Index: compiler-rt/lib/fuzzer/build.sh
===================================================================
--- compiler-rt/lib/fuzzer/build.sh
+++ compiler-rt/lib/fuzzer/build.sh
@@ -14,8 +14,10 @@
 CXX="${CXX:-clang}"
 CFLAGS="${CFLAGS:-${DEFAULT_CFLAGS}}"
 
-for f in $LIBFUZZER_SRC_DIR/*.cpp; do
-  $CXX $CFLAGS $f -c &
+for d in . ipc; do
+  for f in $LIBFUZZER_SRC_DIR/$d/*.cpp; do
+    $CXX $CFLAGS $f -c &
+  done
 done
 
 wait
@@ -71,4 +73,10 @@
   FuzzerUtilWindows.o \
   FuzzerRemote.o
 
-rm -f Fuzzer*.o
+rm -f libFuzzerIPCProxy.a
+ar r libFuzzerIPCProxy.a \
+  IPCLinux.o \
+  IPCProxyLinux.o \
+  IPCQuickExit.o \
+
+rm -f Fuzzer*.o IPC*.o
Index: compiler-rt/lib/fuzzer/ipc/CMakeLists.txt
===================================================================
--- compiler-rt/lib/fuzzer/ipc/CMakeLists.txt
+++ compiler-rt/lib/fuzzer/ipc/CMakeLists.txt
@@ -7,3 +7,30 @@
 add_libfuzzer_objects(RTfuzzer_ipc
   SOURCES IPCQuickExit.cpp
   HEADERS IPCQuickExit.h)
+
+add_libfuzzer_objects(RTfuzzer_ipc_proxy
+  SOURCES IPCProxyLinux.cpp
+  HEADERS IPCProxyLinux.h IPCRemoteLinux.h)
+
+if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
+  function(add_partially_linked_library name arch)
+    cmake_parse_arguments(LOCAL "" "" "OBJECT_LIBS" "" ${ARGN})
+    foreach(object_lib ${LOCAL_OBJECT_LIBS})
+      list(APPEND LOCAL_OBJECTS $<TARGET_OBJECTS:${object_lib}.${arch}>)
+    endforeach()
+    add_library(${name}-${arch} STATIC ${LOCAL_OBJECTS})
+    set_target_properties(${name}-${arch} PROPERTIES
+      ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+    partially_link_libcxx(${name} ${LIBCXX_${arch}_PREFIX} ${arch})
+  endfunction()
+
+  foreach(arch ${FUZZER_SUPPORTED_ARCH})
+    add_partially_linked_library(RTFuzzerIPCProxy ${arch}
+      OBJECT_LIBS RTfuzzer_base
+                  RTfuzzer
+                  RTfuzzer_main
+                  RTfuzzer_ipc_base
+                  RTfuzzer_ipc
+                  RTfuzzer_ipc_proxy)
+  endforeach()
+endif()
Index: compiler-rt/lib/fuzzer/ipc/IPCProxyLinux.h
===================================================================
--- /dev/null
+++ compiler-rt/lib/fuzzer/ipc/IPCProxyLinux.h
@@ -0,0 +1,101 @@
+//===- IPCProxyLinux.h ------------------------------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+// Defines the proxy-related constants shared between both sides of the IPC
+// layer, and the server interface used by FuzzerProxy.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_IPC_PROXY_LINUX_H
+#define LLVM_FUZZER_IPC_PROXY_LINUX_H
+
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_LINUX
+
+#include "FuzzerLock.h"
+#include "IPCLinux.h"
+#include <condition_variable>
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <mutex>
+#include <thread>
+#include <unordered_map>
+
+namespace fuzzer {
+namespace ipc {
+
+// The server address should be easily configurable, yet easy to discover by or
+// pass to the client. An environment variable with a fixed name works.
+constexpr const char *kProxyAddressEnvVar = "FUZZER_IPC_ADDRESS";
+
+// Message types for messages from the proxy to a remote.
+enum ProxyOrdinal : uint8_t {
+  kProxyAck = Connection::kAcknowledgement,
+  kOptions,
+  kStartExecution,
+  kFinishExecution,
+  kPrintPC,
+  kDescribePC,
+  kPrintStackTrace,
+  kPrintMemoryProfile,
+  kDetectLeaksAtExit,
+};
+
+// Maximum length for a PC description.
+constexpr size_t kMaxDescLen = 256;
+
+// This class provides the IPC implementation of the FuzzerRemoteInterface for
+// FuzzerProxy.
+class ProxyConnection;
+class Proxy final : public ConnectionServer {
+public:
+  static Proxy *GetInstance();
+  static std::shared_ptr<ProxyConnection> GetProxyConnection(unsigned long PID);
+
+  ~Proxy() override;
+
+  void Start();
+  void Stop() override;
+
+  // Coverage received by remote processes is added to the static TracePC, and
+  // must remain valid until it is replaced or the program exits, at which point
+  // it should be unmapped. Since it can outlive an individual connection, this
+  // memory region needs to be tracked by the Proxy. Calling AddMapped twice
+  // with the same Idx will cause the first mapping to be replaced.
+  void AddMapped(uintptr_t Idx, void *Addr, size_t Len);
+
+  // When the FuzzerRemote asks the FuzzerProxy to invoke one of the callbacks
+  // defined by the FuzzerMonitor interface, it needs to be executed on a
+  // separate thread to allow the connection to continue receiving messages,
+  // e.g. acknowledgements to PrintStackTrace requests. Only one such error
+  // context is needed, so the thread is owned by the Proxy rather than
+  // individual connections. This method tells the thread to invoke a callback.
