x-pack/filebeat/input/internal/private: add field redaction package

.github/CODEOWNERS

@@ -123,6 +123,7 @@ CHANGELOG*
 /x-pack/filebeat/input/httpjson/ @elastic/security-service-integrations
 /x-pack/filebeat/input/internal/httplog @elastic/security-service-integrations
 /x-pack/filebeat/input/internal/httpmon @elastic/security-service-integrations
+/x-pack/filebeat/input/internal/private @elastic/security-service-integrations
 /x-pack/filebeat/input/lumberjack/ @elastic/security-service-integrations
 /x-pack/filebeat/input/netflow/ @elastic/sec-deployment-and-devices
 /x-pack/filebeat/input/o365audit/ @elastic/security-service-integrations

CHANGELOG-developer.next.asciidoc

@@ -206,6 +206,7 @@ The list below covers the major changes between 7.0.0-rc2 and main only.
 - Added debug logging to parquet reader in x-pack/libbeat/reader. {pull}40651[40651]
 - Added filebeat debug histograms for s3 object size and events per processed s3 object. {pull}40775[40775]
 - Simplified Azure Blob Storage input state checkpoint calculation logic. {issue}40674[40674] {pull}40936[40936]
+- Add field redaction package. {pull}40997[40997]
 
 ==== Deprecated
 

x-pack/filebeat/input/internal/private/private.go

@@ -0,0 +1,268 @@
+// Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+// or more contributor license agreements. Licensed under the Elastic License;
+// you may not use this file except in compliance with the Elastic License.
+
+// Package private implements field redaction in maps and structs.
+package private
+
+import (
+	"fmt"
+	"reflect"
+	"slices"
+	"strings"
+	"unsafe"
+)
+
+const tooDeep = 100
+
+var privateKey = reflect.ValueOf("private")
+
+// Redact returns a copy of val with any fields or map elements that have been
+// marked as private removed. Fields can be marked as private by including a
+// sibling string- or []string-valued field or element with the name of the
+// private field. The names of fields are interpreted through the tag parameter
+// if present. For example if tag is "json", the `json:"<name>"` name would be
+// used, falling back to the field name if not present. The tag parameter is
+// ignored for map values.
+//
+// The global parameter indicates a set of dot-separated paths to redact. Paths
+// originate at the root of val. If global is used, the resultin redaction is on
+// the union of the fields redacted with tags and the fields redacted with the
+// global paths.
+//
+// If a field has a `private:...` tag, its tag value will also be used to
+// determine the list of private fields. If the private tag is empty,
+// `private:""`, the fields with the tag will be marked as private. Otherwise
+// the comma-separated list of names with be used. The list may refer to its
+// own field.
+func Redact[T any](val T, tag string, global []string) (redacted T, err error) {
+	defer func() {
+		switch r := recover().(type) {
+		case nil:
+			return
+		case cycle:
+			// Make the returned type informative in all cases.
+			// If Redact[any](v) is called and we use the zero
+			// value, we would return a nil any, which is less
+			// informative.
+			redacted = reflect.New(reflect.TypeOf(val)).Elem().Interface().(T)
+			err = r
+		default:
+			panic(r)
+		}
+	}()
+	rv := reflect.ValueOf(val)
+	switch rv.Kind() {
+	case reflect.Map, reflect.Pointer, reflect.Struct:
+		return redact(rv, tag, slices.Clone(global), 0, make(map[any]int)).Interface().(T), nil
+	default:
+		return val, nil
+	}
+}
+
+func redact(v reflect.Value, tag string, global []string, depth int, seen map[any]int) reflect.Value {
+	switch v.Kind() {
+	case reflect.Pointer:
+		if v.IsNil() {
+			return v
+		}
+		if depth > tooDeep {
+			ident := v.Interface()
+			if last, ok := seen[ident]; ok && last < depth {
+				panic(cycle{v.Type()})
+			}
+			seen[ident] = depth
+			defer delete(seen, ident)
+		}
+		return redact(v.Elem(), tag, global, depth+1, seen).Addr()
+	case reflect.Interface:
+		if v.IsNil() {
+			return v
+		}
+		return redact(v.Elem(), tag, global, depth+1, seen)
+	case reflect.Array:
+		if v.Len() == 0 {
+			return v
+		}
+		r := reflect.New(v.Type()).Elem()
