CI: Add clang-format checker

Signed-off-by: Andreas Fuchs <andreas.fuchs@infineon.com>

Author: AndreasFuchsTPM

.github/workflows/clang-format.yml

@@ -0,0 +1,26 @@
+# SPDX-FileCopyrightText: Copyright (c) 2025 Infineon Technologies AG
+#
+# SPDX-License-Identifier: BSD-2-Clause
+
+name: Clang Format Checker
+on:
+  [push, pull_request]
+jobs:
+  clang-format-checking:
+    runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/tpm2-software/ubuntu-24.04
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+        with:
+            fetch-depth: 1
+      - name: Run clang-format
+        run: clang-format -i $(find -name '*.h' -or -name '*.c' | xargs realpath)
+      - name: Check for diffs
+        run: git diff --exit-code
+      - name: failure
+        if: ${{ failure() }}
+        run: |
+          echo "Clang format check failed. Please run clang-format on your code or see the diff in the step above."
+          echo "docker run -u $UID -v $PWD:$PWD ghcr.io/tpm2-software/ubuntu-24.04 clang-format -i $(find -name '*.h' -or -name '*.c' | xargs realpath)"

CONTRIBUTING.md

@@ -44,6 +44,14 @@ full text of the DCO is here: https://developercertificate.org/. Contributors
 must add a 'Signed-off-by' line to their commits. This indicates the
 submitters acceptance of the DCO.
 
+Please note that we now require REUSE and clang-format to also pass on PRs.
+You may run
+```sh
+docker run -u 1000 -v $PWD:$PWD xianpengshen/clang-tools:17 clang-format -i $(find -name '*.c' | xargs realpath)
+docker run -u 1000 -v $PWD:$PWD xianpengshen/clang-tools:17 clang-format -i $(find -name '*.h' | xargs realpath)
+```
+before submitting in order to align with our format requirements.
+
 ## Guideline for merging changes
 
 Pull Requests MUST be assigned to an upcoming release tag. If a release milestone does