Fix race condition in GetPreResolvedStrings

DexCache::VisitReferences had a race condition where a null pointer
could be accessed if there was another concurrent caller doing
ClearPreResolvedStrings.

Fix is to add an extra null check that prevents the race.

Bug: 152716138
Test: test-art-host
Change-Id: Ib447ce689e23f7c20d4cda8b457abeb89ade9291

1 files changed, 5 insertions, 3 deletions

diff --git a/runtime/mirror/dex_cache-inl.h b/runtime/mirror/dex_cache-inl.h
index 80b5a346fc..4f23273065 100644
--- a/runtime/mirror/dex_cache-inl.h
+++ b/runtime/mirror/dex_cache-inl.h
@@ -382,9 +382,11 @@ inline void DexCache::VisitReferences(ObjPtr<Class> klass, const Visitor& visito
     }
 
     GcRoot<mirror::String>* const preresolved_strings = GetPreResolvedStrings();
-    const size_t num_preresolved_strings = NumPreResolvedStrings();
-    for (size_t i = 0; i != num_preresolved_strings; ++i) {
-      visitor.VisitRootIfNonNull(preresolved_strings[i].AddressWithoutBarrier());
+    if (preresolved_strings != nullptr) {
+      const size_t num_preresolved_strings = NumPreResolvedStrings();
+      for (size_t i = 0; i != num_preresolved_strings; ++i) {
+        visitor.VisitRootIfNonNull(preresolved_strings[i].AddressWithoutBarrier());
+      }
     }
   }
 }