[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.
+/// with the application memory.
 /// 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);
 

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();