prog,btf: explicitly refuse ambiguous AttachTo targets

See #894 for more context. Explicitly refuse loading programs with
multiple AttachTo candidates. BTF needs to carry more information to allow
disambiguating between them. The kernel API likely needs to be extended to
allow specifying which candidate to pick.

Signed-off-by: Timo Beckers <[email protected]>

btf/btf.go

@@ -22,9 +22,10 @@ const btfMagic = 0xeB9F
 
 // Errors returned by BTF functions.
 var (
-	ErrNotSupported   = internal.ErrNotSupported
-	ErrNotFound       = errors.New("not found")
-	ErrNoExtendedInfo = errors.New("no extended info")
+	ErrNotSupported    = internal.ErrNotSupported
+	ErrNotFound        = errors.New("not found")
+	ErrNoExtendedInfo  = errors.New("no extended info")
+	ErrMultipleMatches = errors.New("multiple matching types")
 )
 
 // ID represents the unique ID of a BTF object.
@@ -564,16 +565,15 @@ func (s *Spec) AnyTypeByName(name string) (Type, error) {
 	return types[0], nil
 }
 
-// TypeByName searches for a Type with a specific name. Since multiple
-// Types with the same name can exist, the parameter typ is taken to
-// narrow down the search in case of a clash.
+// TypeByName searches for a Type with a specific name. Since multiple Types
+// with the same name can exist, the parameter typ is taken to narrow down the
+// search in case of a clash.
 //
-// typ must be a non-nil pointer to an implementation of a Type.
-// On success, the address of the found Type will be copied to typ.
+// typ must be a non-nil pointer to an implementation of a Type. On success, the
+// address of the found Type will be copied to typ.
 //
-// Returns an error wrapping ErrNotFound if no matching
-// Type exists in the Spec. If multiple candidates are found,
-// an error is returned.
+// Returns an error wrapping ErrNotFound if no matching Type exists in the Spec.
+// Returns an error wrapping ErrMultipleTypes if multiple candidates are found.
 func (s *Spec) TypeByName(name string, typ interface{}) error {
 	typeInterface := reflect.TypeOf((*Type)(nil)).Elem()
 
@@ -610,7 +610,7 @@ func (s *Spec) TypeByName(name string, typ interface{}) error {
 		}
 
 		if candidate != nil {
-			return fmt.Errorf("type %s: multiple candidates for %T", name, typ)
+			return fmt.Errorf("type %s(%T): %w", name, typ, ErrMultipleMatches)
 		}
 
 		candidate = typ

prog.go

@@ -891,6 +891,12 @@ func findTargetInKernel(name string, progType ProgramType, attachType AttachType
 		}
 		return module, id, nil
 	}
+	// See cilium/ebpf#894. Until we can disambiguate between equally-named kernel
+	// symbols, we should explicitly refuse program loads. They will not reliably
+	// do what the caller intended.
+	if errors.Is(err, btf.ErrMultipleMatches) {
+		return nil, 0, fmt.Errorf("attaching to ambiguous kernel symbol is not supported: %w", err)
+	}
 	if err != nil {
 		return nil, 0, fmt.Errorf("find target for %s in vmlinux: %w", featureName, err)
 	}