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

3 files changed, 41 insertions, 27 deletions

diff --git a/src/pkg/debug/elf/file.go b/src/pkg/debug/elf/file.go
index d38da4bf8e..25b04d7959 100644
--- a/src/pkg/debug/elf/file.go
+++ b/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
diff --git a/src/pkg/debug/elf/file_test.go b/src/pkg/debug/elf/file_test.go
index 12036e816b..810a2f9b9a 100644
--- a/src/pkg/debug/elf/file_test.go
+++ b/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
+	file    string
+	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}}}}},
+		},
 	},
 }
 
@@ -207,20 +225,24 @@ func TestDWARFRelocations(t *testing.T) {
 			t.Error(err)
 			continue
 		}
-		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()
-		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)
-			continue
+		for _, testEntry := range test.entries {
+			reader := dwarf.Reader()
+			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(testEntry.entry, entry) {
+				t.Errorf("#%d/%d: mismatch: got:%#v want:%#v", i, testEntry.entryNumber, entry, testEntry.entry)
+				continue
+			}
 		}
 	}
 }
diff --git a/src/pkg/debug/elf/testdata/gcc-amd64-openbsd-debug-with-rela.obj b/src/pkg/debug/elf/testdata/gcc-amd64-openbsd-debug-with-rela.obj
new file mode 100644
index 0000000000..f62b1ea1ca
--- /dev/null
+++ b/src/pkg/debug/elf/testdata/gcc-amd64-openbsd-debug-with-rela.obj