previously had an
   // allocation before all the slots have been utilised.
   size_t NumSampledAllocations = 0;
   // Pointer to the pool of guarded slots. Note that this points to the start of
   // the pool (which is a guard page), not a pointer to the first guarded page.
-  uintptr_t GuardedPagePool = UINTPTR_MAX;
+  uintptr_t GuardedPagePool = 0;
   uintptr_t GuardedPagePoolEnd = 0;
   // Pointer to the allocation metadata (allocation/deallocation stack traces),
   // if any.
   AllocationMetadata *Metadata = nullptr;
 
   // Pointer to an array of free slot indexes.
   size_t *FreeSlots = nullptr;
   // The current length of the list of free slots.
private:
   options::PrintBacktrace_t PrintBacktrace = nullptr;
 
   // The adjusted sample rate for allocation sampling. Default *must* be
   // nonzero, as dynamic initialisation may call malloc (e.g. from libstdc++)
   // before GPA::init() is called. This would cause an error in shouldSample(),
   // where we would calculate modulo zero. This value is set UINT32_MAX, as when
   // GWP-ASan is disabled, we wish to never spend wasted cycles recalculating
   // the sample rate.
-  uint32_t AdjustedSampleRate = UINT32_MAX;
+  uint32_t AdjustedSampleRatePlusOne = 0;
 
   // Pack the thread local variables into a struct to ensure that they're in
   // the same cache line for performance reasons. These are the most touched
   // variables in GWP-ASan.
   struct alignas(8) ThreadLocalPackedVariables {
     constexpr ThreadLocalPackedVariables() {}
     // Thread-local decrementing counter that indicates that a given allocation
     // should be sampled when it reaches zero.

compiler-rt/lib/gwp_asan/guarded_pool_allocator.cpp

for (size_t i = 0; i < TraceLength; ++i) {
     Printf("  #%zu 0x%zx in <unknown>\n", i, Trace[i]);
   }
   Printf("\n");
 }
 } // anonymous namespace
 
 // Gets the singleton implementation of this class. Thread-compatible until
 // init() is called, thread-safe afterwards.
-GuardedPoolAllocator *getSingleton() { return SingletonPtr; }
+GuardedPoolAllocator *GuardedPoolAllocator::getSingleton() {
+  return SingletonPtr;
+}
 
 void GuardedPoolAllocator::AllocationMetadata::RecordAllocation(
     uintptr_t AllocAddr, size_t AllocSize, options::Backtrace_t Backtrace) {
   Addr = AllocAddr;
   Size = AllocSize;
   IsDeallocated = false;
 
   // TODO(hctim): Ask the caller to provide the thread ID, so we don't waste
void GuardedPoolAllocator::init(const options::Options &Opts) {
   // Allocate memory and set up the free pages queue.
   BytesRequired = MaxSimultaneousAllocations * sizeof(*FreeSlots);
   FreeSlots = reinterpret_cast<size_t *>(mapMemory(BytesRequired));
   markReadWrite(FreeSlots, BytesRequired);
 
   // Multiply the sample rate by 2 to give a good, fast approximation for (1 /
   // SampleRate) chance of sampling.
   if (Opts.SampleRate != 1)
-    AdjustedSampleRate = static_cast<uint32_t>(Opts.SampleRate) * 2;
+    AdjustedSampleRatePlusOne = static_cast<uint32_t>(Opts.SampleRate) * 2 + 1;
   else
-    AdjustedSampleRate = 1;
+    AdjustedSampleRatePlusOne = 2;
 
   GuardedPagePool = reinterpret_cast<uintptr_t>(GuardedPoolMemory);
   GuardedPagePoolEnd =
       reinterpret_cast<uintptr_t>(GuardedPoolMemory) + PoolBytesRequired;
 
