Index: include/sanitizer/dfsan_interface.h
===================================================================
--- include/sanitizer/dfsan_interface.h
+++ include/sanitizer/dfsan_interface.h
@@ -52,6 +52,13 @@
 /// Sets the label for each address in [addr,addr+size) to \c label.
 void dfsan_set_label(dfsan_label label, void *addr, size_t size);
 
+// Reset labels and shadow memory for dfsan to restart from clean.
+// This is intended to work in a single threaded setting (main use
+// case is to avoid running out of labels in fuzzing).
+// This function is not safe to use in multithreaded code or when
+// there are active stack frames processing non-zero labels.
+void dfsan_reset(void);
+
 /// Sets the label for each address in [addr,addr+size) to the union of the
 /// current label for that address and \c label.
 void dfsan_add_label(dfsan_label label, void *addr, size_t size);
Index: lib/dfsan/dfsan.h
===================================================================
--- lib/dfsan/dfsan.h
+++ lib/dfsan/dfsan.h
@@ -34,6 +34,7 @@
 extern "C" {
 void dfsan_add_label(dfsan_label label, void *addr, uptr size);
 void dfsan_set_label(dfsan_label label, void *addr, uptr size);
+void dfsan_reset(void);
 dfsan_label dfsan_read_label(const void *addr, uptr size);
 dfsan_label dfsan_union(dfsan_label l1, dfsan_label l2);
 }  // extern "C"
Index: lib/dfsan/dfsan.cc
===================================================================
--- lib/dfsan/dfsan.cc
+++ lib/dfsan/dfsan.cc
@@ -26,6 +26,7 @@
 #include "sanitizer_common/sanitizer_libc.h"
 
 #include "dfsan/dfsan.h"
+#include <assert.h>
 
 using namespace __dfsan;
 
@@ -158,6 +159,20 @@
   }
 }
 
+// Reset labels and shadow memory for dfsan to restart from clean.
+// This is intended to work in a single threaded setting (to avoid
+// running out of labels in fuzzing).
+// This function is not safe to use in multithreaded code or when
+// there are active stack frames processing non-zero labels.
+extern "C" SANITIZER_INTERFACE_ATTRIBUTE
+void dfsan_reset(void) {
+  // reset label count
+  atomic_store(&__dfsan_last_label, 0, memory_order_relaxed);
+  // reset shadow memory
+  MmapFixedNoReserve(ShadowAddr(), UnusedAddr() - ShadowAddr());
+  assert(atomic_load(&__dfsan_last_label, memory_order_relaxed) == 0);
+}
+
 // Resolves the union of two unequal labels.  Nonequality is a precondition for
 // this function (the instrumentation pass inlines the equality test).
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE
Index: lib/dfsan/done_abilist.txt
===================================================================
--- lib/dfsan/done_abilist.txt
+++ lib/dfsan/done_abilist.txt
@@ -6,6 +6,8 @@
 ###############################################################################
 fun:dfsan_union=uninstrumented
 fun:dfsan_union=discard
+fun:dfsan_reset=uninstrumented
+fun:dfsan_reset=discard
 fun:dfsan_create_label=uninstrumented
 fun:dfsan_create_label=discard
 fun:dfsan_set_label=uninstrumented
Index: test/dfsan/reset.cc
===================================================================
--- /dev/null
+++ test/dfsan/reset.cc
@@ -0,0 +1,26 @@
+// RUN: %clang_dfsan %s -o %t && %run %t
+// RUN: %clang_dfsan -mllvm -dfsan-args-abi %s -o %t && %run %t
+
+// Tests that dfsan is reset correctly in single threaded code.
+
+#include <sanitizer/dfsan_interface.h>
+
+#include <assert.h>
+
+void foo() {
+  int i = 1;
+  int j = 1;
+  dfsan_label i_label = dfsan_create_label("i", 0);
+  dfsan_set_label(i_label, &i, sizeof(i));
+  dfsan_label j_label = dfsan_create_label("j", 0);
+  dfsan_add_label(j_label, &j, sizeof(j));
+}
+
+int main(void) {
+  foo();
+  assert(dfsan_get_label_count() == 2);
+  dfsan_reset();
+  assert(dfsan_get_label_count() == 0);
+
+  return 0;
+}
Index: test/dfsan/reset_multithread.cc
===================================================================
--- /dev/null
+++ test/dfsan/reset_multithread.cc
@@ -0,0 +1,30 @@
+// RUN: %clang_dfsan %s -o %t && %run %t
+// RUN: %clang_dfsan -mllvm -dfsan-args-abi %s -o %t && %run %t
+// XFAIL: *
+// Tests that dfsan runtime is reset can be violated in multithreaded settings.
+
+#include <sanitizer/dfsan_interface.h>
+
+#include <assert.h>
+#include <pthread.h>
+
+void *baz(void* unused) {
+  for(int i = 0; i < 1000000000; i++) {
+    for(int k = 0; k < 100; k++) {
+      int j = 1;
+      dfsan_label j_label = dfsan_create_label("j", 0);
+      dfsan_set_label(j_label, &j, sizeof(j));
+    }
+    dfsan_reset();
+    assert(dfsan_get_label_count() == 0);
+  }
+}
+
+int main(void) {
+  pthread_t t1, t2;
+  pthread_create(&t1, NULL, baz, NULL);
+  pthread_create(&t2, NULL, baz, NULL);
+  pthread_join(t1, NULL);
+  pthread_join(t2, NULL);
+  return 0;
+}