+		for i := 0; i < v.Len(); i++ {
+			r.Index(i).Set(redact(v.Index(i), tag, global, depth+1, seen))
+		}
+		return r
+	case reflect.Slice:
+		if v.Len() == 0 {
+			return v
+		}
+		if depth > tooDeep {
+			ident := struct {
+				data unsafe.Pointer
+				len  int
+			}{
+				v.UnsafePointer(),
+				v.Len(),
+			}
+			if last, ok := seen[ident]; ok && last < depth {
+				panic(cycle{v.Type()})
+			}
+			seen[ident] = depth
+			defer delete(seen, ident)
+		}
+		r := reflect.MakeSlice(v.Type(), v.Len(), v.Cap())
+		for i := 0; i < v.Len(); i++ {
+			r.Index(i).Set(redact(v.Index(i), tag, global, depth+1, seen))
+		}
+		return r
+	case reflect.Map:
+		if v.IsNil() {
+			return v
+		}
+		if depth > tooDeep {
+			ident := v.UnsafePointer()
+			if last, ok := seen[ident]; ok && last < depth {
+				panic(cycle{v.Type()})
+			}
+			seen[ident] = depth
+			defer delete(seen, ident)
+		}
+		private := nextStep(global)
+		if privateKey.CanConvert(v.Type().Key()) {
+			p := v.MapIndex(privateKey.Convert(v.Type().Key()))
+			if p.IsValid() && p.CanInterface() {
+				switch p := p.Interface().(type) {
+				case string:
+					private = append(private, p)
+				case []string:
+					private = append(private, p...)
+				case []any:
+					for _, s := range p {
+						private = append(private, fmt.Sprint(s))
+					}
+				}
+			}
+		}
+		r := reflect.MakeMap(v.Type())
+		it := v.MapRange()
+		for it.Next() {
+			name := it.Key().String()
+			if slices.Contains(private, name) {
+				continue
+			}
+			r.SetMapIndex(it.Key(), redact(it.Value(), tag, nextPath(name, global), depth+1, seen))
+		}
+		return r
+	case reflect.Struct:
+		private := nextStep(global)
+		rt := v.Type()
+		names := make([]string, rt.NumField())
+		for i := range names {
+			f := rt.Field(i)
+
+			// Look for `private:` tags.
+			p, ok := f.Tag.Lookup("private")
+			if ok {
+				if p != "" {
+					private = append(private, strings.Split(p, ",")...)
+				} else {
+					if tag == "" {
+						names[i] = f.Name
+						private = append(private, f.Name)
+					} else {
+						p = f.Tag.Get(tag)
+						if p != "" {
+							name, _, _ := strings.Cut(p, ",")
+							names[i] = name
+							private = append(private, name)
+						}
+					}
+				}
+			}
+
+			// Look after Private fields if we are not using a tag.
+			if tag == "" {
+				names[i] = f.Name
+				if f.Name == "Private" {
+					switch p := v.Field(i).Interface().(type) {
+					case string:
+						private = append(private, p)
+					case []string:
+						private = append(private, p...)
+					}
+				}
+				continue
+			}
+
+			// If we are using a tag, look for `tag:"<private>"`
+			// falling back to fields named Private if no tag is
+			// present.
+			p = f.Tag.Get(tag)
+			var name string
+			if p == "" {
+				name = f.Name
+			} else {
+				name, _, _ = strings.Cut(p, ",")
+			}
+			names[i] = name
+			if name == "private" {
+				switch p := v.Field(i).Interface().(type) {
+				case string:
+					private = append(private, p)
+				case []string:
+					private = append(private, p...)
+				}
+			}
+		}
+
+		r := reflect.New(v.Type()).Elem()
+		for i := 0; i < v.NumField(); i++ {
+			f := v.Field(i)
+			if f.IsZero() || !rt.Field(i).IsExported() {
+				continue
+			}
+			if slices.Contains(private, names[i]) {
+				continue
+			}
+			if r.Field(i).CanSet() {
+				r.Field(i).Set(redact(f, tag, nextPath(names[i], global), depth+1, seen))
+			}
+		}
+		return r
+	}
+	return v
+}
+
+func nextStep(global []string) (private []string) {
+	if len(global) == 0 {
+		return nil
+	}
+	private = make([]string, 0, len(global))
+	for _, s := range global {
+		key, _, more := strings.Cut(s, ".")
+		if !more {
+			private = append(private, key)
+		}
+	}
+	return private
+}
+
+func nextPath(step string, global []string) []string {
+	if len(global) == 0 {
+		return nil
+	}
+	step += "."
+	next := make([]string, 0, len(global))
+	for _, s := range global {
+		if !strings.HasPrefix(s, step) {
+			continue
+		}
+		next = append(next, s[len(step):])
+	}
+	return next
+}
+
+type cycle struct {
+	typ reflect.Type
+}
+
+func (e cycle) Error() string {
+	return fmt.Sprintf("cycle including %s", e.typ)
+}