debug/elf: Add support for getting DynTag string table values.

R=rsc
CC=golang-dev
https://golang.org/cl/6430064

src/pkg/debug/elf/file.go

@@ -726,6 +726,20 @@ func (f *File) gnuVersion(i int, sym *ImportedSymbol) {
 // referred to by the binary f that are expected to be
 // linked with the binary at dynamic link time.
 func (f *File) ImportedLibraries() ([]string, error) {
+	return f.DynString(DT_NEEDED)
+}
+
+// DynString returns the strings listed for the given tag in the file's dynamic
+// section.
+//
+// The tag must be one that takes string values: DT_NEEDED, DT_SONAME, DT_RPATH, or
+// DT_RUNPATH.
+func (f *File) DynString(tag DynTag) ([]string, error) {
+	switch tag {
+	case DT_NEEDED, DT_SONAME, DT_RPATH, DT_RUNPATH:
+	default:
+		return nil, fmt.Errorf("non-string-valued tag %v", tag)
+	}
 	ds := f.SectionByType(SHT_DYNAMIC)
 	if ds == nil {
 		// not dynamic, so no libraries
@@ -741,25 +755,24 @@ func (f *File) ImportedLibraries() ([]string, error) {
 	}
 	var all []string
 	for len(d) > 0 {
-		var tag DynTag
-		var value uint64
+		var t DynTag
+		var v uint64
 		switch f.Class {
 		case ELFCLASS32:
-			tag = DynTag(f.ByteOrder.Uint32(d[0:4]))
-			value = uint64(f.ByteOrder.Uint32(d[4:8]))
+			t = DynTag(f.ByteOrder.Uint32(d[0:4]))
+			v = uint64(f.ByteOrder.Uint32(d[4:8]))
 			d = d[8:]
 		case ELFCLASS64:
-			tag = DynTag(f.ByteOrder.Uint64(d[0:8]))
-			value = f.ByteOrder.Uint64(d[8:16])
+			t = DynTag(f.ByteOrder.Uint64(d[0:8]))
+			v = f.ByteOrder.Uint64(d[8:16])
 			d = d[16:]
 		}
-		if tag == DT_NEEDED {
-			s, ok := getString(str, int(value))
+		if t == tag {
+			s, ok := getString(str, int(v))
 			if ok {
 				all = append(all, s)
 			}
 		}
 	}
-
 	return all, nil
 }

src/pkg/debug/elf/file_test.go

@@ -19,6 +19,7 @@ type fileTest struct {
 	hdr      FileHeader
 	sections []SectionHeader
 	progs    []ProgHeader
+	needed   []string
 }
 
 var fileTests = []fileTest{
@@ -64,6 +65,7 @@ var fileTests = []fileTest{
 			{PT_LOAD, PF_R + PF_W, 0x5fc, 0x80495fc, 0x80495fc, 0xd8, 0xf8, 0x1000},
 			{PT_DYNAMIC, PF_R + PF_W, 0x60c, 0x804960c, 0x804960c, 0x98, 0x98, 0x4},
 		},
+		[]string{"libc.so.6"},
 	},
 	{
 		"testdata/gcc-amd64-linux-exec",
@@ -117,6 +119,7 @@ var fileTests = []fileTest{
 			{PT_LOOS + 0x474E550, PF_R, 0x5b8, 0x4005b8, 0x4005b8, 0x24, 0x24, 0x4},
 			{PT_LOOS + 0x474E551, PF_R + PF_W, 0x0, 0x0, 0x0, 0x0, 0x0, 0x8},
 		},
+		[]string{"libc.so.6"},
 	},
 }
 
@@ -161,6 +164,14 @@ func TestOpen(t *testing.T) {
 		if tn != fn {
 			t.Errorf("open %s: len(Progs) = %d, want %d", tt.file, fn, tn)
 		}
+		tl := tt.needed
+		fl, err := f.ImportedLibraries()
+		if err != nil {
+			t.Error(err)
+		}
+		if !reflect.DeepEqual(tl, fl) {
+			t.Errorf("open %s: DT_NEEDED = %v, want %v", tt.file, tl, fl)
+		}
 	}
 }