[dfsan] Introduce dfsan_flush().

[kcc]

Summary

dfsan_flush() allows to restart tain tracking from scratch in the same process.
The primary purpose right now is to allow more efficient data flow tracing
for DFT fuzzing: https://github.com/google/oss-fuzz/issues/1632

[pcc]

LGTM

include/sanitizer/dfsan_interface.h

 
 /// If the given label label contains a label with the description desc, returns
 /// that label, else returns 0.
 dfsan_label dfsan_has_label_with_desc(dfsan_label label, const char *desc);
 
 /// Returns the number of labels allocated.
 size_t dfsan_get_label_count(void);
 
+/// Flushes the DFSan shadow, i.e. forgets about all labels currently associated
+/// with the application memory. Will work only if there are no other
+/// threads executing DFSan-instrumented code concurrently.
+/// Use this call to start over the taint tracking within the same procces.
+void dfsan_flush(void);
+
 /// Sets a callback to be invoked on calls to write().  The callback is invoked
 /// before the write is done.  The write is not guaranteed to succeed when the
 /// callback executes.  Pass in NULL to remove any callback.
 void dfsan_set_write_callback(dfsan_write_callback_t labeled_write_callback);
 
 /// Writes the labels currently used by the program to the given file
 /// descriptor. The lines of the output have the following format:
 ///

lib/dfsan/dfsan.cc

if (internal_strcmp(flags().dump_labels_at_exit, "") != 0) {
 
     Report("INFO: DataFlowSanitizer: dumping labels to %s\n",
            flags().dump_labels_at_exit);
     dfsan_dump_labels(fd);
     CloseFile(fd);
   }
 }
 
+extern "C" void dfsan_flush() {
+  UnmapOrDie((void*)ShadowAddr(), UnusedAddr() - ShadowAddr());
+  if (!MmapFixedNoReserve(ShadowAddr(), UnusedAddr() - ShadowAddr()))
+    Die();
+}
+
 static void dfsan_init(int argc, char **argv, char **envp) {
   InitializeFlags();
 
   ::InitializePlatformEarly();
 
   if (!MmapFixedNoReserve(ShadowAddr(), UnusedAddr() - ShadowAddr()))
     Die();
 

lib/dfsan/done_abilist.txt

 fun:dfsan_get_label_info=uninstrumented
 fun:dfsan_get_label_info=discard
 fun:dfsan_has_label=uninstrumented
 fun:dfsan_has_label=discard
 fun:dfsan_has_label_with_desc=uninstrumented
 fun:dfsan_has_label_with_desc=discard
 fun:dfsan_set_write_callback=uninstrumented
 fun:dfsan_set_write_callback=custom
+fun:dfsan_flush=uninstrumented
+fun:dfsan_flush=discard
 
 ###############################################################################
 # glibc
 ###############################################################################
 fun:malloc=discard
 fun:realloc=discard
 fun:free=discard
 

test/dfsan/flush.c

+// Tests dfsan_flush().
+// RUN: %clang_dfsan %s -o %t && %run %t
+#include <sanitizer/dfsan_interface.h>
+#include <assert.h>
+#include <stdlib.h>
+
+int global;
+
+int main() {
+  int local;
+  int *heap = (int*)malloc(sizeof(int));
+
+  dfsan_set_label(10, &global, sizeof(global));
+  dfsan_set_label(20, &local, sizeof(local));
+  dfsan_set_label(30, heap, sizeof(*heap));
+
+  assert(dfsan_get_label(global) == 10);
+  assert(dfsan_get_label(local) == 20);
+  assert(dfsan_get_label(*heap) == 30);
+
+  dfsan_flush();
+
+  assert(dfsan_get_label(global) == 0);
+  assert(dfsan_get_label(local) == 0);
+  assert(dfsan_get_label(*heap) == 0);
+
+  free(heap);
+}