Index: llvm/include/llvm/DebugInfo/MSF/MSFError.h
===================================================================
--- llvm/include/llvm/DebugInfo/MSF/MSFError.h
+++ llvm/include/llvm/DebugInfo/MSF/MSFError.h
@@ -23,7 +23,8 @@
   not_writable,
   no_stream,
   invalid_format,
-  block_in_use
+  block_in_use,
+  stream_directory_overflow
 };
 } // namespace msf
 } // namespace llvm
Index: llvm/lib/DebugInfo/MSF/MSFBuilder.cpp
===================================================================
--- llvm/lib/DebugInfo/MSF/MSFBuilder.cpp
+++ llvm/lib/DebugInfo/MSF/MSFBuilder.cpp
@@ -364,6 +364,18 @@
                 FileSize, Layout.SB->BlockSize));
   }
 
+  uint64_t NumDirectoryBlocks =
+      bytesToBlocks(Layout.SB->NumDirectoryBytes, Layout.SB->BlockSize);
+  uint64_t DirectoryBlockMapSize =
+      NumDirectoryBlocks * sizeof(support::ulittle32_t);
+  if (DirectoryBlockMapSize > Layout.SB->BlockSize) {
+    return make_error<MSFError>(msf_error_code::stream_directory_overflow,
+                                formatv("The directory block map ({0} bytes) "
+                                        "doesn't fit in a block ({1} bytes)",
+                                        DirectoryBlockMapSize,
+                                        Layout.SB->BlockSize));
+  }
+
   auto OutFileOrError = FileOutputBuffer::create(Path, FileSize);
   if (auto EC = OutFileOrError.takeError())
     return std::move(EC);
Index: llvm/lib/DebugInfo/MSF/MSFError.cpp
===================================================================
--- llvm/lib/DebugInfo/MSF/MSFError.cpp
+++ llvm/lib/DebugInfo/MSF/MSFError.cpp
@@ -43,6 +43,8 @@
       return "The data is in an unexpected format.";
     case msf_error_code::block_in_use:
       return "The block is already in use.";
+    case msf_error_code::stream_directory_overflow:
+      return "PDB Stream directory too large.";
     }
     llvm_unreachable("Unrecognized msf_error_code");
   }