btf: return an error if type has no signedness

Refuse to create a fixup for reloFieldSigned if the type isn't an
Int or an Enum. On clang-14 it's not even possible to create such
a relocation, so no need to be extra lenient.

Add a unit test since we can't get clang to emit a wonky relocation.

Signed-off-by: Lorenz Bauer <[email protected]>

Author: lmb

btf/core.go

@@ -303,6 +303,8 @@ func coreCalculateFixups(relos []*CORERelocation, targetSpec *Spec, targets []Ty
 	return bestFixups, nil
 }
 
+var errNoSignedness = errors.New("no signedness")
+
 // coreCalculateFixup calculates the fixup for a single local type, target type
 // and relocation.
 func coreCalculateFixup(relo *CORERelocation, target Type, targetID TypeID, bo binary.ByteOrder) (COREFixup, error) {
@@ -461,7 +463,7 @@ func coreCalculateFixup(relo *CORERelocation, target Type, targetID TypeID, bo b
 					uint32(target.Encoding&Signed),
 				)
 			default:
-				return fixupWithoutValidation(0, 0)
+				return zero, fmt.Errorf("type %T: %w", local, errNoSignedness)
 			}
 		}
 	}

btf/core_test.go

@@ -673,6 +673,14 @@ func TestCOREReloFieldSigned(t *testing.T) {
 			qt.Assert(t, err, qt.IsNil)
 		})
 	}
+
+	t.Run("type without signedness", func(t *testing.T) {
+		relo := &CORERelocation{
+			&Array{}, coreAccessor{0}, reloFieldSigned, 0,
+		}
+		_, err := coreCalculateFixup(relo, &Array{}, 0, internal.NativeEndian)
+		qt.Assert(t, err, qt.ErrorIs, errNoSignedness)
+	})
 }
 
 func TestCOREReloFieldShiftU64(t *testing.T) {