   // Ensure that signal handlers are installed as late as possible, as the class
   // is not thread-safe until init() is finished, and thus a SIGSEGV may cause a
   // race to members if received during init().
   if (Opts.InstallSignalHandlers)
     installSignalHandlers();
+
+  if (Opts.InstallForkHandlers)
+    installAtFork();
+}
+
+void GuardedPoolAllocator::disable() { PoolMutex.lock(); }
+
+void GuardedPoolAllocator::enable() { PoolMutex.unlock(); }
+
+void GuardedPoolAllocator::uninitTestOnly() {
+  if (GuardedPagePool) {
+    unmapMemory(reinterpret_cast<void *>(GuardedPagePool),
+                GuardedPagePoolEnd - GuardedPagePool);
+    GuardedPagePool = 0;
+    GuardedPagePoolEnd = 0;
+  }
+  if (Metadata) {
+    unmapMemory(Metadata, MaxSimultaneousAllocations * sizeof(*Metadata));
+    Metadata = nullptr;
+  }
+  if (FreeSlots) {
+    unmapMemory(FreeSlots, MaxSimultaneousAllocations * sizeof(*FreeSlots));
+    FreeSlots = nullptr;
+  }
+  uninstallSignalHandlers();
 }
 
 void *GuardedPoolAllocator::allocate(size_t Size) {
   // GuardedPagePoolEnd == 0 when GWP-ASan is disabled. If we are disabled, fall
   // back to the supporting allocator.
   if (GuardedPagePoolEnd == 0)
     return nullptr;
 

compiler-rt/lib/gwp_asan/options.inc

GWP_ASAN_OPTION(
     bool, InstallSignalHandlers, true,
     "Install GWP-ASan signal handlers for SIGSEGV during dynamic loading. This "
     "allows better error reports by providing stack traces for allocation and "
     "deallocation when reporting a memory error. GWP-ASan's signal handler "
     "will forward the signal to any previously-installed handler, and user "
     "programs that install further signal handlers should make sure they do "
     "the same. Note, if the previously installed SIGSEGV handler is SIG_IGN, "
     "we terminate the process after dumping the error report.")
+
+GWP_ASAN_OPTION(bool, InstallForkHandlers, true,
+                "Install GWP-ASan atfork handlers to acquire internal locks "
+                "before fork and release them after.")

compiler-rt/lib/gwp_asan/platform_specific/guarded_pool_allocator_posix.cpp

 //===-- guarded_pool_allocator_posix.cpp ------------------------*- C++ -*-===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #include "gwp_asan/guarded_pool_allocator.h"
 
 #include <stdlib.h>
+#include <string.h>
 #include <errno.h>
 #include <signal.h>
 #include <sys/mman.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
 #include <unistd.h>
 
 namespace gwp_asan {
 
 void *GuardedPoolAllocator::mapMemory(size_t Size) const {
   void *Ptr =
       mmap(nullptr, Size, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
 
   if (Ptr == MAP_FAILED) {
     Printf("Failed to map guarded pool allocator memory, errno: %d\n", errno);
     Printf("  mmap(nullptr, %zu, ...) failed.\n", Size);
     exit(EXIT_FAILURE);
   }
   return Ptr;
 }
 
+void GuardedPoolAllocator::unmapMemory(void *Addr, size_t Size) const {
+  int Res = munmap(Addr, Size);
+
+  if (Res != 0) {
+    Printf("Failed to unmap guarded pool allocator memory, errno: %d\n", errno);
+    Printf("  unmmap(%p, %zu, ...) failed.\n", Addr, Size);
+    exit(EXIT_FAILURE);
+  }
+}
+
 void GuardedPoolAllocator::markReadWrite(void *Ptr, size_t Size) const {
   if (mprotect(Ptr, Size, PROT_READ | PROT_WRITE) != 0) {
     Printf("Failed to set guarded pool allocator memory at as RW, errno: %d\n",
            errno);
     Printf("  mprotect(%p, %zu, RW) failed.\n", Ptr, Size);
     exit(EXIT_FAILURE);
   }
 }
void GuardedPoolAllocator::markInaccessible(void *Ptr, size_t Size) const {
   }
 }
 
 size_t GuardedPoolAllocator::getPlatformPageSize() {
   return sysconf(_SC_PAGESIZE);
 }
 
 struct sigaction PreviousHandler;
+bool SignalHandlerInstalled;
 
 static void sigSegvHandler(int sig, siginfo_t *info, void *ucontext) {
   gwp_asan::GuardedPoolAllocator::reportError(
       reinterpret_cast<uintptr_t>(info->si_addr));
 
