Index: llvm/lib/BinaryFormat/Magic.cpp
===================================================================
--- llvm/lib/BinaryFormat/Magic.cpp
+++ llvm/lib/BinaryFormat/Magic.cpp
@@ -14,6 +14,7 @@
 #include "llvm/BinaryFormat/MachO.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/MemoryBuffer.h"
 
 #if !defined(_MSC_VER) && !defined(__MINGW32__)
 #include <unistd.h>
@@ -214,6 +215,22 @@
   if (close(FD) != 0 || Length < 0)
     return std::error_code(errno, std::generic_category());
 
-  Result = identify_magic(StringRef(Buffer, Length));
+  std::unique_ptr<MemoryBuffer> FileBuffer;
+  StringRef BufferStr(Buffer, Length);
+  // If this might be a PE file, we need to read the whole file, because
+  // verification requires seeking to a certain offset, which is only determined
+  // by examining a field in the file.  Since we don't know what that offset
+  // will be we have to assume it could be anywhere, and thus we need to read
+  // the entire file.
+  if (BufferStr.startswith("MZ")) {
+    auto FileOrError = MemoryBuffer::getFile(Path);
+    if (!FileOrError)
+      return FileOrError.getError();
+    FileBuffer = std::move(*FileOrError);
+    BufferStr = FileBuffer->getBuffer();
+  }
+
+  Result = identify_magic(BufferStr);
+
   return std::error_code();
 }