debug/pe: introduce Section.Relocs

cmd/link reads PE object files when building programs with cgo.
cmd/link accesses object relocations. Add new Section.Relocs that
provides similar functionality in debug/pe.

Updates #15345

Change-Id: I34de91b7f18cf1c9e4cdb3aedd685486a625ac92
Reviewed-on: https://go-review.googlesource.com/22332
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/debug/pe/file.go

@@ -192,6 +192,14 @@ func NewFile(r io.ReaderAt) (*File, error) {
 		s.ReaderAt = s.sr
 		f.Sections[i] = s
 	}
+	for i := range f.Sections {
+		var err error
+		f.Sections[i].Relocs, err = readRelocs(&f.Sections[i].SectionHeader, sr)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	return f, nil
 }
 

src/debug/pe/section.go

@@ -5,6 +5,8 @@
 package pe
 
 import (
+	"encoding/binary"
+	"fmt"
 	"io"
 	"strconv"
 )
@@ -37,6 +39,30 @@ func (sh *SectionHeader32) fullName(st StringTable) (string, error) {
 	return st.String(uint32(i))
 }
 
+// Reloc represents a PE COFF relocation.
+// Each section contains its own relocation list.
+type Reloc struct {
+	VirtualAddress   uint32
+	SymbolTableIndex uint32
+	Type             uint16
+}
+
+func readRelocs(sh *SectionHeader, r io.ReadSeeker) ([]Reloc, error) {
+	if sh.NumberOfRelocations <= 0 {
+		return nil, nil
+	}
+	_, err := r.Seek(int64(sh.PointerToRelocations), io.SeekStart)
+	if err != nil {
+		return nil, fmt.Errorf("fail to seek to %q section relocations: %v", sh.Name, err)
+	}
+	relocs := make([]Reloc, sh.NumberOfRelocations)
+	err = binary.Read(r, binary.LittleEndian, relocs)
+	if err != nil {
+		return nil, fmt.Errorf("fail to read section relocations: %v", err)
+	}
+	return relocs, nil
+}
+
 // SectionHeader is similar to SectionHeader32 with Name
 // field replaced by Go string.
 type SectionHeader struct {
@@ -55,6 +81,7 @@ type SectionHeader struct {
 // Section provides access to PE COFF section.
 type Section struct {
 	SectionHeader
+	Relocs []Reloc
 
 	// Embed ReaderAt for ReadAt method.
 	// Do not embed SectionReader directly