   // Process any previous handlers.
   if (PreviousHandler.sa_flags & SA_SIGINFO) {
     PreviousHandler.sa_sigaction(sig, info, ucontext);
   } else if (PreviousHandler.sa_handler == SIG_IGN ||
              PreviousHandler.sa_handler == SIG_DFL) {
     // If the previous handler was the default handler, or was ignoring this
     // signal, install the default handler and re-raise the signal in order to
     // get a core dump and terminate this process.
     signal(SIGSEGV, SIG_DFL);
     raise(SIGSEGV);
   } else {
     PreviousHandler.sa_handler(sig);
   }
 }
 
+void GuardedPoolAllocator::installAtFork() {
+  auto Disable = []() {
+    if (auto *S = getSingleton())
+      S->disable();
+  };
+  auto Enable = []() {
+    if (auto *S = getSingleton())
+      S->enable();
+  };
+  pthread_atfork(Disable, Enable, Enable);
+}
+
 void GuardedPoolAllocator::installSignalHandlers() {
   struct sigaction Action;
   Action.sa_sigaction = sigSegvHandler;
   Action.sa_flags = SA_SIGINFO;
   sigaction(SIGSEGV, &Action, &PreviousHandler);
+  SignalHandlerInstalled = true;
+}
+
+void GuardedPoolAllocator::uninstallSignalHandlers() {
+  if (SignalHandlerInstalled) {
+    sigaction(SIGSEGV, &PreviousHandler, nullptr);
+    SignalHandlerInstalled = false;
+  }
 }
 
 uint64_t GuardedPoolAllocator::getThreadID() {
 #ifdef SYS_gettid
   return syscall(SYS_gettid);
 #else
   return kInvalidThreadID;
 #endif
 }
 
 } // namespace gwp_asan

compiler-rt/lib/gwp_asan/tests/CMakeLists.txt

set(GWP_ASAN_UNITTESTS
   optional/printf_sanitizer_common.cpp
   alignment.cpp
   backtrace.cpp
   basic.cpp
   compression.cpp
   driver.cpp
   mutex_test.cpp
   slot_reuse.cpp
-  thread_contention.cpp)
+  thread_contention.cpp
+  harness.cpp
+  enable_disable.cpp)
 
 set(GWP_ASAN_UNIT_TEST_HEADERS
   ${GWP_ASAN_HEADERS}
   harness.h)
 
 add_custom_target(GwpAsanUnitTests)
 set_target_properties(GwpAsanUnitTests PROPERTIES FOLDER "Compiler-RT Tests")
 

compiler-rt/lib/gwp_asan/tests/enable_disable.cpp

+//===-- enable_disable.cpp --------------------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "gwp_asan/tests/harness.h"
+
+constexpr size_t Size = 100;
+
+TEST_F(DefaultGuardedPoolAllocator, Fork) {
+  void *P;
+  pid_t Pid = fork();
+  EXPECT_GE(Pid, 0);
+  if (Pid == 0) {
+    P = GPA.allocate(Size);
+    EXPECT_NE(P, nullptr);
+    memset(P, 0x42, Size);
+    GPA.deallocate(P);
+    _exit(0);
+  }
+  waitpid(Pid, nullptr, 0);
+  P = GPA.allocate(Size);
+  EXPECT_NE(P, nullptr);
+  memset(P, 0x42, Size);
+  GPA.deallocate(P);
+
+  // fork should stall if the allocator has been disabled.
+  EXPECT_DEATH(
+      {
+        GPA.disable();
+        alarm(1);
+        Pid = fork();
+        EXPECT_GE(Pid, 0);
+      },
+      "");
+}
+
+static pthread_mutex_t Mutex;
+static pthread_cond_t Conditional = PTHREAD_COND_INITIALIZER;
+
+static void *enableMalloc(void *arg) {
+  auto &GPA = *(gwp_asan::GuardedPoolAllocator *)arg;
+  // Initialize the allocator for this thread.
+  void *P = GPA.allocate(Size);
+  EXPECT_NE(P, nullptr);
+  memset(P, 0x42, Size);
+  GPA.deallocate(P);
+
+  // Signal the main thread we are ready.
+  pthread_mutex_lock(&Mutex);
+  pthread_cond_signal(&Conditional);
+  pthread_mutex_unlock(&Mutex);
+
+  // Wait for the malloc_disable & fork, then enable the allocator again.
+  sleep(1);
+  GPA.enable();
+
+  return nullptr;
+}
+
+TEST_F(DefaultGuardedPoolAllocator, DisableForkEnable) {
+  pthread_t ThreadId;
+  EXPECT_EQ(pthread_create(&ThreadId, nullptr, &enableMalloc, &GPA), 0);
+
+  // Wait for the thread to be warmed up.
+  pthread_mutex_lock(&Mutex);
+  pthread_cond_wait(&Conditional, &Mutex);
+  pthread_mutex_unlock(&Mutex);
+
+  // Disable the allocator and fork. fork should succeed after malloc_enable.
+  GPA.disable();
+  pid_t Pid = fork();
+  EXPECT_GE(Pid, 0);
+  if (Pid == 0) {
+    void *P = GPA.allocate(Size);
+    EXPECT_NE(P, nullptr);
+    memset(P, 0x42, Size);
+    GPA.deallocate(P);
+    _exit(0);
+  }
+  waitpid(Pid, nullptr, 0);
+  EXPECT_EQ(pthread_join(ThreadId, 0), 0);
+}

