1 files changed, 21 insertions, 0 deletions
diff --git a/llvm/lib/Fuzzer/afl/afl_driver.cpp b/llvm/lib/Fuzzer/afl/afl_driver.cpp
index 63aebab469c..228317ca9e3 100644
--- a/llvm/lib/Fuzzer/afl/afl_driver.cpp
+++ b/llvm/lib/Fuzzer/afl/afl_driver.cpp
@@ -60,6 +60,25 @@ static volatile char suppress_warning1 = AFL_DEFER_FORKSVR[0];
 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 @@ int main(int argc, char **argv) {
     LLVMFuzzerInitialize(&argc, &argv);
   // Do any other expensive one-time initialization here.
 
+  maybe_duplicate_stderr();
+
   __afl_manual_init();
 
   int N = 1000;