Index: lib/sanitizer_common/sanitizer_flags.inc
===================================================================
--- lib/sanitizer_common/sanitizer_flags.inc
+++ lib/sanitizer_common/sanitizer_flags.inc
@@ -207,3 +207,5 @@
             "(asan only).")
 COMMON_FLAG(bool, html_cov_report, false, "Generate html coverage report.")
 COMMON_FLAG(const char *, sancov_path, "sancov", "Sancov tool location.")
+COMMON_FLAG(bool, disable_reexec, false, "Don't re-execute the program even "
+            " when interceptors aren't installed.")
Index: lib/sanitizer_common/sanitizer_mac.cc
===================================================================
--- lib/sanitizer_common/sanitizer_mac.cc
+++ lib/sanitizer_common/sanitizer_mac.cc
@@ -573,7 +573,7 @@
 }
 
 void MaybeReexec() {
-  if (reexec_disabled) return;
+  if (reexec_disabled || common_flags()->disable_reexec) return;
 
   // Make sure the dynamic runtime library is preloaded so that the
   // wrappers work. If it is not, set DYLD_INSERT_LIBRARIES and re-exec
@@ -627,6 +627,24 @@
     CHECK("execv failed" && 0);
   }
 
+  // Verify that interceptors really work.  We'll use dlsym to locate
+  // "pthread_create", if interceptors are working, it should really point to
+  // "wrap_pthread_create" within our own dylib.
+  Dl_info info_pthread_create;
+  void *dlopen_addr = dlsym(RTLD_DEFAULT, "pthread_create");
+  CHECK(dladdr(dlopen_addr, &info_pthread_create));
+  Report("info.dli_fname = %p = %s, info_pthread_create.dli_fname = %p = %s\n",
+         info.dli_fname, info.dli_fname, info_pthread_create.dli_fname,
+         info_pthread_create.dli_fname);
+  if (internal_strcmp(info.dli_fname, info_pthread_create.dli_fname) != 0) {
+    Report(
+        "ERROR: Interceptors are not working. This may be because %s is "
+        "loaded too late (e.g. via dlopen). Please launch the executable "
+        "with:\n%s=%s\n",
+        SanitizerToolName, kDyldInsertLibraries, info.dli_fname);
+    CHECK("interceptors not installed" && 0);
+  }
+
   if (!lib_is_in_env)
     return;
 
Index: lib/tsan/tests/rtl/tsan_test.cc
===================================================================
--- lib/tsan/tests/rtl/tsan_test.cc
+++ lib/tsan/tests/rtl/tsan_test.cc
@@ -49,8 +49,10 @@
 
 #ifdef __APPLE__
 // On Darwin, turns off symbolication and crash logs to make tests faster.
+// We also need "disable_reexec=1" to make sure unit tests can run even
+// when interceptors are not properly installed.
 extern "C" const char* __tsan_default_options() {
-  return "symbolize=false:abort_on_error=0";
+  return "symbolize=false:abort_on_error=0:disable_reexec=1";
 }
 #endif
 
Index: lib/tsan/tests/unit/tsan_flags_test.cc
===================================================================
--- lib/tsan/tests/unit/tsan_flags_test.cc
+++ lib/tsan/tests/unit/tsan_flags_test.cc
@@ -132,10 +132,8 @@
   EXPECT_EQ(f->die_after_fork, false);
 }
 
-static const char *test_default_options;
-extern "C" const char *__tsan_default_options() {
-  return test_default_options;
-}
+// Defined in tsan_unit_test_main.cc.
+extern const char *test_default_options;
 
 TEST(Flags, ParseDefaultOptions) {
   Flags f;
Index: lib/tsan/tests/unit/tsan_unit_test_main.cc
===================================================================
--- lib/tsan/tests/unit/tsan_unit_test_main.cc
+++ lib/tsan/tests/unit/tsan_unit_test_main.cc
@@ -12,6 +12,24 @@
 //===----------------------------------------------------------------------===//
 #include "gtest/gtest.h"
 