compiler-rt/lib/gwp_asan/tests/harness.h

 
 namespace gwp_asan {
 namespace test {
 // This printf-function getter allows other platforms (e.g. Android) to define
 // their own signal-safe Printf function. In LLVM, we use
 // `optional/printf_sanitizer_common.cpp` which supplies the __sanitizer::Printf
 // for this purpose.
 options::Printf_t getPrintfFunction();
+
+extern gwp_asan::GuardedPoolAllocator GPA;
+
+// First call returns true, all the following calls return false.
+bool OnlyOnce();
+
 }; // namespace test
 }; // namespace gwp_asan
 
 class DefaultGuardedPoolAllocator : public ::testing::Test {
 public:
-  DefaultGuardedPoolAllocator() {
+  void SetUp() override {
     gwp_asan::options::Options Opts;
     Opts.setDefaults();
     MaxSimultaneousAllocations = Opts.MaxSimultaneousAllocations;
 
     Opts.Printf = gwp_asan::test::getPrintfFunction();
+    Opts.InstallForkHandlers = gwp_asan::test::OnlyOnce();
     GPA.init(Opts);
   }
 
+  void TearDown() override {
+    GPA.uninitTestOnly();
+  }
 protected:
-  gwp_asan::GuardedPoolAllocator GPA;
+  gwp_asan::GuardedPoolAllocator &GPA = gwp_asan::test::GPA;
   decltype(gwp_asan::options::Options::MaxSimultaneousAllocations)
       MaxSimultaneousAllocations;
 };
 
 class CustomGuardedPoolAllocator : public ::testing::Test {
 public:
   void
   InitNumSlots(decltype(gwp_asan::options::Options::MaxSimultaneousAllocations)
                    MaxSimultaneousAllocationsArg) {
     gwp_asan::options::Options Opts;
     Opts.setDefaults();
 
     Opts.MaxSimultaneousAllocations = MaxSimultaneousAllocationsArg;
     MaxSimultaneousAllocations = MaxSimultaneousAllocationsArg;
 
     Opts.Printf = gwp_asan::test::getPrintfFunction();
+    Opts.InstallForkHandlers = gwp_asan::test::OnlyOnce();
     GPA.init(Opts);
   }
 
+  void TearDown() override {
+    GPA.uninitTestOnly();
+  }
+
  protected:
-  gwp_asan::GuardedPoolAllocator GPA;
+  gwp_asan::GuardedPoolAllocator &GPA = gwp_asan::test::GPA;
   decltype(gwp_asan::options::Options::MaxSimultaneousAllocations)
       MaxSimultaneousAllocations;
 };
 
 class BacktraceGuardedPoolAllocator : public ::testing::Test {
 public:
-  BacktraceGuardedPoolAllocator() {
+  void SetUp() override {
     gwp_asan::options::Options Opts;
     Opts.setDefaults();
 
     Opts.Printf = gwp_asan::test::getPrintfFunction();
     Opts.Backtrace = gwp_asan::options::getBacktraceFunction();
     Opts.PrintBacktrace = gwp_asan::options::getPrintBacktraceFunction();
+    Opts.InstallForkHandlers = gwp_asan::test::OnlyOnce();
     GPA.init(Opts);
   }
 