+  void InvokeCallback(uint8_t CB, unsigned long RemotePID = 0, size_t Size = 0);
+
+protected:
+  std::unique_ptr<Connection> CreateConnection() override;
+
+private:
+  std::mutex Mutex;
+  std::unordered_map<uintptr_t, std::pair<void *, size_t>>
+      MappedByIdx GUARDED_BY(Mutex);
+
+  // See InvokeCallback, above. These variables are used to communicate details
+  // of a fatal callback from a remote process to the per-proxy CallbackThread.
+  uint8_t Callback GUARDED_BY(Mutex);
+  unsigned long CallbackPID GUARDED_BY(Mutex);
+  size_t CallbackSize GUARDED_BY(Mutex);
+
+  std::thread CallbackThread;
+  std::condition_variable CallbackCV;
+};
+
+} // namespace ipc
+} // namespace fuzzer
+
+#endif // LIBFUZZER_LINUX
+#endif // LLVM_FUZZER_IPC_PROXY_LINUX_H
Index: compiler-rt/lib/fuzzer/ipc/IPCProxyLinux.cpp
===================================================================
--- /dev/null
+++ compiler-rt/lib/fuzzer/ipc/IPCProxyLinux.cpp
@@ -0,0 +1,386 @@
+//===- IPCProxyLinux.cpp --------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+// IPC routines used by the FuzzerProxy on Linux.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerPlatform.h"
+
+#if LIBFUZZER_LINUX
+
+#include "FuzzerExtFunctions.h"
+#include "FuzzerMonitor.h"
+#include "FuzzerProxy.h"
+#include "FuzzerRemoteInterface.h"
+#include "IPCLinux.h"
+#include "IPCProxyLinux.h"
+#include "IPCRemoteLinux.h"
+#include <memory>
+#include <sys/mman.h>
+
+namespace fuzzer {
+namespace ipc {
+
+// This class represents a single FuzzerProxy-side endpoint for a connection to
+// a FuzzerRemote process.
+class ProxyConnection final : public Connection {
+public:
+  explicit ProxyConnection(ConnectionServer *S)
+      : Connection(S), Proxy(Proxy::GetInstance()) {
+    assert(Proxy == S);
+  }
+
+  ~ProxyConnection() override { Reset(); }
+
+  bool DoHandshake() override EXCLUDES(GetTxMutex()) {
+    Connection::DoHandshake();
+    ProxiedOptions Options;
+    FuzzerProxyConnect(this->GetPid(), &Options, sizeof(Options));
+    {
+      std::lock_guard<std::mutex> TxLock(GetTxMutex());
+      return SendLocked(kOptions) && SendLocked(&Options, sizeof(Options));
+    }
+  }
+
+  void StartExecution(uint32_t Options) EXCLUDES(GetTxMutex()) {
+    std::lock_guard<std::mutex> TxLock(GetTxMutex());
+    SendLocked(kStartExecution);
+    SendLocked(Options);
+  }
+
+  void FinishExecution() EXCLUDES(GetTxMutex()) {
+    std::lock_guard<std::mutex> TxLock(GetTxMutex());
+    SendLocked(kFinishExecution);
+  }
+
+  void PrintPC(const char *SymbolizedFMT, const char *FallbackFMT, uintptr_t PC)
+      EXCLUDES(GetTxMutex()) {
+    {
+      std::lock_guard<std::mutex> TxLock(GetTxMutex());
+      SendLocked(kPrintPC);
+      SendStringLocked(SymbolizedFMT);
+      SendStringLocked(FallbackFMT);
+      SendLocked(PC);
+    }
+    WaitForAcknowledgement();
+  }
+
+  void DescribePC(const char *SymbolizedFMT, uintptr_t PC, char *Desc,
+                  size_t DescLen) EXCLUDES(Mutex, GetTxMutex()) {
+    assert(Desc && DescLen);
+    {
+      std::lock_guard<std::mutex> Lock(Mutex);
+      Desc[0] = '\0';
+      PCDescription = Desc;
+      PCDescriptionLen = std::min(DescLen, kMaxDescLen);
+    }
+    {
+      std::lock_guard<std::mutex> TxLock(GetTxMutex());
+      SendLocked(kDescribePC);
+      SendStringLocked(SymbolizedFMT);
+      SendLocked(PC);
+      SendLocked(DescLen);
+    }
+    WaitForAcknowledgement();
+  }
+
+  void PrintStackTrace() EXCLUDES(GetTxMutex()) {
+    {
+      std::lock_guard<std::mutex> TxLock(GetTxMutex());
+      SendLocked(kPrintStackTrace);
+    }
+    WaitForAcknowledgement();
+  }
+
+  void PrintMemoryProfile() EXCLUDES(GetTxMutex()) {
+    {
+      std::lock_guard<std::mutex> TxLock(GetTxMutex());
+      SendLocked(kPrintMemoryProfile);
+    }
+    WaitForAcknowledgement();
+  }
+
+  void DetectLeaksAtExit() EXCLUDES(GetTxMutex()) {
+    {
+      std::lock_guard<std::mutex> TxLock(GetTxMutex());
+      SendLocked(kDetectLeaksAtExit);
+    }
+    WaitForAcknowledgement();
+  }
+
+private:
+  bool DispatchOneLocked() override REQUIRES(GetRxMutex()) {
+    unsigned long PID = this->GetPid();
+    RemoteOrdinal O;
+    if (!ReceiveLocked(&O))
+      return false;
+    size_t Size = 0;
+    switch (O) {
+    case kRemoteAck:
+      HandleAcknowledgement();
+      break;
+    case kAddCoverage: {
+      void *Addr1, *Addr2;
+      size_t Len1, Len2;
+      if (!ReceiveMemoryLocked(&Addr1, &Len1) ||
+          !ReceiveMemoryLocked(&Addr2, &Len2))
+        return false;
+      auto *Start = reinterpret_cast<uint8_t *>(Addr1);
+      auto *Stop = Start + (Len1 / sizeof(Start[0]));
+      auto *Begin = reinterpret_cast<uintptr_t *>(Addr2);
+      auto *End = Begin + (Len2 / sizeof(Begin[0]));
+      uintptr_t Idx = FuzzerProxyAddCoverage(PID, Start, Stop, Begin, End);
+      //  Counters are added to TracePC and must be kept mapped, even beyond
+      // this connection.