+namespace __tsan {
+
+// Used for testing in tsan_flags_test.cc.
+const char *test_default_options = nullptr;
+
+extern "C" const char* __tsan_default_options() {
+  if (test_default_options) return test_default_options;
+#ifdef __APPLE__
+  // On Darwin, we need "disable_reexec=1" to make sure unit tests can run even
+  // when interceptors are not properly installed.
+  return "disable_reexec=1";
+#else
+  return "";
+#endif
+}
+
+}  // namespace __tsan
+
 int main(int argc, char **argv) {
   testing::GTEST_FLAG(death_test_style) = "threadsafe";
   testing::InitGoogleTest(&argc, argv);
Index: test/lit.common.cfg
===================================================================
--- test/lit.common.cfg
+++ test/lit.common.cfg
@@ -118,6 +118,14 @@
 
 lit.util.usePlatformSdkOnDarwin(config, lit_config)
 
+if config.host_os == 'Darwin':
+  ld_cmd = subprocess.Popen(["bash", "-c", "sw_vers -productVersion | awk -F '.' '{print $1 \".\" $2}'"], stdout = subprocess.PIPE)
+  ld_out = ld_cmd.stdout.read().decode()
+  ld_cmd.wait()
+  osx_version = float(ld_out)
+  if osx_version >= 10.11:
+    config.available_features.add('osx-autointerception')
+
 sancovcc_path = os.path.join(llvm_tools_dir, "sancov") 
 if os.path.exists(sancovcc_path):
   config.available_features.add("has_sancovcc")
Index: test/tsan/Darwin/dlopen.cc
===================================================================
--- test/tsan/Darwin/dlopen.cc
+++ test/tsan/Darwin/dlopen.cc
@@ -0,0 +1,41 @@
+// Checks that on OS X 10.11+ (where we do not re-exec anymore, because
+// interceptors work automatically), dlopen'ing a TSanified library from a
+// non-instrumented program exits with a user-friendly message.
+
+// REQUIRES: osx-autointerception
+
+// RUN: %clangxx_tsan %s -o %t.so -shared -DSHARED_LIB
+// RUN: %clangxx_tsan -fno-sanitize=thread %s -o %t
+
+// RUN: TSAN_DYLIB_PATH=`%clangxx_tsan %s -### 2>&1 \
+// RUN:   | grep "libclang_rt.tsan_osx_dynamic.dylib" \
+// RUN:   | sed -e 's/.*"\(.*libclang_rt.tsan_osx_dynamic.dylib\)".*/\1/'`
+
+// Launching a non-instrumented binary that dlopen's an instrumented library should fail.
+// RUN: not %run %t %t.so 2>&1 | FileCheck %s --check-prefix=CHECK-FAIL
+// Launching a non-instrumented binary with an explicit DYLD_INSERT_LIBRARIES should work.
+// RUN: DYLD_INSERT_LIBRARIES=$TSAN_DYLIB_PATH %run %t %t.so 2>&1 | FileCheck %s
+
+#include <dlfcn.h>
+#include <pthread.h>
+#include <stdio.h>
+
+#if defined(SHARED_LIB)
+extern "C" void foo() {
+  fprintf(stderr, "Hello world.\n");
+}
+#else  // defined(SHARED_LIB)
+int main(int argc, char *argv[]) {
+  void *handle = dlopen(argv[1], RTLD_NOW);
+  fprintf(stderr, "handle = %p\n", handle);
+  void (*foo)() = (void (*)())dlsym(handle, "foo");
+  fprintf(stderr, "foo = %p\n", foo);
+  foo();
+}
+#endif  // defined(SHARED_LIB)
+
+// CHECK: Hello world.
+// CHECK-NOT: ERROR: Interceptors are not working.
+
+// CHECK-FAIL-NOT: Hello world.
+// CHECK-FAIL: ERROR: Interceptors are not working.