+  void TearDown() override {
+    GPA.uninitTestOnly();
+  }
 protected:
-  gwp_asan::GuardedPoolAllocator GPA;
+  gwp_asan::GuardedPoolAllocator &GPA = gwp_asan::test::GPA;
 };
 
 #endif // GWP_ASAN_TESTS_HARNESS_H_

compiler-rt/lib/gwp_asan/tests/harness.cpp

+#include "harness.h"
+
+namespace gwp_asan {
+namespace test {
+gwp_asan::GuardedPoolAllocator GPA;
+
+bool FirstTrueThenFalse() {
+  static int x = 0;
+  return !x++;
+}
+}
+}

compiler-rt/lib/scudo/standalone/CMakeLists.txt

 add_compiler_rt_component(scudo_standalone)
-# FIXME: GWP-ASan is temporarily disabled, re-enable once issues are fixed.
-if (FALSE AND COMPILER_RT_HAS_GWP_ASAN)
+if (COMPILER_RT_HAS_GWP_ASAN)
   add_dependencies(scudo_standalone gwp_asan)
 endif()
 
 include_directories(../..)
 
 set(SCUDO_CFLAGS)
 
 list(APPEND SCUDO_CFLAGS
set(SCUDO_SOURCES_C_WRAPPERS
   )
 
 set(SCUDO_SOURCES_CXX_WRAPPERS
   wrappers_cpp.cpp
   )
 
 set(SCUDO_OBJECT_LIBS)
 
-if (FALSE AND COMPILER_RT_HAS_GWP_ASAN)
+if (COMPILER_RT_HAS_GWP_ASAN)
   list(APPEND SCUDO_OBJECT_LIBS RTGwpAsan)
   list(APPEND SCUDO_CFLAGS -DGWP_ASAN_HOOKS)
 endif()
 
 if(COMPILER_RT_HAS_SCUDO_STANDALONE)
   add_compiler_rt_object_libraries(RTScudoStandalone
     ARCHS ${SCUDO_STANDALONE_SUPPORTED_ARCH}
     SOURCES ${SCUDO_SOURCES}

compiler-rt/lib/scudo/standalone/combined.h

 #include "quarantine.h"
 #include "report.h"
 #include "secondary.h"
 #include "string_utils.h"
 #include "tsd.h"
 
 #ifdef GWP_ASAN_HOOKS
 #include "gwp_asan/guarded_pool_allocator.h"
-// GWP-ASan is declared here in order to avoid indirect call overhead. It's also
-// instantiated outside of the Allocator class, as the allocator is only
-// zero-initialised. GWP-ASan requires constant initialisation, and the Scudo
-// allocator doesn't have a constexpr constructor (see discussion here:
-// https://reviews.llvm.org/D69265#inline-624315).
-static gwp_asan::GuardedPoolAllocator GuardedAlloc;
 #endif // GWP_ASAN_HOOKS
 
 extern "C" inline void EmptyCallback() {}
 
 namespace scudo {
 
 template <class Params, void (*PostInitCallback)(void) = EmptyCallback>
 class Allocator {
void initLinkerInitialized() {
 
     Stats.initLinkerInitialized();
     Primary.initLinkerInitialized(getFlags()->release_to_os_interval_ms);
     Secondary.initLinkerInitialized(&Stats);
 
     Quarantine.init(
         static_cast<uptr>(getFlags()->quarantine_size_kb << 10),
         static_cast<uptr>(getFlags()->thread_local_quarantine_size_kb << 10));
+  }
 
+  // Initialize the embedded GWP-ASan instance. Requires the main allocator to
+  // be functional, best called from PostInitCallback.
+  void initGwpAsan() {
 #ifdef GWP_ASAN_HOOKS
     gwp_asan::options::Options Opt;
     Opt.Enabled = getFlags()->GWP_ASAN_Enabled;
     // Bear in mind - Scudo has its own alignment guarantees that are strictly
     // enforced. Scudo exposes the same allocation function for everything from
     // malloc() to posix_memalign, so in general this flag goes unused, as Scudo
     // will always ask GWP-ASan for an aligned amount of bytes.
     Opt.PerfectlyRightAlign = getFlags()->GWP_ASAN_PerfectlyRightAlign;
     Opt.MaxSimultaneousAllocations =
         getFlags()->GWP_ASAN_MaxSimultaneousAllocations;
     Opt.SampleRate = getFlags()->GWP_ASAN_SampleRate;
     Opt.InstallSignalHandlers = getFlags()->GWP_ASAN_InstallSignalHandlers;
+    // Embedded GWP-ASan is locked through the Scudo atfork handler (via
+    // Allocator::disable calling GWPASan.disable). Disable GWP-ASan's atfork
+    // handler.
+    Opt.InstallForkHandlers = false;
     Opt.Printf = Printf;
     GuardedAlloc.init(Opt);
 #endif // GWP_ASAN_HOOKS
   }
 
