ci: disallow directional formatting

As described in https://trojansource.codes/trojan-source.pdf, it is
possible to abuse directional formatting (a feature of Unicode) to
deceive human readers into interpreting code differently from compilers.

For example, an "if ()" expression could be enclosed in a comment, but
rendered as if it was outside of that comment. In effect, this could
fool a reviewer into misinterpreting the code flow as benign when it is
not.

It is highly unlikely that Git's source code wants to contain such
directional formatting in the first place, so let's just disallow it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -287,6 +287,7 @@ jobs:
     - uses: actions/checkout@v2
     - run: ci/install-dependencies.sh
     - run: ci/run-static-analysis.sh
+    - run: ci/check-directional-formatting.bash
   sparse:
     needs: ci-config
     if: needs.ci-config.outputs.enabled == 'yes'

ci/check-directional-formatting.bash

@@ -0,0 +1,27 @@
+#!/bin/bash
+
+# This script verifies that the non-binary files tracked in the Git index do
+# not contain any Unicode directional formatting: such formatting could be used
+# to deceive reviewers into interpreting code differently from the compiler.
+# This is intended to run on an Ubuntu agent in a GitHub workflow.
+#
+# To allow translated messages to introduce such directional formatting in the
+# future, we exclude the `.po` files from this validation.
+#
+# Neither GNU grep nor `git grep` (not even with `-P`) handle `\u` as a way to
+# specify UTF-8.
+#
+# To work around that, we use `printf` to produce the pattern as a byte
+# sequence, and then feed that to `git grep` as a byte sequence (setting
+# `LC_CTYPE` to make sure that the arguments are interpreted as intended).
+#
+# Note: we need to use Bash here because its `printf` interprets `\uNNNN` as
+# UTF-8 code points, as desired. Running this script through Ubuntu's `dash`,
+# for example, would use a `printf` that does not understand that syntax.
+
+# U+202a..U+2a2e: LRE, RLE, PDF, LRO and RLO
+# U+2066..U+2069: LRI, RLI, FSI and PDI
+regex='(\u202a|\u202b|\u202c|\u202d|\u202e|\u2066|\u2067|\u2068|\u2069)'
+
+! LC_CTYPE=C git grep -El "$(LC_CTYPE=C.UTF-8 printf "$regex")" \
+	-- ':(exclude,attr:binary)' ':(exclude)*.po'