Index: lib/sanitizer_common/sanitizer_mac.cc
===================================================================
--- lib/sanitizer_common/sanitizer_mac.cc
+++ lib/sanitizer_common/sanitizer_mac.cc
@@ -627,6 +627,21 @@
     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));
+  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: test/tsan/Darwin/dlopen.cc
===================================================================
--- test/tsan/Darwin/dlopen.cc
+++ test/tsan/Darwin/dlopen.cc
@@ -0,0 +1,46 @@
+// 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.
+
+// RUN: %clangxx_tsan %s -o %t-instr -DINSTRUMENTED_PART=1
+// RUN: %clangxx_tsan -fno-sanitize=thread %s -o %t-noninstr -DINSTRUMENTED_PART=0 -DINSTRUMENTED_PART_LIB_NAME="\"%t-instr\""
+
+// Launching a regular instrumented binary just works.  Either interceptors
+// already work or we re-exec.
+// RUN: %run %t-instr 2>&1 | FileCheck %s
+
+// RUN: IS_OSX_10_11_OR_HIGHER=$([ `sw_vers -productVersion | cut -d'.' -f2` -lt 11 ]; echo $?)
+// 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/'`
+
+// RUN: if [ $IS_OSX_10_11_OR_HIGHER == 1 ]; then \
+//        Launching a non-instrumented binary that dlopen's an instrumented library should fail.
+// RUN:   not %run %t-noninstr 2>&1 | FileCheck %s --check-prefix=CHECK-FAIL; \
+//        Launching a non-instrumented binary with an explicit DYLD_INSERT_LIBRARIES should work.
+// RUN:   DYLD_INSERT_LIBRARIES=$TSAN_DYLIB_PATH %run %t-noninstr 2>&1 | FileCheck %s; \
+// RUN: fi
+
+#include <dlfcn.h>
+#include <pthread.h>
+#include <stdio.h>
+
+#if INSTRUMENTED_PART == 1
+int main() {
+  fprintf(stderr, "Hello world.\n");
+}
+#else // INSTRUMENTED_PART == 1
+int main() {
+  void *handle = dlopen(INSTRUMENTED_PART_LIB_NAME, RTLD_NOW);
+  fprintf(stderr, "handle = %p\n", handle);
+  int (*other_main)() = (int (*)())dlsym(handle, "main");
+  fprintf(stderr, "other_main = %p\n", other_main);
+  other_main();
+}
+#endif
+
+// CHECK: Hello world.
+// CHECK-NOT: ERROR: Interceptors are not working.
+
+// CHECK-FAIL-NOT: Hello world.
+// CHECK-FAIL: ERROR: Interceptors are not working.