debug/elf: restore Go 1.0 semantics for (*File).Symbols

Also adjust the implementation of applyRelocationsAMD64
so that the test added in CL 6848044 still passes.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7686049

doc/go1.1.html

@@ -258,28 +258,6 @@ TODO introduction
 TODO
 </p>
 
-<h3 id="debug_elf">debug/elf</h3>
-
-<p>
-TODO: Decide whether to keep this change. See CL 6848044.
-</p>
-
-<p>
-Previous versions of the <a href="/debug/elf/"><code>debug/elf</code></a> package
-intentionally skipped over the first
-symbol in the ELF symbol table, since it is always an empty symbol.
-This symbol
-is no longer skipped since indexes into the symbol table returned by <code>debug/elf</code>
-will be different from indexes into the original ELF symbol table.
-Any code that calls the methods
-<a href="/pkg/debug/elf/#File.Symbols"><code>Symbols</code></a>
-or
-<a href="/pkg/debug/elf/#File.ImportedSymbols"><code>ImportedSymbols</code></a>
-of the
-<a href="/pkg/debug/elf/#File"><code>elf.File</code></a>
-type may need to be adjusted to account for the additional symbol and the change in symbol offsets.
-</p>
-
 <h3 id="net">net</h3>
 
 <p>

src/pkg/debug/elf/file.go

@@ -422,6 +422,10 @@ func (f *File) getSymbols32(typ SectionType) ([]Symbol, []byte, error) {
 		return nil, nil, errors.New("cannot load string table section")
 	}
 
+	// The first entry is all zeros.
+	var skip [Sym32Size]byte
+	symtab.Read(skip[:])
+
 	symbols := make([]Symbol, symtab.Len()/Sym32Size)
 
 	i := 0
@@ -461,6 +465,10 @@ func (f *File) getSymbols64(typ SectionType) ([]Symbol, []byte, error) {
 		return nil, nil, errors.New("cannot load string table section")
 	}
 
+	// The first entry is all zeros.
+	var skip [Sym64Size]byte
+	symtab.Read(skip[:])
+
 	symbols := make([]Symbol, symtab.Len()/Sym64Size)
 
 	i := 0
@@ -533,10 +541,10 @@ func (f *File) applyRelocationsAMD64(dst []byte, rels []byte) error {
 		symNo := rela.Info >> 32
 		t := R_X86_64(rela.Info & 0xffff)
 
-		if symNo >= uint64(len(symbols)) {
+		if symNo == 0 || symNo > uint64(len(symbols)) {
 			continue
 		}
-		sym := &symbols[symNo]
+		sym := &symbols[symNo-1]
 		if SymType(sym.Info&0xf) != STT_SECTION {
 			// We don't handle non-section relocations for now.
 			continue
@@ -597,6 +605,10 @@ func (f *File) DWARF() (*dwarf.Data, error) {
 }
 
 // Symbols returns the symbol table for f.
+//
+// For compatibility with Go 1.0, Symbols omits the null symbol at index 0.
+// After retrieving the symbols as symtab, an externally supplied index x
+// corresponds to symtab[x-1], not symtab[x].
 func (f *File) Symbols() ([]Symbol, error) {
 	sym, _, err := f.getSymbols(SHT_SYMTAB)
 	return sym, err