debug/pe: support PE files which contain no symbol table (if NumberOfSymbols is…

debug/pe: support PE files which contain no symbol table (if NumberOfSymbols is equal to 0 in the IMAGE_FILE_HEADER structure).

No longer assume that e_lfanew (in the IMAGE_DOS_HEADER strcuture) is always one byte. It is now regarded as a 4 byte uint32.

Fixes #4177.

R=golang-dev, alex.brainman, dave, minux.ma
CC=golang-dev
https://golang.org/cl/6587048

src/pkg/debug/pe/file.go

@@ -131,12 +131,13 @@ func NewFile(r io.ReaderAt) (*File, error) {
 	}
 	var base int64
 	if dosheader[0] == 'M' && dosheader[1] == 'Z' {
+		signoff := int64(binary.LittleEndian.Uint32(dosheader[0x3c:]))
 		var sign [4]byte
-		r.ReadAt(sign[0:], int64(dosheader[0x3c]))
+		r.ReadAt(sign[:], signoff)
 		if !(sign[0] == 'P' && sign[1] == 'E' && sign[2] == 0 && sign[3] == 0) {
 			return nil, errors.New("Invalid PE File Format.")
 		}
-		base = int64(dosheader[0x3c]) + 4
+		base = signoff + 4
 	} else {
 		base = int64(0)
 	}
@@ -148,13 +149,15 @@ func NewFile(r io.ReaderAt) (*File, error) {
 		return nil, errors.New("Invalid PE File Format.")
 	}
 
+	var ss []byte
+	if f.FileHeader.NumberOfSymbols > 0 {
 		// Get COFF string table, which is located at the end of the COFF symbol table.
 		sr.Seek(int64(f.FileHeader.PointerToSymbolTable+COFFSymbolSize*f.FileHeader.NumberOfSymbols), os.SEEK_SET)
 		var l uint32
 		if err := binary.Read(sr, binary.LittleEndian, &l); err != nil {
 			return nil, err
 		}
-	ss := make([]byte, l)
+		ss = make([]byte, l)
 		if _, err := r.ReadAt(ss, int64(f.FileHeader.PointerToSymbolTable+COFFSymbolSize*f.FileHeader.NumberOfSymbols)); err != nil {
 			return nil, err
 		}
@@ -188,6 +191,7 @@ func NewFile(r io.ReaderAt) (*File, error) {
 			}
 			f.Symbols = append(f.Symbols, s)
 		}
+	}
 
 	// Process sections.
 	sr.Seek(base, os.SEEK_SET)