pe: parse overlay at end of PE file

Author: mewmew

Diff for: overlay.go

@@ -0,0 +1,46 @@
+package pe
+
+import (
+	"io"
+
+	"github.com/pkg/errors"
+)
+
+// Overlay returns the overlay of the PE fil (i.e. any optional bytes directly
+// succeeding the image).
+func (file *File) Overlay() ([]byte, error) {
+	if file.overlay == nil {
+		if err := file.parseOverlay(); err != nil {
+			return nil, err
+		}
+	}
+	return file.overlay, nil
+}
+
+// parseOverlay parses the overlay of the PE file.
+func (file *File) parseOverlay() error {
+	// Locate start of overlay (i.e. end of image).
+	overlayStart := int64(0)
+	sectHdrs, err := file.SectHeaders()
+	if err != nil {
+		return errors.WithStack(err)
+	}
+	for _, sectHdr := range sectHdrs {
+		sectStart := int64(sectHdr.Offset + sectHdr.Size)
+		if sectStart > overlayStart {
+			overlayStart = sectStart
+		}
+	}
+	// Locate end of overlay (i.e. end of file).
+	overlayEnd, err := file.r.Seek(0, io.SeekEnd)
+	if err != nil {
+		return errors.WithStack(err)
+	}
+	overlaySize := overlayEnd - overlayStart
+	overlay := make([]byte, overlaySize)
+	if _, err := file.r.ReadAt(overlay, overlayStart); err != nil {
+		return errors.WithStack(err)
+	}
+	file.overlay = overlay
+	return nil
+}

Diff for: pe.go

@@ -18,8 +18,10 @@ type File struct {
 	opthdr *OptHeader
 	// Section headers.
 	sectHdrs []*SectHeader
+	// Overlay.
+	overlay []byte
 	// Underlying reader.
-	r io.ReaderAt
+	r ReadAtSeeker
 	io.Closer
 }
 
@@ -40,8 +42,14 @@ func Open(path string) (file *File, err error) {
 	return file, nil
 }
 
+// ReadAtSeeker is the interface that wraps the basic ReadAt and Seek methods.
+type ReadAtSeeker interface {
+	io.ReaderAt
+	io.Seeker
+}
+
 // New returns a new File for accessing the PE binary of r.
-func New(r io.ReaderAt) (file *File, err error) {
+func New(r ReadAtSeeker) (file *File, err error) {
 	// TODO(u): Figure out which headers that should always be parsed.
 	//    * DOS header
 	//       - Contains no relevant information, but is required to locate the