sysenc: special case unmarshaling into []byte

The library has somewhat complicated semantics when unmarshaling
map keys and values. It turns out that the following was supported
(but not documented) until the zero-allocation marshaling work
broke it:

    var s []byte
    m.Lookup(key, &s)

The important thing is that the slice here is empty. The current
code treats this as "the key should be zero length" while originally
we'd assign the temporary buffer used by the syscall to s as a way to
reduce allocations. In hindsight this wasn't a great idea, but it is
what it is.

Reintroduce the byte slice special case.

Fixes #1175

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

internal/sysenc/marshal.go

@@ -11,6 +11,8 @@ import (
 	"unsafe"
 
 	"github.com/cilium/ebpf/internal"
+
+	"golang.org/x/exp/slices"
 )
 
 // Marshal turns data into a byte slice using the system's native endianness.
@@ -88,6 +90,11 @@ func Unmarshal(data interface{}, buf []byte) error {
 		*value = string(buf)
 		return nil
 
+	case *[]byte:
+		// Backwards compat: unmarshaling into a slice replaces the whole slice.
+		*value = slices.Clone(buf)
+		return nil
+
 	default:
 		if dataBuf := unsafeBackingMemory(data); len(dataBuf) == len(buf) {
 			copy(dataBuf, buf)

map.go

@@ -1411,11 +1411,7 @@ func (mi *MapIterator) Next(keyOut, valueOut interface{}) bool {
 			return false
 		}
 
-		// The user can get access to nextKey since unmarshalBytes
-		// does not copy when unmarshaling into a []byte.
-		// Make a copy to prevent accidental corruption of
-		// iterator state.
-		copy(mi.curKey, nextKey)
+		mi.curKey = nextKey
 
 		mi.count++
 		mi.err = mi.target.Lookup(nextKey, valueOut)

map_test.go

@@ -75,6 +75,10 @@ func TestMap(t *testing.T) {
 		t.Error("Want value 42, got", v)
 	}
 
+	var slice []byte
+	qt.Assert(t, m.Lookup(uint32(0), &slice), qt.IsNil)
+	qt.Assert(t, slice, qt.DeepEquals, internal.NativeEndian.AppendUint32(nil, 42))
+
 	var k uint32
 	if err := m.NextKey(uint32(0), &k); err != nil {
 		t.Fatal("Can't get:", err)