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

diff --git a/doc/go1.1.html b/doc/go1.1.html
index df495ee81ea15b4d64e6eaf18c42f1864c858edf..31d6d9b020ac9e1291e21f9c3e15ffe5d7fba439 100644 (file)
--- a/doc/go1.1.html
+++ b/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>

diff --git a/src/pkg/debug/elf/file.go b/src/pkg/debug/elf/file.go
index acb9817af024432e9af1da25ef20abd4b9401847..0d022ab910a80765d7e3c52507b33a744e5b19f3 100644 (file)
--- a/src/pkg/debug/elf/file.go
+++ b/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