   void reset() { memset(this, 0, sizeof(*this)); }
 
   void unmapTestOnly() {
     TSDRegistry.unmapTestOnly();
     Primary.unmapTestOnly();
+#ifdef GWP_ASAN_HOOKS
+    GuardedAlloc.uninitTestOnly();
+#endif // GWP_ASAN_HOOKS
   }
 
   TSDRegistryT *getTSDRegistry() { return &TSDRegistry; }
 
   // The Cache must be provided zero-initialized.
   void initCache(CacheT *Cache) {
     Cache->initLinkerInitialized(&Stats, &Primary);
   }
#endif // GWP_ASAN_HOOKS
     return NewPtr;
   }
 
   // TODO(kostyak): disable() is currently best-effort. There are some small
   //                windows of time when an allocation could still succeed after
   //                this function finishes. We will revisit that later.
   void disable() {
     initThreadMaybe();
+#ifdef GWP_ASAN_HOOKS
+    GuardedAlloc.disable();
+#endif
     TSDRegistry.disable();
     Stats.disable();
     Quarantine.disable();
     Primary.disable();
     Secondary.disable();
   }
 
   void enable() {
     initThreadMaybe();
     Secondary.enable();
     Primary.enable();
     Quarantine.enable();
     Stats.enable();
     TSDRegistry.enable();
+#ifdef GWP_ASAN_HOOKS
+    GuardedAlloc.enable();
+#endif
   }
 
   // The function returns the amount of bytes required to store the statistics,
   // which might be larger than the amount of bytes provided. Note that the
   // statistics buffer is not necessarily constant between calls to this
   // function. This can be called with a null buffer or zero size for buffer
   // sizing purposes.
   uptr getStats(char *Buffer, uptr Size) {
private:
   struct {
     u8 MayReturnNull : 1;       // may_return_null
     u8 ZeroContents : 1;        // zero_contents
     u8 DeallocTypeMismatch : 1; // dealloc_type_mismatch
     u8 DeleteSizeMismatch : 1;  // delete_size_mismatch
     u32 QuarantineMaxChunkSize; // quarantine_max_chunk_size
   } Options;
 
+#ifdef GWP_ASAN_HOOKS
+  gwp_asan::GuardedPoolAllocator GuardedAlloc;
+#endif // GWP_ASAN_HOOKS
+
   // The following might get optimized out by the compiler.
   NOINLINE void performSanityChecks() {
     // Verify that the header offset field can hold the maximum offset. In the
     // case of the Secondary allocator, it takes care of alignment and the
     // offset will always be small. In the case of the Primary, the worst case
     // scenario happens in the last size class, when the backend allocation
     // would already be aligned on the requested alignment, which would happen
     // to be the maximum alignment that would fit in that size class. As a

compiler-rt/lib/scudo/standalone/tests/CMakeLists.txt

set(SCUDO_UNITTEST_CFLAGS
   # Extra flags for the C++ tests
   # TODO(kostyak): find a way to make -fsized-deallocation work
   -Wno-mismatched-new-delete)
 
 if(ANDROID)
   list(APPEND SCUDO_UNITTEST_CFLAGS -fno-emulated-tls)
 endif()
 
-# FIXME: GWP-ASan is temporarily disabled, re-enable once issues are fixed.
-if (FALSE AND COMPILER_RT_HAS_GWP_ASAN)
+if (COMPILER_RT_HAS_GWP_ASAN)
   list(APPEND SCUDO_UNITTEST_CFLAGS -DGWP_ASAN_HOOKS)
 endif()
 
 set(SCUDO_TEST_ARCH ${SCUDO_STANDALONE_SUPPORTED_ARCH})
 