+      if (Idx == kInvalidIdx)
+        munmap(Addr1, Len1);
+      else
+        Proxy->AddMapped(Idx, Addr1, Len1);
+      // PCs have been copied and can be unmapped.
+      munmap(Addr2, Len2);
+      Acknowledge();
+      break;
+    }
+    case kExecutionStarted:
+      FuzzerProxyExecutionStarted(PID);
+      break;
+    case kExecutionFinished: {
+      int HasMoreMallocsThanFrees;
+      if (!ReceiveLocked(&HasMoreMallocsThanFrees))
+        return false;
+      FuzzerProxyExecutionFinished(PID, HasMoreMallocsThanFrees);
+      break;
+    }
+    case kCrashCallback:
+    case kExitCallback:
+    case kLeakCallback:
+    case kRssLimitCallback:
+      Proxy->InvokeCallback(O, PID, Size);
+      break;
+    case kDeathCallback:
+      if (EF->__sanitizer_acquire_crash_state &&
+          !EF->__sanitizer_acquire_crash_state())
+        break;
+      Proxy->InvokeCallback(O, PID, Size);
+      break;
+    case kMallocLimitCallback:
+      if (!ReceiveLocked(&Size))
+        return false;
+      Proxy->InvokeCallback(O, PID, Size);
+      break;
+    case kDescribePCResponse: {
+      std::lock_guard<std::mutex> Lock(Mutex);
+      if (!ReceiveStringLocked(PCDescription, PCDescriptionLen))
+        return false;
+    }
+      HandleAcknowledgement();
+      break;
+    }
+    return true;
+  }
+
+  void OnClose() override {
+    unsigned long PID = this->GetPid();
+    FuzzerProxyDisconnect(PID);
+    Connection::OnClose();
+    {
+      std::lock_guard<std::mutex> Lock(Mutex);
+      AwaitingResponse = false;
+    }
+    DescriptionCV.notify_one();
+  }
+
+  fuzzer::ipc::Proxy *const Proxy;
+
+  std::mutex Mutex;
+  char *PCDescription GUARDED_BY(Mutex) = nullptr;
+  size_t PCDescriptionLen GUARDED_BY(Mutex) = 0;
+  bool AwaitingResponse GUARDED_BY(Mutex) = false;
+  std::condition_variable DescriptionCV;
+};
+
+// fuzzer::ipc::Proxy methods
+
+Proxy *Proxy::GetInstance() {
+  static Proxy Singleton;
+  return &Singleton;
+}
+
+Proxy::~Proxy() {
+  Stop();
+  std::lock_guard<std::mutex> Lock(Mutex);
+  for (auto &I : MappedByIdx)
+    if (munmap(I.second.first, I.second.second) == -1)
+      Printf("ERROR: munmap: %s\n", strerror(errno));
+}
+
+void Proxy::Start() {
+  {
+    std::lock_guard<std::mutex> Lock(Mutex);
+    assert(Callback != kExecutionStarted);
+    Callback = kExecutionStarted;
+  }
+  CallbackThread = std::thread([this]() EXCLUDES(Mutex) {
+    uint8_t CB;
+    unsigned long RemotePID;
+    size_t Size;
+    {
+      UniqueLock Lock(Mutex);
+      while (Callback == kExecutionStarted)
+        CallbackCV.wait(Lock.get());
+      CB = Callback;
+      RemotePID = CallbackPID;
+      Size = CallbackSize;
+    }
+    switch (CB) {
+    case kCrashCallback:
+      FuzzerCrashSignalCallback(RemotePID);
+      break;
+    case kDeathCallback:
+      FuzzerDeathCallback();
+      break;
+    case kExitCallback:
+      FuzzerExitCallback(RemotePID);
+      break;
+    case kLeakCallback:
+      FuzzerLeakCallback(RemotePID);
+      break;
+    case kMallocLimitCallback:
+      FuzzerMallocLimitCallback(RemotePID, Size);
+      break;
+    case kRssLimitCallback:
+      FuzzerRssLimitCallback(RemotePID);
+      break;
+    case kExecutionFinished:
+      // Normal exit
+      break;
+    default:
+      Printf("ERROR: Unknown callback: %u.\n", CB);
+      break;
+    }
+  });
+  ConnectionServer::Start(kProxyAddressEnvVar);
+}
+
+void Proxy::Stop() {
+  ConnectionServer::Stop();
+  {
+    std::lock_guard<std::mutex> Lock(Mutex);
+    Callback = kExecutionFinished;
+  }
+  CallbackCV.notify_one();
+  if (CallbackThread.joinable())
+    CallbackThread.join();
+}
+
+std::unique_ptr<Connection> Proxy::CreateConnection() {
+  return std::unique_ptr<Connection>(new ProxyConnection(this));
+}
+
+std::shared_ptr<ProxyConnection> Proxy::GetProxyConnection(unsigned long PID) {
+  auto C = GetInstance()->GetConnection(PID);
+  return std::static_pointer_cast<ProxyConnection>(C);
+}
+
+void Proxy::AddMapped(uintptr_t Idx, void *Addr, size_t Len) {
+  assert(Idx != kInvalidIdx);
+  assert(Addr != nullptr);
+  assert(Len != 0);
+  std::lock_guard<std::mutex> Lock(Mutex);
+  auto I = MappedByIdx.find(Idx);
+  if (I != MappedByIdx.end())
+    if (munmap(I->second.first, I->second.second) == -1)
+      Printf("ERROR: munmap: %s\n", strerror(errno));
+  MappedByIdx[Idx] = {Addr, Len};
+}
+
+void Proxy::InvokeCallback(uint8_t CB, unsigned long RemotePID, size_t Size) {
+  {
+    std::lock_guard<std::mutex> Lock(Mutex);
+    Callback = CB;
+    CallbackPID = RemotePID;
+    CallbackSize = Size;
+  }
+  CallbackCV.notify_one();
+}
+
+} // namespace ipc
+} // namespace fuzzer
+
+// FuzzerRemoteInterface implementations. The IPC layer looks like FuzzerRemote
+// to the FuzzerProxy.
