[DFSan] Don't unmap during dfsan_flush().

[morehouse]

Summary

Unmapping and remapping is dangerous since another thread could touch
the shadow memory while it is unmapped. But there is really no need to
unmap anyway, since mmap(MAP_FIXED) will happily clobber the existing
mapping with zeroes. This is thread-safe since the mmap() is done under
the same kernel lock as page faults are done.

[morehouse]

• Add note about use in multithreaded environment.

[kcc]

Would it be possible to add a threaded test that fails w/o this change?
LGTM otherwise, thanks!

[morehouse]

• Add multithreaded dfsan_flush() test.

compiler-rt/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.
+/// with the application memory.  Use this call to start over the taint tracking
+/// within the same process.
+///
+/// Note: If another thread is working with tainted data during the flush, that
+/// taint could still be written to shadow after the flush.
 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

compiler-rt/lib/dfsan/dfsan.cpp

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();

compiler-rt/test/dfsan/threaded_flush.c

+// Tests that doing dfsan_flush() while another thread is executing doesn't
+// segfault.
+// RUN: %clang_dfsan %s -o %t && %run %t
+#include <assert.h>
+#include <pthread.h>
+#include <sanitizer/dfsan_interface.h>
+#include <stdlib.h>
+
+static unsigned char GlobalBuf[4096];
+static int ShutDownThread;
+static int StartFlush;
+
+// Access GlobalBuf continuously, causing its shadow to be touched as well.
+// When main() calls dfsan_flush(), no segfault should be triggered.
+static void *accessGlobalInBackground(void *Arg) {
+  __atomic_store_n(&StartFlush, 1, __ATOMIC_RELEASE);
+
+  while (!__atomic_load_n(&ShutDownThread, __ATOMIC_ACQUIRE))
+    for (unsigned I = 0; I < sizeof(GlobalBuf); ++I)
+      ++GlobalBuf[I];
+
+  return NULL;
+}
+
+int main() {
+  pthread_t Thread;
+  pthread_create(&Thread, NULL, accessGlobalInBackground, NULL);
+  while (!__atomic_load_n(&StartFlush, __ATOMIC_ACQUIRE))
+    ; // Spin
+
+  dfsan_flush();
+
+  __atomic_store_n(&ShutDownThread, 1, __ATOMIC_RELEASE);
+  pthread_join(Thread, NULL);
+  return 0;
+}