debug/elf: retrieve values for dynamic section tags

Add functionality to retrieve values for .dynamic entries that don't
correspond to entries in the string table.

Fixes #56892

Change-Id: I6edabc8ca331c819e442d06e19b7f4df8343372b
Reviewed-on: https://go-review.googlesource.com/c/go/+/452617
Auto-Submit: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: florinpapa

api/next/56892.txt

@@ -0,0 +1 @@
+pkg debug/elf, method (*File) DynValue(DynTag) ([]uint64, error) #56892

src/debug/elf/file.go

@@ -1642,6 +1642,40 @@ func (f *File) DynString(tag DynTag) ([]string, error) {
 	return all, nil
 }
 
+// DynValue returns the values listed for the given tag in the file's dynamic
+// section.
+func (f *File) DynValue(tag DynTag) ([]uint64, error) {
+	ds := f.SectionByType(SHT_DYNAMIC)
+	if ds == nil {
+		return nil, nil
+	}
+	d, err := ds.Data()
+	if err != nil {
+		return nil, err
+	}
+
+	// Parse the .dynamic section as a string of bytes.
+	var vals []uint64
+	for len(d) > 0 {
+		var t DynTag
+		var v uint64
+		switch f.Class {
+		case ELFCLASS32:
+			t = DynTag(f.ByteOrder.Uint32(d[0:4]))
+			v = uint64(f.ByteOrder.Uint32(d[4:8]))
+			d = d[8:]
+		case ELFCLASS64:
+			t = DynTag(f.ByteOrder.Uint64(d[0:8]))
+			v = f.ByteOrder.Uint64(d[8:16])
+			d = d[16:]
+		}
+		if t == tag {
+			vals = append(vals, v)
+		}
+	}
+	return vals, nil
+}
+
 type nobitsSectionReader struct{}
 
 func (*nobitsSectionReader) ReadAt(p []byte, off int64) (n int, err error) {

src/debug/elf/file_test.go

@@ -1224,3 +1224,21 @@ func TestIssue10996(t *testing.T) {
 		t.Fatalf("opening invalid ELF file unexpectedly succeeded")
 	}
 }
+
+func TestDynValue(t *testing.T) {
+	const testdata = "testdata/gcc-amd64-linux-exec"
+	f, err := Open(testdata)
+	if err != nil {
+		t.Fatalf("could not read %s: %v", testdata, err)
+	}
+	defer f.Close()
+
+	vals, err := f.DynValue(DT_VERNEEDNUM)
+	if err != nil {
+		t.Fatalf("DynValue(DT_VERNEEDNUM): got unexpected error %v", err)
+	}
+
+	if len(vals) != 1 || vals[0] != 1 {
+		t.Errorf("DynValue(DT_VERNEEDNUM): got %v, want [1]", vals)
+	}
+}