Add xcm fuzzer (#3869)

* Extend xcm-simulator with a fuzzer for xcm exec

This commit adds a very basic fuzzer that fuzzes the xcm exectution,
in particular the process_instruction function.

* Update cargo.toml in xcm-simulator-fuzzer

* Add xcm-fuzzer to honggfuzz workflow

* Update Cargo.lock

* Update xcm/xcm-simulator/fuzzer/Cargo.toml so honggfuzz shows up on top

Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

* Update relay_chain.rs in xcm-fuzzer

* Use MAX_XCM_DECODE_DEPTH instead of hardcoded decode limit in xcm-fuzzer

* Add comment on how to generate coverage report in xcm-fuzzer

* fix warnings, fmt, and unused result

* fix compiler

Co-authored-by: Vincent Ulitzsch <vincent.ulitzsch@srlabs.de>
Co-authored-by: Vincent Ulitzsch <vincent@srlabs.de>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

polkadot/.github/workflows/honggfuzz.yml

@@ -5,6 +5,45 @@ on:
     - cron: '0 0 * * *'
 
 jobs:
+  xcm-fuzzer:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 1
+
+      - name: Install minimal stable Rust
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+
+      - name: Install minimal nightly Rust
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: nightly
+          target: wasm32-unknown-unknown
+
+      - name: Install honggfuzz deps
+        run: sudo apt-get install --no-install-recommends binutils-dev libunwind8-dev
+
+      - name: Install honggfuzz
+        uses: actions-rs/cargo@v1
+        with:
+          command: install
+          args: honggfuzz --version "0.5.54"
+
+      - name: Build fuzzer binaries
+        working-directory: xcm/xcm-simulator/fuzzer/
+        run: cargo hfuzz build
+
+      - name: Run fuzzer
+        working-directory: xcm/xcm-simulator/fuzzer/
+        run: bash $GITHUB_WORKSPACE/scripts/github/run_fuzzer.sh xcm-fuzzer
+
   erasure-coding-round-trip:
     runs-on: ubuntu-latest
     steps: