debug/elf: do not skip first symbol in the symbol table

Do not skip the first symbol in the symbol table. Any other indexes
into the symbol table (for example, indexes in relocation entries)
will now refer to the symbol following the one that was intended.

Add an object that contains debug relocations, which debug/dwarf
failed to decode correctly. Extend the relocation tests to cover
this case.

Note that the existing tests passed since the symbol following the
symbol that required relocation is also of type STT_SECTION.

Fixes #4107.

R=golang-dev, mikioh.mikioh, iant, iant
CC=golang-dev
https://golang.org/cl/6848044

doc/go1.1.html

@@ -62,4 +62,14 @@ to adjust frame pointer offsets.
 
 <h2 id="library">Changes to the standard library</h2>
 
+<h3 id="debug/elf">debug/elf</h3>
+<p>
+Previous versions of the debug/elf 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 debug/elf,
+will be different to indexes into the original ELF symbol table. Any code that
+calls the debug/elf functions Symbols or ImportedSymbols may need to be
+adjusted to account for the additional symbol and the change in symbol offsets.
+</p>
+
 TODO

src/pkg/debug/elf/file.go

@@ -417,10 +417,6 @@ 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[0:])
-
 	symbols := make([]Symbol, symtab.Len()/Sym32Size)
 
 	i := 0
@@ -460,10 +456,6 @@ 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[0:])
-
 	symbols := make([]Symbol, symtab.Len()/Sym64Size)
 
 	i := 0

src/pkg/debug/elf/file_test.go

@@ -175,23 +175,41 @@ func TestOpen(t *testing.T) {
 	}
 }
 
+type relocationTestEntry struct {
+	entryNumber int
+	entry       *dwarf.Entry
+}
+
 type relocationTest struct {
 	file    string
-	firstEntry *dwarf.Entry
+	entries []relocationTestEntry
 }
 
 var relocationTests = []relocationTest{
 	{
 		"testdata/go-relocation-test-gcc441-x86-64.obj",
-		&dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{{Attr: dwarf.AttrProducer, Val: "GNU C 4.4.1"}, {Attr: dwarf.AttrLanguage, Val: int64(1)}, {Attr: dwarf.AttrName, Val: "go-relocation-test.c"}, {Attr: dwarf.AttrCompDir, Val: "/tmp"}, {Attr: dwarf.AttrLowpc, Val: uint64(0x0)}, {Attr: dwarf.AttrHighpc, Val: uint64(0x6)}, {Attr: dwarf.AttrStmtList, Val: int64(0)}}},
+		[]relocationTestEntry{
+			{0, &dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{{Attr: dwarf.AttrProducer, Val: "GNU C 4.4.1"}, {Attr: dwarf.AttrLanguage, Val: int64(1)}, {Attr: dwarf.AttrName, Val: "go-relocation-test.c"}, {Attr: dwarf.AttrCompDir, Val: "/tmp"}, {Attr: dwarf.AttrLowpc, Val: uint64(0x0)}, {Attr: dwarf.AttrHighpc, Val: uint64(0x6)}, {Attr: dwarf.AttrStmtList, Val: int64(0)}}}},
+		},
 	},
 	{
 		"testdata/go-relocation-test-gcc441-x86.obj",
-		&dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{{Attr: dwarf.AttrProducer, Val: "GNU C 4.4.1"}, {Attr: dwarf.AttrLanguage, Val: int64(1)}, {Attr: dwarf.AttrName, Val: "t.c"}, {Attr: dwarf.AttrCompDir, Val: "/tmp"}, {Attr: dwarf.AttrLowpc, Val: uint64(0x0)}, {Attr: dwarf.AttrHighpc, Val: uint64(0x5)}, {Attr: dwarf.AttrStmtList, Val: int64(0)}}},
+		[]relocationTestEntry{
+			{0, &dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{{Attr: dwarf.AttrProducer, Val: "GNU C 4.4.1"}, {Attr: dwarf.AttrLanguage, Val: int64(1)}, {Attr: dwarf.AttrName, Val: "t.c"}, {Attr: dwarf.AttrCompDir, Val: "/tmp"}, {Attr: dwarf.AttrLowpc, Val: uint64(0x0)}, {Attr: dwarf.AttrHighpc, Val: uint64(0x5)}, {Attr: dwarf.AttrStmtList, Val: int64(0)}}}},
+		},
 	},
 	{
 		"testdata/go-relocation-test-gcc424-x86-64.obj",
-		&dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{{Attr: dwarf.AttrProducer, Val: "GNU C 4.2.4 (Ubuntu 4.2.4-1ubuntu4)"}, {Attr: dwarf.AttrLanguage, Val: int64(1)}, {Attr: dwarf.AttrName, Val: "go-relocation-test-gcc424.c"}, {Attr: dwarf.AttrCompDir, Val: "/tmp"}, {Attr: dwarf.AttrLowpc, Val: uint64(0x0)}, {Attr: dwarf.AttrHighpc, Val: uint64(0x6)}, {Attr: dwarf.AttrStmtList, Val: int64(0)}}},
+		[]relocationTestEntry{
+			{0, &dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{{Attr: dwarf.AttrProducer, Val: "GNU C 4.2.4 (Ubuntu 4.2.4-1ubuntu4)"}, {Attr: dwarf.AttrLanguage, Val: int64(1)}, {Attr: dwarf.AttrName, Val: "go-relocation-test-gcc424.c"}, {Attr: dwarf.AttrCompDir, Val: "/tmp"}, {Attr: dwarf.AttrLowpc, Val: uint64(0x0)}, {Attr: dwarf.AttrHighpc, Val: uint64(0x6)}, {Attr: dwarf.AttrStmtList, Val: int64(0)}}}},
+		},
+	},
+	{
+		"testdata/gcc-amd64-openbsd-debug-with-rela.obj",
+		[]relocationTestEntry{
+			{203, &dwarf.Entry{Offset: 0xc62, Tag: dwarf.TagMember, Children: false, Field: []dwarf.Field{{Attr: dwarf.AttrName, Val: "it_interval"}, {Attr: dwarf.AttrDeclFile, Val: int64(7)}, {Attr: dwarf.AttrDeclLine, Val: int64(236)}, {Attr: dwarf.AttrType, Val: dwarf.Offset(0xb7f)}, {Attr: dwarf.AttrDataMemberLoc, Val: []byte{0x23, 0x0}}}}},
+			{204, &dwarf.Entry{Offset: 0xc70, Tag: dwarf.TagMember, Children: false, Field: []dwarf.Field{{Attr: dwarf.AttrName, Val: "it_value"}, {Attr: dwarf.AttrDeclFile, Val: int64(7)}, {Attr: dwarf.AttrDeclLine, Val: int64(237)}, {Attr: dwarf.AttrType, Val: dwarf.Offset(0xb7f)}, {Attr: dwarf.AttrDataMemberLoc, Val: []byte{0x23, 0x10}}}}},
+		},
 	},
 }
 
@@ -207,22 +225,26 @@ func TestDWARFRelocations(t *testing.T) {
 			t.Error(err)
 			continue
 		}
+		for _, testEntry := range test.entries {
 			reader := dwarf.Reader()
-		// Checking only the first entry is sufficient since it has
-		// many different strings. If the relocation had failed, all
-		// the string offsets would be zero and all the strings would
-		// end up being the same.
-		firstEntry, err := reader.Next()
+			for j := 0; j < testEntry.entryNumber; j++ {
+				entry, err := reader.Next()
+				if entry == nil || err != nil {
+					t.Errorf("Failed to skip to entry %d: %v", testEntry.entryNumber, err)
+					continue
+				}
+			}
+			entry, err := reader.Next()
 			if err != nil {
 				t.Error(err)
 				continue
 			}
-
-		if !reflect.DeepEqual(test.firstEntry, firstEntry) {
-			t.Errorf("#%d: mismatch: got:%#v want:%#v", i, firstEntry, test.firstEntry)
+			if !reflect.DeepEqual(testEntry.entry, entry) {
+				t.Errorf("#%d/%d: mismatch: got:%#v want:%#v", i, testEntry.entryNumber, entry, testEntry.entry)
 				continue
 			}
 		}
+	}
 }
 
 func TestNoSectionOverlaps(t *testing.T) {