--- a/ipc/glue/Shmem.cpp
+++ b/ipc/glue/Shmem.cpp
@@ -311,16 +311,23 @@ Shmem::RevokeRights(IHadBetterBeIPDLCode
 
   if (!header->mUnsafe) {
     Protect(mSegment);
   } else {
     mSegment->Protect(reinterpret_cast<char*>(header), pageSize, RightsNone);
   }
 }
 
+void
+Shmem::ResurrectReadOnly()
+{
+  mSegment->Protect(reinterpret_cast<char*>(mSegment->memory()),
+                    mSegment->Size(), RightsRead);
+}
+
 // static
 already_AddRefed<Shmem::SharedMemory>
 Shmem::Alloc(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
              size_t aNBytes,
              SharedMemoryType aType,
              bool aUnsafe,
              bool aProtect)
 {

--- a/ipc/glue/Shmem.h
+++ b/ipc/glue/Shmem.h
@@ -179,16 +179,27 @@ public:
 #ifndef DEBUG
   void RevokeRights(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead)
   {
   }
 #else
   void RevokeRights(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead);
 #endif
 
+  // Violates the invariant that only one side has read access right
+  // to the shmem. Make sure the other side has no access to the shmem
+  // when using this. In other words, don't use it.
+#ifndef DEBUG
+  void ResurrectReadOnly()
+  {
+  }
+#else
+  void ResurrectReadOnly();
+#endif
+
   void forget(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead)
   {
     mSegment = nullptr;
     mData = nullptr;
     mSize = 0;
     mId = 0;
   }