Index: llvm/trunk/lib/Fuzzer/afl/afl_driver.cpp
===================================================================
--- llvm/trunk/lib/Fuzzer/afl/afl_driver.cpp
+++ llvm/trunk/lib/Fuzzer/afl/afl_driver.cpp
@@ -60,6 +60,25 @@
 static const size_t kMaxAflInputSize = 1 << 20;
 static uint8_t AflInputBuf[kMaxAflInputSize];
 
+// If the user asks us to duplicate stderr, then do it.
+static void maybe_duplicate_stderr() {
+  char* stderr_duplicate_filename =
+      getenv("AFL_DRIVER_STDERR_DUPLICATE_FILENAME");
+
+  if (!stderr_duplicate_filename)
+    return;
+
+  FILE* stderr_duplicate_stream =
+      freopen(stderr_duplicate_filename, "a+", stderr);
+
+  if (!stderr_duplicate_stream) {
+    fprintf(stderr,
+            "Failed to duplicate stderr to AFL_DRIVER_STDERR_DUPLICATE_FILENAME"
+            );
+    abort();
+  }
+}
+
 int main(int argc, char **argv) {
   fprintf(stderr, "Running in AFl-fuzz mode\nUsage:\n"
                   "afl-fuzz [afl-flags] %s [N] "
@@ -70,6 +89,8 @@
     LLVMFuzzerInitialize(&argc, &argv);
   // Do any other expensive one-time initialization here.
 
+  maybe_duplicate_stderr();
+
   __afl_manual_init();
 
   int N = 1000;
Index: llvm/trunk/lib/Fuzzer/test/AFLDriverTest.cpp
===================================================================
--- llvm/trunk/lib/Fuzzer/test/AFLDriverTest.cpp
+++ llvm/trunk/lib/Fuzzer/test/AFLDriverTest.cpp
@@ -0,0 +1,12 @@
+#include <stdint.h>
+#include <stdlib.h>
+
+extern "C" void __afl_manual_init() {}
+
+extern "C" int __afl_persistent_loop(unsigned int) {
+  return 0;
+}
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+  return 0;
+}
Index: llvm/trunk/lib/Fuzzer/test/CMakeLists.txt
===================================================================
--- llvm/trunk/lib/Fuzzer/test/CMakeLists.txt
+++ llvm/trunk/lib/Fuzzer/test/CMakeLists.txt
@@ -110,6 +110,19 @@
 endforeach()
 
 ###############################################################################
+# AFL Driver test
+###############################################################################
+
+add_executable(AFLDriverTest
+  AFLDriverTest.cpp ../afl/afl_driver.cpp)
+
+set_target_properties(AFLDriverTest
+    PROPERTIES RUNTIME_OUTPUT_DIRECTORY
+    "${CMAKE_BINARY_DIR}/lib/Fuzzer/test"
+    )
+set(TestBinaries ${TestBinaries} AFLDriverTest)
+
+###############################################################################
 # Unit tests
 ###############################################################################
 
Index: llvm/trunk/lib/Fuzzer/test/afl-driver.test
===================================================================
--- llvm/trunk/lib/Fuzzer/test/afl-driver.test
+++ llvm/trunk/lib/Fuzzer/test/afl-driver.test
@@ -0,0 +1,10 @@
+; Test that not specifying a file isn't broken.
+RUN: unset AFL_DRIVER_STDERR_DUPLICATE_FILENAME
+RUN: AFLDriverTest
+
+; Test that specifying an invalid file causes a crash.
+RUN: AFL_DRIVER_STDERR_DUPLICATE_FILENAME="%T" not --crash AFLDriverTest
+
+; Test that a file is created when specified as the duplicate stderr.
+RUN: AFL_DRIVER_STDERR_DUPLICATE_FILENAME=%t AFLDriverTest
+RUN: stat %t