+
+extern "C" {
+
+ATTRIBUTE_INTERFACE
+void FuzzerAcceptRemotes() {
+  auto *Proxy = fuzzer::ipc::Proxy::GetInstance();
+  Proxy->Start();
+}
+
+ATTRIBUTE_INTERFACE
+void FuzzerShutdownRemotes() {
+  auto *Proxy = fuzzer::ipc::Proxy::GetInstance();
+  Proxy->Stop();
+}
+
+ATTRIBUTE_INTERFACE
+void FuzzerRemoteStartExecution(unsigned long PID, uint32_t Options) {
+  auto C = fuzzer::ipc::Proxy::GetProxyConnection(PID);
+  if (C)
+    C->StartExecution(Options);
+}
+
+ATTRIBUTE_INTERFACE
+void FuzzerRemoteFinishExecution(unsigned long PID) {
+  auto C = fuzzer::ipc::Proxy::GetProxyConnection(PID);
+  if (C)
+    C->FinishExecution();
+}
+
+ATTRIBUTE_INTERFACE
+void FuzzerRemotePrintPC(unsigned long PID, const char *SymbolizedFMT,
+                         const char *FallbackFMT, uintptr_t PC) {
+  auto C = fuzzer::ipc::Proxy::GetProxyConnection(PID);
+  if (C)
+    C->PrintPC(SymbolizedFMT, FallbackFMT, PC);
+}
+
+ATTRIBUTE_INTERFACE
+void FuzzerRemoteDescribePC(unsigned long PID, const char *SymbolizedFMT,
+                            uintptr_t PC, char *Desc, size_t DescLen) {
+  auto C = fuzzer::ipc::Proxy::GetProxyConnection(PID);
+  if (C)
+    C->DescribePC(SymbolizedFMT, PC, Desc, DescLen);
+}
+
+ATTRIBUTE_INTERFACE
+void FuzzerRemotePrintStackTrace(unsigned long PID) {
+  auto C = fuzzer::ipc::Proxy::GetProxyConnection(PID);
+  if (C)
+    C->PrintStackTrace();
+}
+
+ATTRIBUTE_INTERFACE
+void FuzzerRemotePrintMemoryProfile(unsigned long PID) {
+  auto C = fuzzer::ipc::Proxy::GetProxyConnection(PID);
+  if (C)
+    C->PrintMemoryProfile();
+}
+
+ATTRIBUTE_INTERFACE
+void FuzzerRemoteDetectLeaksAtExit(unsigned long PID) {
+  auto C = fuzzer::ipc::Proxy::GetProxyConnection(PID);
+  if (C)
+    C->DetectLeaksAtExit();
+}
+
+int my_hack;
+
+} // extern "C"
+
+#endif // LIBFUZZER_LINUX
Index: compiler-rt/lib/fuzzer/ipc/IPCRemoteLinux.h
===================================================================
--- /dev/null
+++ compiler-rt/lib/fuzzer/ipc/IPCRemoteLinux.h
@@ -0,0 +1,42 @@
+//===- IPCRemoteLinux.h -----------------------------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+// Defines the remote-related constants shared between both sides of the IPC
+// layer.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_IPC_REMOTE_LINUX_H
+#define LLVM_FUZZER_IPC_REMOTE_LINUX_H
+
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_LINUX
+
+#include <cstdint>
+
+namespace fuzzer {
+namespace ipc {
+
+// Message types for messages from a remote to the proxy.
