utilize Pattern Tree to decide the policy to use and modify HTTP response headers according to that policy

This commit also enhances the build script so that preprocessor conditionals can now use operators '&&' and '||'.
The features being developed are not yet included in the actual Haketilo build.
Some of the new source files contain similar functionality to other ones already existing in the source tree. At some point the latter will be removed.

1 files changed, 106 insertions, 0 deletions

diff --git a/common/policy.js b/common/policy.js
new file mode 100644
index 0000000..ebd663f
--- /dev/null
+++ b/common/policy.js
@@ -0,0 +1,106 @@
+/**
+ * This file is part of Haketilo.
+ *
+ * Function: Determining what to do on a given web page.
+ *
+ * Copyright (C) 2021 Wojtek Kosior
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * As additional permission under GNU GPL version 3 section 7, you
+ * may distribute forms of that code without the copy of the GNU
+ * GPL normally required by section 4, provided you include this
+ * license notice and, in case of non-source distribution, a URL
+ * through which recipients can access the Corresponding Source.
+ * If you modify file(s) with this exception, you may extend this
+ * exception to your version of the file(s), but you are not
+ * obligated to do so. If you do not wish to do so, delete this
+ * exception statement from your version.
+ *
+ * As a special exception to the GPL, any HTML file which merely
+ * makes function calls to this code, and for that purpose
+ * includes it by reference shall be deemed a separate work for
+ * copyright law purposes. If you modify this code, you may extend
+ * this exception to your version of the code, but you are not
+ * obligated to do so. If you do not wish to do so, delete this
+ * exception statement from your version.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ * I, Wojtek Kosior, thereby promise not to sue for violation of this file's
+ * license. Although I request that you do not make use this code in a
+ * proprietary program, I am not going to enforce this in court.
+ */
+
+#IMPORT common/patterns_query_tree.js AS pqt
+
+#FROM common/sha256.js IMPORT sha256
+
+/*
+ * CSP rule that either blocks all scripts or only allows scripts with specified
+ * nonce attached.
+ */
+function make_csp(nonce)
+{
+    const rule = nonce ? `nonce-${nonce}` : "none";
+    const csp_dict = {"prefetch-src": "none", "script-src-attr": "none"};
+    Object.assign(csp_dict, {"script-src": rule, "script-src-elem": rule});
+    return Object.entries(csp_dict).map(([a, b]) => `${a} '${b}';`).join(" ");
+}
+
+function decide_policy(patterns_tree, url, default_allow, secret)
+{
+    const policy = {allow: default_allow};
+
+    try {
+	var payloads = pqt.search(patterns_tree, url).next().value;
+    } catch (e) {
+	console.error(e);
+	policy.allow = false;
+	policy.error = true;
+    }
+
+    if (payloads !== undefined) {
+	policy.mapping = Object.keys(payloads).sort()[0];
+	const payload = payloads[policy.mapping];
+	if (payload.allow !== undefined) {
+	    policy.allow = payload.allow;
+	} else /* if (payload.identifier) */ {
+	    policy.allow = false;
+	    policy.payload = payload;
+	    /*
+	     * Hash a secret and other values into a string that's unpredictable
+	     * to someone who does not know these values. What we produce here
+	     * is not a true "nonce" because it might get produced multiple
+	     * times given the same url and mapping choice. Nevertheless, this
+	     * is reasonably good given the limitations WebExtension APIs and
+	     * environments give us. If we were using a true nonce, we'd have no
+	     * reliable way of passing it to our content scripts.
+	     */
+	    const nonce_source = [
+		policy.mapping,
+		policy.payload.identifier,
+		url,
+		secret
+	    ];
+	    policy.nonce = sha256(nonce_source.join(":"));
+	}
+    }
+
+    if (!policy.allow)
+	policy.csp = make_csp(policy.nonce);
+
+    return policy;
+}
+#EXPORT decide_policy
+
+#EXPORT  () => ({allow: false, csp: make_csp()})  AS fallback_policy