Add a basic fuzzing project

src/Markdig.Fuzzing/.gitignore

@@ -0,0 +1,4 @@
+corpus
+libfuzzer-dotnet-windows.exe
+crash-*
+timeout-*

src/Markdig.Fuzzing/Markdig.Fuzzing.csproj

@@ -0,0 +1,19 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="SharpFuzz" Version="2.2.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Markdig\Markdig.csproj" />
+  </ItemGroup>
+
+</Project>

src/Markdig.Fuzzing/Program.cs

@@ -0,0 +1,71 @@
+using Markdig;
+using Markdig.Renderers.Roundtrip;
+using Markdig.Syntax;
+using SharpFuzz;
+using System.Diagnostics;
+using System.Text;
+
+ReadOnlySpanAction fuzzTarget = ParseRenderFuzzer.FuzzTarget;
+
+if (args.Length > 0)
+{
+    // Run the target on existing inputs
+    string[] files = Directory.Exists(args[0])
+        ? Directory.GetFiles(args[0])
+        : [args[0]];
+
+    Debugger.Launch();
+
+    foreach (string inputFile in files)
+    {
+        fuzzTarget(File.ReadAllBytes(inputFile));
+    }
+}
+else
+{
+    Fuzzer.LibFuzzer.Run(fuzzTarget);
+}
+
+sealed class ParseRenderFuzzer
+{
+    private static readonly MarkdownPipeline s_advancedPipeline = new MarkdownPipelineBuilder()
+        .UseAdvancedExtensions()
+        .Build();
+
+    private static readonly ResettableRoundtripRenderer _roundtripRenderer = new();
+
+    public static void FuzzTarget(ReadOnlySpan<byte> bytes)
+    {
+        string text = Encoding.UTF8.GetString(bytes);
+
+        try
+        {
+            MarkdownDocument document = Markdown.Parse(text);
+            _ = document.ToHtml();
+
+            document = Markdown.Parse(text, s_advancedPipeline);
+            _ = document.ToHtml(s_advancedPipeline);
+
+            document = Markdown.Parse(text, trackTrivia: true);
+            _ = document.ToHtml();
+            _roundtripRenderer.Reset();
+            _roundtripRenderer.Render(document);
+
+            _ = Markdown.Normalize(text);
+            _ = Markdown.ToPlainText(text);
+        }
+        catch (Exception ex) when (IsIgnorableException(ex)) { }
+    }
+
+    private static bool IsIgnorableException(Exception exception)
+    {
+        return exception.Message.Contains("Markdown elements in the input are too deeply nested", StringComparison.Ordinal);
+    }
+
+    private sealed class ResettableRoundtripRenderer : RoundtripRenderer
+    {
+        public ResettableRoundtripRenderer() : base(new StringWriter(new StringBuilder(1024 * 1024))) { }
+
+        public new void Reset() => base.Reset();
+    }
+}

src/Markdig.Fuzzing/run-fuzzer.ps1

@@ -0,0 +1,86 @@
+param (
+    [string]$configuration = $null
+)
+
+Set-StrictMode -Version Latest
+
+$libFuzzer = "libfuzzer-dotnet-windows.exe"
+$outputDir = "bin"
+
+function Get-LibFuzzer {
+    param (
+        [string]$Path
+    )
+
+    $libFuzzerUrl = "https://github.com/Metalnem/libfuzzer-dotnet/releases/download/v2025.05.02.0904/libfuzzer-dotnet-windows.exe"
+    $expectedHash = "17af5b3f6ff4d2c57b44b9a35c13051b570eb66f0557d00015df3832709050bf"
+
+    Write-Output "Downloading libFuzzer from $libFuzzerUrl..."
+
+    try {
+        $tempFile = "$Path.tmp"
+        Invoke-WebRequest -Uri $libFuzzerUrl -OutFile $tempFile -UseBasicParsing
+
+        $downloadedHash = (Get-FileHash -Path $tempFile -Algorithm SHA256).Hash
+
+        if ($downloadedHash -eq $ExpectedHash) {
+            Move-Item -Path $tempFile -Destination $Path -Force
+            Write-Output "libFuzzer downloaded successfully to $Path"
+        }
+        else {
+            Write-Error "Hash validation failed."
+            Remove-Item -Path $tempFile -Force -ErrorAction SilentlyContinue
+            exit 1
+        }
+    }
+    catch {
+        Write-Error "Failed to download libFuzzer: $($_.Exception.Message)"
+        Remove-Item -Path $tempFile -Force -ErrorAction SilentlyContinue
+        exit 1
+    }
+}
+
+# Check if libFuzzer exists, download if not
+if (-not (Test-Path $libFuzzer)) {
+    Get-LibFuzzer -Path $libFuzzer
+}
+
+$toolListOutput = dotnet tool list --global sharpFuzz.CommandLine 2>$null
+if (-not ($toolListOutput -match "sharpfuzz")) {
+    Write-Output "Installing sharpfuzz CLI"
+    dotnet tool install --global sharpFuzz.CommandLine
+}
+
+if (Test-Path $outputDir) {
+    Remove-Item -Recurse -Force $outputDir
+}
+
+if ($configuration -eq $null) {
+    $configuration = "Debug"
+}
+
+dotnet publish -c $configuration -o $outputDir
+
+$project = Join-Path $outputDir "Markdig.Fuzzing.dll"
+
+$fuzzingTarget = Join-Path $outputDir "Markdig.dll"
+
+Write-Output "Instrumenting $fuzzingTarget"
+& sharpfuzz $fuzzingTarget
+
+if ($LastExitCode -ne 0) {
+    Write-Error "An error occurred while instrumenting $fuzzingTarget"
+    exit 1
+}
+
+New-Item -ItemType Directory -Force -Path corpus | Out-Null
+
+$libFuzzerArgs = @("--target_path=dotnet", "--target_arg=$project", "-timeout=10", "corpus")
+
+# Add any additional arguments passed to the script
+if ($args) {
+    $libFuzzerArgs += $args
+}
+
+Write-Output "Starting libFuzzer with arguments: $libFuzzerArgs"
+& ./$libFuzzer @libFuzzerArgs

src/markdig.slnx

@@ -12,6 +12,9 @@
   <Project Path="Markdig.Benchmarks/Markdig.Benchmarks.csproj">
     <BuildDependency Project="Markdig/Markdig.csproj" />
   </Project>
+  <Project Path="Markdig.Fuzzing/Markdig.Fuzzing.csproj">
+    <BuildDependency Project="Markdig/Markdig.csproj" />
+  </Project>
   <Project Path="Markdig.Tests/Markdig.Tests.csproj">
     <BuildDependency Project="Markdig/Markdig.csproj" />
   </Project>