+enum RemoteOrdinal : uint8_t {
+  kRemoteAck = Connection::kAcknowledgement,
+  kAddCoverage,
+  kExecutionStarted,
+  kExecutionFinished,
+  kCrashCallback,
+  kDeathCallback,
+  kExitCallback,
+  kLeakCallback,
+  kMallocLimitCallback,
+  kRssLimitCallback,
+  kDescribePCResponse,
+};
+
+} // namespace ipc
+} // namespace fuzzer
+
+#endif // LIBFUZZER_LINUX
+#endif // LLVM_FUZZER_IPC_REMOTE_LINUX_H
Index: compiler-rt/lib/fuzzer/tests/ipc/CMakeLists.txt
===================================================================
--- compiler-rt/lib/fuzzer/tests/ipc/CMakeLists.txt
+++ compiler-rt/lib/fuzzer/tests/ipc/CMakeLists.txt
@@ -4,34 +4,25 @@
   IPCTestUtilGeneric.cpp
   IPCTestUtilLinux.cpp)
 
-set(LIBFUZZER_IPC_TEST_HEADERS
-  IPCTestUtil.h)
-
-add_compiler_rt_object_libraries(RTfuzzer_ipc_test_util
-  OS ${FUZZER_SUPPORTED_OS}
-  ARCHS ${FUZZER_SUPPORTED_ARCH}
+add_libfuzzer_objects(RTfuzzer_ipc_test_util
   SOURCES ${LIBFUZZER_IPC_TEST_SOURCES}
-  ADDITIONAL_HEADERS ${LIBFUZZER_IPC_TEST_HEADERS}
-  CFLAGS ${LIBFUZZER_UNITTEST_CFLAGS}
-  DEPS ${LIBFUZZER_DEPS})
-
-if(OS_NAME MATCHES "Linux" AND
-   COMPILER_RT_LIBCXX_PATH AND
-   COMPILER_RT_LIBCXXABI_PATH)
-  foreach(arch ${FUZZER_SUPPORTED_ARCH})
-    target_compile_options(RTfuzzer_ipc_test_util.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
-    add_dependencies(RTfuzzer_ipc_test_util.${arch} libcxx_fuzzer_${arch}-build)
-  endforeach()
-endif()
+  HEADERS IPCTestUtil.h
+  CFLAGS ${LIBFUZZER_UNITTEST_CFLAGS})
 
 if(COMPILER_RT_DEFAULT_TARGET_ARCH IN_LIST FUZZER_SUPPORTED_ARCH)
   # libFuzzer unit tests are only run on the host machine.
   set(arch ${COMPILER_RT_DEFAULT_TARGET_ARCH})
 
-  generate_libfuzzer_test_runtime(RTFuzzerIPCTest ${arch}
-    OBJECT_LIBS RTfuzzer_base RTfuzzer_ipc_base RTfuzzer_ipc_test_util)
-
-  generate_libfuzzer_unittests(FuzzerIPC ${arch}
+  generate_libfuzzer_unittests(FuzzerIPCUnitTests ${arch}
     SOURCES IPCLinuxUnittest.cpp
-    RUNTIME RTFuzzerIPCTest)
+    OBJECT_LIBS RTfuzzer_base
+                RTfuzzer_ipc_base
+                RTfuzzer_ipc_test_util)
+
+  generate_libfuzzer_unittests(FuzzerIPCProxyUnitTests ${arch}
+    SOURCES IPCProxyLinuxUnittest.cpp
+    OBJECT_LIBS RTfuzzer_base
+                RTfuzzer_ipc_base
+                RTfuzzer_ipc_test_util
+                RTfuzzer_ipc_proxy)
 endif()
Index: compiler-rt/lib/fuzzer/tests/ipc/IPCProxyLinuxUnittest.cpp
===================================================================
--- /dev/null
+++ compiler-rt/lib/fuzzer/tests/ipc/IPCProxyLinuxUnittest.cpp
@@ -0,0 +1,407 @@
+//===- IPCRemoteLinuxUnittest.cpp -----------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+// Unit test of IPCProxyLinux.
+//
+// This is kept separate from other unit tests as it must link against the
+// ipc_proxy library, which has symbols that conflict with the fuzzer and
+// fuzzer_remote runtimes and with the ipc_remote library, e.g. the
+// FuzzerRemoteInterface symbols.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_LINUX
+
+#include "FuzzerRemoteInterface.h"
+#include "ipc/IPCProxyLinux.h"
+#include "ipc/IPCRemoteLinux.h"
+#include "tests/FuzzerTestUtil.h"
+#include "tests/ipc/IPCTestUtilLinux.h"
+#include "gtest/gtest.h"
+
+namespace fuzzer {
+namespace ipc {
+namespace {
+
+// Test fixtures
+
+class IPCProxyLinuxTest : public TestBase {
+public:
+  template <typename Responder> static void OnProxyReceive(Responder R) {
+    ReceiveAtNear(R);
+  }
+
+protected:
+  void SetUp() override {
+    TestBase::SetUp();
+    FarConnection.reset(new TestConnection());
+  }
+
+  void Connect() {
+    FuzzerAcceptRemotes();
+    DoRemoteSend([&](TestConnection *TC) {
+      if (TC->Connect(kProxyAddressEnvVar))
+        TC->StopReceiving();
+    });
+    auto TC = GetFarConnection();
+    ProxyOrdinal Ordinal;
+    ProxiedOptions Options;
+    EXPECT_TRUE(TC->Receive(&Ordinal));
+    EXPECT_TRUE(TC->Receive(&Options, sizeof(Options)));
+  }
+
+  std::shared_ptr<TestConnection> GetFarConnection() override {
+    return FarConnection;
+  }
+
+  // Rename the base names for clarity.