 # gtests requires c++
 set(LINK_FLAGS ${COMPILER_RT_UNITTEST_LINK_FLAGS})
 foreach(lib ${SANITIZER_TEST_CXX_LIBRARIES})
   list(APPEND LINK_FLAGS -l${lib})
 endforeach()
 list(APPEND LINK_FLAGS -pthread)
 # Linking against libatomic is required with some compilers
 list(APPEND LINK_FLAGS -latomic)
 
 set(SCUDO_TEST_HEADERS)
 foreach (header ${SCUDO_HEADERS})
   list(APPEND SCUDO_TEST_HEADERS ${CMAKE_CURRENT_SOURCE_DIR}/../${header})
 endforeach()
 
 macro(add_scudo_unittest testname)
   cmake_parse_arguments(TEST "" "" "SOURCES;ADDITIONAL_RTOBJECTS" ${ARGN})
-  if (FALSE AND COMPILER_RT_HAS_GWP_ASAN)
+  if (COMPILER_RT_HAS_GWP_ASAN)
     list(APPEND TEST_ADDITIONAL_RTOBJECTS RTGwpAsan)
   endif()
 
   if(COMPILER_RT_HAS_SCUDO_STANDALONE)
     foreach(arch ${SCUDO_TEST_ARCH})
       # Additional runtime objects get added along RTScudoStandalone
       set(SCUDO_TEST_RTOBJECTS $<TARGET_OBJECTS:RTScudoStandalone.${arch}>)
       foreach(rtobject ${TEST_ADDITIONAL_RTOBJECTS})

compiler-rt/lib/scudo/standalone/tests/combined_test.cpp

template <class Config> static void testAllocator() {
   std::string Stats(Buffer.begin(), Buffer.end());
   // Basic checks on the contents of the statistics output, which also allows us
   // to verify that we got it all.
   EXPECT_NE(Stats.find("Stats: SizeClassAllocator"), std::string::npos);
   EXPECT_NE(Stats.find("Stats: MapAllocator"), std::string::npos);
   EXPECT_NE(Stats.find("Stats: Quarantine"), std::string::npos);
 }
 
+// Test that multiple instantiations of the allocator have not messed up the
+// process's signal handlers (GWP-ASan used to do this).
+void testSEGV() {
+  const scudo::uptr Size = 4 * scudo::getPageSizeCached();
+  scudo::MapPlatformData Data = {};
+  void *P = scudo::map(nullptr, Size, "testSEGV", MAP_NOACCESS, &Data);
+  EXPECT_NE(P, nullptr);
+  EXPECT_DEATH(memset(P, 0xaa, Size), "");
+  scudo::unmap(P, Size, UNMAP_ALL, &Data);
+}
+
 TEST(ScudoCombinedTest, BasicCombined) {
   UseQuarantine = false;
   testAllocator<scudo::AndroidSvelteConfig>();
 #if SCUDO_FUCHSIA
   testAllocator<scudo::FuchsiaConfig>();
 #else
   testAllocator<scudo::DefaultConfig>();
   UseQuarantine = true;
   testAllocator<scudo::AndroidConfig>();
+  testSEGV();
 #endif
 }
 
 template <typename AllocatorT> static void stressAllocator(AllocatorT *A) {
   {
     std::unique_lock<std::mutex> Lock(Mutex);
     while (!Ready)
       Cv.wait(Lock);

compiler-rt/lib/scudo/standalone/wrappers_c.inc

 
 INTERFACE WEAK void SCUDO_PREFIX(malloc_enable)() { SCUDO_ALLOCATOR.enable(); }
 
 INTERFACE WEAK void SCUDO_PREFIX(malloc_disable)() {
   SCUDO_ALLOCATOR.disable();
 }
 
 void SCUDO_PREFIX(malloc_postinit)() {
+  SCUDO_ALLOCATOR.initGwpAsan();
   pthread_atfork(SCUDO_PREFIX(malloc_disable), SCUDO_PREFIX(malloc_enable),
                  SCUDO_PREFIX(malloc_enable));
 }
 
 INTERFACE WEAK int SCUDO_PREFIX(mallopt)(int param, UNUSED int value) {
   if (param == M_DECAY_TIME) {
     // TODO(kostyak): set release_to_os_interval_ms accordingly.
     return 1;