Implementing $csp modifier from uBlockOrigin

networkrules/rule/rule.go

@@ -83,6 +83,8 @@ func (rm *Rule) ParseModifiers(modifiers []string) error {
 			modifier = &rulemodifiers.ScrambleJSModifier{}
 		case isKind("jsonprune"):
 			modifier = &rulemodifiers.JSONPruneModifier{}
+		case isKind("csp"):
+			modifier = &rulemodifiers.CSPModifier{}
 		case isKind("all"):
 			// TODO: should act as "popup" modifier once it gets implemented
 			continue

networkrules/rulemodifiers/csp.go

@@ -0,0 +1,44 @@
+package rulemodifiers
+
+import (
+	"errors"
+	"net/http"
+	"strings"
+)
+
+type CSPModifier struct {
+	policy string
+}
+
+var _ ModifyingModifier = (*CSPModifier)(nil)
+
+func (m *CSPModifier) Parse(modifier string) error {
+	if !strings.HasPrefix(modifier, "csp=") {
+		return errors.New("invalid csp modifier")
+	}
+	m.policy = strings.TrimPrefix(modifier, "csp=")
+	if m.policy == "" {
+		return errors.New("empty csp policy")
+	}
+	return nil
+}
+
+func (m *CSPModifier) ModifyReq(req *http.Request) bool {
+	// CSP is a response header, so we do nothing to the request.
+	return false
+}
+
+func (m *CSPModifier) ModifyRes(res *http.Response) (bool, error) {
+	// We use Add() instead of Set() because if the site already has a CSP,
+	// browsers enforce the "intersection" of all policies (most restrictive wins).
+	res.Header.Add("Content-Security-Policy", m.policy)
+	return true, nil
+}
+
+func (m *CSPModifier) Cancels(modifier Modifier) bool {
+	other, ok := modifier.(*CSPModifier)
+	if !ok {
+		return false
+	}
+	return m.policy == other.policy
+}