+  template <typename Requester> void DoProxySend(Requester R) {
+    SendFromNear(R);
+  }
+
+  template <typename Responder> void OnRemoteReceive(Responder R) {
+    ReceiveAtFar(R);
+  }
+
+  template <typename Requester> void DoRemoteSend(Requester R) {
+    SendFromFar(R);
+  }
+
+  void TearDown() override {
+    FuzzerShutdownRemotes();
+    FarConnection.reset();
+    TestBase::TearDown();
+  }
+
+  std::shared_ptr<TestConnection> FarConnection;
+};
+
+using IPCProxyLinuxDeathTest = IPCProxyLinuxTest;
+
+// Unit tests
+
+TEST_F(IPCProxyLinuxDeathTest, ProxyNotListening) {
+  auto TC = GetFarConnection();
+  EXPECT_DEATH(TC->Connect("kProxyAddressEnvVar"),
+               "environment variable is not set");
+}
+
+static StaticTestMessage ConnectReq;
+
+extern "C" void FuzzerProxyConnect(unsigned long PID, void *Options,
+                                   size_t OptionsLen) {
+  IPCProxyLinuxTest::OnProxyReceive([&]() {
+    ConnectReq.PID = PID;
+    memset(Options, 0x41, OptionsLen);
+  });
+}
+
+TEST_F(IPCProxyLinuxTest, Connect) {
+  FuzzerAcceptRemotes();
+  DoRemoteSend([&](TestConnection *TC) { TC->Connect(kProxyAddressEnvVar); });
+
+  std::vector<uint8_t> Expected(sizeof(ProxiedOptions), 0x41);
+  auto TC = GetFarConnection();
+  ProxyOrdinal Ordinal;
+  ProxiedOptions Options;
+  EXPECT_TRUE(TC->Receive(&Ordinal));
+  EXPECT_TRUE(TC->Receive(&Options, sizeof(Options)));
+  EXPECT_EQ(Ordinal, kOptions);
+  EXPECT_EQ(memcmp(&Options, &Expected[0], sizeof(Options)), 0);
+}
+
+static struct : public StaticTestMessage {
+  size_t NumCounters;
+  uint8_t *Counters;
+  size_t NumPCs;
+  std::unique_ptr<uintptr_t[]> PCs;
+} AddCoverageReq;
+
+extern "C" uintptr_t FuzzerProxyAddCoverage(unsigned long PID,
+                                            uint8_t *CountersBegin,
+                                            uint8_t *CountersEnd,
+                                            const uintptr_t *PCsBegin,
+                                            const uintptr_t *PCsEnd) {
+  IPCProxyLinuxTest::OnProxyReceive([&]() {
+    AddCoverageReq.PID = PID;
+    AddCoverageReq.NumCounters = CountersEnd - CountersBegin;
+    AddCoverageReq.Counters = CountersBegin;
+    size_t N = PCsEnd - PCsBegin;
+    AddCoverageReq.NumPCs = N;
+    AddCoverageReq.PCs.reset(new uintptr_t[N]);
+    memcpy(AddCoverageReq.PCs.get(), PCsBegin, N * sizeof(uintptr_t));
+  });
+  return 0; // Fake index.
+}
+
+TEST_F(IPCProxyLinuxTest, AddCoverage) {
+  Connect();
+
+  FakeDSO<0xcafe> DSO;
+  DoRemoteSend([&](TestConnection *TC) {
+    EXPECT_TRUE(TC->Send(kAddCoverage));
+    EXPECT_TRUE(TC->SendMemory(DSO.Counters, sizeof(DSO.Counters), true));
+    EXPECT_TRUE(TC->SendMemory(DSO.PCs, sizeof(DSO.PCs), false));
+  });
+  EXPECT_EQ(AddCoverageReq.PID, PID);
+  ASSERT_EQ(AddCoverageReq.NumCounters, sizeof(DSO.Counters));
+  ASSERT_EQ(AddCoverageReq.NumPCs, sizeof(DSO.PCs) / sizeof(DSO.PCs[0]));
+  EXPECT_EQ(memcmp(AddCoverageReq.Counters, DSO.Counters, sizeof(DSO.Counters)),
+            0);
+  EXPECT_EQ(memcmp(AddCoverageReq.PCs.get(), DSO.PCs, sizeof(DSO.PCs)), 0);
+}
+
+TEST_F(IPCProxyLinuxTest, StartExecution) {
+  Connect();
+  auto ExecOptionsIn = Pick<uint32_t>();
+  DoProxySend([&]() { FuzzerRemoteStartExecution(PID, ExecOptionsIn); });
+
+  ProxyOrdinal Ordinal;
+  uint32_t ExecOptionsOut;
+  OnRemoteReceive([&](TestConnection *TC) {
+    EXPECT_TRUE(TC->Receive(&Ordinal));
+    EXPECT_TRUE(TC->Receive(&ExecOptionsOut));
+  });
+  EXPECT_EQ(Ordinal, kStartExecution);
+  EXPECT_EQ(ExecOptionsIn, ExecOptionsOut);
+}
+
+static StaticTestMessage ExecutionStartedReq;
+
+extern "C" void FuzzerProxyExecutionStarted(unsigned long PID) {
+  IPCProxyLinuxTest::OnProxyReceive([&]() { ExecutionStartedReq.PID = PID; });
+}
+
+TEST_F(IPCProxyLinuxTest, ExecutionStarted) {
+  Connect();
+  DoRemoteSend(
+      [&](TestConnection *TC) { EXPECT_TRUE(TC->Send(kExecutionStarted)); });
+  EXPECT_EQ(ExecutionStartedReq.PID, PID);
+}
+
+TEST_F(IPCProxyLinuxTest, FinishExecution) {
+  Connect();
+  DoProxySend([&]() { FuzzerRemoteFinishExecution(PID); });
+  ProxyOrdinal Ordinal;
+  OnRemoteReceive(
+      [&](TestConnection *TC) { EXPECT_TRUE(TC->Receive(&Ordinal)); });
+  EXPECT_EQ(Ordinal, kFinishExecution);
+}
+
+static struct : public StaticTestMessage {
+  int HasMoreMallocsThanFrees;
+} ExecutionFinishedReq;
+
+extern "C" void FuzzerProxyExecutionFinished(unsigned long PID,
+                                             int HasMoreMallocsThanFrees) {
+  IPCProxyLinuxTest::OnProxyReceive([&]() {
+    ExecutionFinishedReq.PID = PID;
+    ExecutionFinishedReq.HasMoreMallocsThanFrees = HasMoreMallocsThanFrees;
+  });
+}
+
+TEST_F(IPCProxyLinuxTest, ExecutionFinished) {
+  Connect();
+  int HasMoreMallocsThanFrees = Pick<int>();
+  DoRemoteSend([&](TestConnection *TC) {
+    EXPECT_TRUE(TC->Send(kExecutionFinished));
+    EXPECT_TRUE(TC->Send(HasMoreMallocsThanFrees));
+  });
+  EXPECT_EQ(ExecutionFinishedReq.PID, PID);
+  EXPECT_EQ(ExecutionFinishedReq.HasMoreMallocsThanFrees,
+            HasMoreMallocsThanFrees);
+}
+
+static StaticTestMessage CrashCallbackReq;
+
+extern "C" void FuzzerCrashSignalCallback(unsigned long PID) {
+  IPCProxyLinuxTest::OnProxyReceive([&]() { CrashCallbackReq.PID = PID; });
+}
+
+TEST_F(IPCProxyLinuxTest, CrashCallback) {
+  Connect();
+  DoRemoteSend(
+      [&](TestConnection *TC) { EXPECT_TRUE(TC->Send(kCrashCallback)); });
+  EXPECT_EQ(CrashCallbackReq.PID, PID);
+}
+
+static StaticTestMessage DeathCallbackReq;
+
+extern "C" void FuzzerDeathCallback() {
+  IPCProxyLinuxTest::OnProxyReceive([&]() { DeathCallbackReq.PID = GetPid(); });
+}
+
+TEST_F(IPCProxyLinuxTest, DeathCallback) {
+  Connect();
+  DoRemoteSend(
+      [&](TestConnection *TC) { EXPECT_TRUE(TC->Send(kDeathCallback)); });
+  EXPECT_EQ(DeathCallbackReq.PID, GetPid());
+}
+
+static struct : public StaticTestMessage { int ExitCode; } ExitCallbackReq;
+
+extern "C" void FuzzerExitCallback(unsigned long PID) {
+  IPCProxyLinuxTest::OnProxyReceive([&]() { ExitCallbackReq.PID = PID; });
+}
+
+TEST_F(IPCProxyLinuxTest, ExitCallback) {
+  Connect();
+  DoRemoteSend(
+      [&](TestConnection *TC) { EXPECT_TRUE(TC->Send(kExitCallback)); });
+  EXPECT_EQ(ExitCallbackReq.PID, PID);
+}
+
+static StaticTestMessage LeakCallbackReq;
+
+extern "C" void FuzzerLeakCallback(unsigned long PID) {
+  IPCProxyLinuxTest::OnProxyReceive([&]() { LeakCallbackReq.PID = PID; });
+}
+
+TEST_F(IPCProxyLinuxTest, LeakCallback) {
+  Connect();
+  DoRemoteSend(
+      [&](TestConnection *TC) { EXPECT_TRUE(TC->Send(kLeakCallback)); });
+  EXPECT_EQ(LeakCallbackReq.PID, PID);
+}
+
+static struct : public StaticTestMessage {
+  size_t MallocSize;
+} MallocLimitCallbackReq;
+
+extern "C" void FuzzerMallocLimitCallback(unsigned long PID,
+                                          size_t MallocSize) {
+  IPCProxyLinuxTest::OnProxyReceive([&]() {
+    MallocLimitCallbackReq.PID = PID;
+    MallocLimitCallbackReq.MallocSize = MallocSize;
+  });
+}
+
+TEST_F(IPCProxyLinuxTest, MallocLimitCallback) {
+  Connect();
+  auto MallocSize = Pick<size_t>();
+  DoRemoteSend([&](TestConnection *TC) {
+    EXPECT_TRUE(TC->Send(kMallocLimitCallback));
+    EXPECT_TRUE(TC->Send(MallocSize));
+  });
+  EXPECT_EQ(MallocLimitCallbackReq.PID, PID);
+  EXPECT_EQ(MallocLimitCallbackReq.MallocSize, MallocSize);
+}
+
+static StaticTestMessage RssLimitCallbackReq;
+
+extern "C" void FuzzerRssLimitCallback(unsigned long PID) {
+  IPCProxyLinuxTest::OnProxyReceive([&]() { RssLimitCallbackReq.PID = PID; });
+}
+
+TEST_F(IPCProxyLinuxTest, RssLimitCallback) {
+  Connect();
+  DoRemoteSend(
+      [&](TestConnection *TC) { EXPECT_TRUE(TC->Send(kRssLimitCallback)); });
+  EXPECT_EQ(RssLimitCallbackReq.PID, PID);
+}
+
+TEST_F(IPCProxyLinuxTest, PrintPC) {
+  Connect();
+  const char *SymbolizedFMTIn = "symbolized format: %lu";
+  const char *FallbackFMTIn = "fallback format: %lu";
+  auto PCIn = Pick<uintptr_t>();
+  DoProxySend([&]() {
+    FuzzerRemotePrintPC(PID, SymbolizedFMTIn, FallbackFMTIn, PCIn);
+  });
+
+  ProxyOrdinal Ordinal;
+  char SymbolizedFMTOut[kMaxDescLen];
+  char FallbackFMTOut[kMaxDescLen];
+  uintptr_t PCOut;
+  OnRemoteReceive([&](TestConnection *TC) {
+    EXPECT_TRUE(TC->Receive(&Ordinal));
+    EXPECT_TRUE(TC->ReceiveString(SymbolizedFMTOut, sizeof(SymbolizedFMTOut)));
+    EXPECT_TRUE(TC->ReceiveString(FallbackFMTOut, sizeof(FallbackFMTOut)));
+    EXPECT_TRUE(TC->Receive(&PCOut));
+    EXPECT_TRUE(TC->Send(Connection::kAcknowledgement));
+  });
+  EXPECT_EQ(Ordinal, kPrintPC);
+  EXPECT_STREQ(SymbolizedFMTIn, SymbolizedFMTOut);
+  EXPECT_STREQ(FallbackFMTIn, FallbackFMTOut);
+  EXPECT_EQ(PCIn, PCOut);
+}
+
+TEST_F(IPCProxyLinuxTest, DescribePC) {
+  Connect();
+  const char *SymbolizedFMTIn = "symbolized format: %lu";
+  auto PCIn = Pick<uintptr_t>();
+  char Expected[kMaxDescLen];
+  snprintf(Expected, sizeof(Expected), SymbolizedFMTIn, PCIn);
+  char Actual[kMaxDescLen];
+  DoProxySend([&]() {
+    FuzzerRemoteDescribePC(PID, SymbolizedFMTIn, PCIn, Actual, sizeof(Actual));
+  });
+
+  ProxyOrdinal Ordinal;
+  char SymbolizedFMTOut[kMaxDescLen];
+  uintptr_t PCOut;
+  char Desc[kMaxDescLen];
+  size_t DescLen;
+  OnRemoteReceive([&](TestConnection *TC) {
+    EXPECT_TRUE(TC->Receive(&Ordinal));
+    EXPECT_TRUE(TC->ReceiveString(SymbolizedFMTOut, sizeof(SymbolizedFMTOut)));
+    EXPECT_TRUE(TC->Receive(&PCOut));
+    EXPECT_TRUE(TC->Receive(&DescLen));
+    ASSERT_LE(DescLen, sizeof(Desc));
+    snprintf(Desc, DescLen, SymbolizedFMTOut, PCOut);
+    EXPECT_TRUE(TC->Send(kDescribePCResponse));
+    EXPECT_TRUE(TC->SendString(Desc));
+  });
+
+  EXPECT_EQ(Ordinal, kDescribePC);
+  EXPECT_STREQ(SymbolizedFMTIn, SymbolizedFMTOut);
+  EXPECT_EQ(PCIn, PCOut);
+  EXPECT_EQ(DescLen, sizeof(Actual));
+  EXPECT_STREQ(Expected, Actual);
+}
+
+TEST_F(IPCProxyLinuxTest, PrintStackTrace) {
+  Connect();
+  DoProxySend([&]() { FuzzerRemotePrintStackTrace(PID); });
+
+  ProxyOrdinal Ordinal;
+  OnRemoteReceive([&](TestConnection *TC) {
+    EXPECT_TRUE(TC->Receive(&Ordinal));
+    EXPECT_TRUE(TC->Send(Connection::kAcknowledgement));
+  });
+  EXPECT_EQ(Ordinal, kPrintStackTrace);
+}
+
+TEST_F(IPCProxyLinuxTest, PrintMemoryProfile) {
+  Connect();
+  DoProxySend([&]() { FuzzerRemotePrintMemoryProfile(PID); });
+
+  ProxyOrdinal Ordinal;
+  OnRemoteReceive([&](TestConnection *TC) {
+    EXPECT_TRUE(TC->Receive(&Ordinal));
+    EXPECT_TRUE(TC->Send(Connection::kAcknowledgement));
+  });
+  EXPECT_EQ(Ordinal, kPrintMemoryProfile);
+}
+
+TEST_F(IPCProxyLinuxTest, DetectLeaksAtExit) {
+  Connect();
+  DoProxySend([&]() { FuzzerRemoteDetectLeaksAtExit(PID); });
+
+  ProxyOrdinal Ordinal;
+  OnRemoteReceive([&](TestConnection *TC) {
+    EXPECT_TRUE(TC->Receive(&Ordinal));
+    EXPECT_TRUE(TC->Send(Connection::kAcknowledgement));
+  });
+  EXPECT_EQ(Ordinal, kDetectLeaksAtExit);
+}
+
+} // namespace
+} // namespace ipc
+} // namespace fuzzer
+
+#endif // LIBFUZZER_LINUX
Index: compiler-rt/test/fuzzer/CMakeLists.txt
===================================================================
--- compiler-rt/test/fuzzer/CMakeLists.txt
+++ compiler-rt/test/fuzzer/CMakeLists.txt
@@ -22,6 +22,7 @@
   list(APPEND LIBFUZZER_TEST_DEPS FuzzedDataProviderUnitTests)
   list(APPEND LIBFUZZER_TEST_DEPS FuzzerRemoteUnitTests)
   list(APPEND LIBFUZZER_TEST_DEPS FuzzerIPCUnitTests)
+  list(APPEND LIBFUZZER_TEST_DEPS FuzzerIPCProxyUnitTests)
 endif()
 
 add_custom_target(check-fuzzer)