Fix missing Mono.Posix.NETStandard assembly load on Windows ARM64

Lib/Utils/AssemblyInfo.cs

@@ -1,7 +1,9 @@
 // Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT License.
 using System.Resources;
+using System.Runtime.CompilerServices;
 
 [assembly: NeutralResourcesLanguage("en")]
+[assembly: InternalsVisibleTo("AsaTests")]
 
 namespace Microsoft.CST.AttackSurfaceAnalyzer.Utils
 {

Lib/Utils/MonoPosixAssemblyResolver.cs

@@ -0,0 +1,83 @@
+// Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT License.
+using System;
+using System.IO;
+using System.Reflection;
+using System.Runtime.CompilerServices;
+using System.Runtime.Loader;
+
+namespace Microsoft.CST.AttackSurfaceAnalyzer.Utils
+{
+    /// <summary>
+    ///     Fallback resolver for the Mono.Posix.NETStandard assembly.
+    ///
+    ///     Mono.Posix.NETStandard 1.0.0 only ships its managed assembly under
+    ///     runtimes/{rid}/lib/netstandard2.0/ and does not provide a win-arm64 runtime asset
+    ///     (only win-x64, win-x86, linux-* and osx). Because the default host resolves that
+    ///     assembly per-RID, on Windows ARM64 it cannot be found and a FileNotFoundException is
+    ///     thrown whenever code that references Mono.Unix is JIT compiled (for example during
+    ///     file system collection or monitoring).
+    ///
+    ///     The managed assembly is platform independent IL, so we resolve it from one of the
+    ///     runtime folders that actually ships with the application. This only runs when the
+    ///     default resolution fails, so it has no effect on platforms where the assembly already
+    ///     resolves normally.
+    /// </summary>
+    internal static class MonoPosixAssemblyResolver
+    {
+        internal const string MonoPosixAssemblyName = "Mono.Posix.NETStandard";
+
+        // RID-specific subfolders to probe, ordered by preference. win-arm64 is listed first so a
+        // matching asset is preferred should one ever ship (the 1.0.0 package does not provide one);
+        // the remaining RIDs all ship a managed Mono.Posix.NETStandard.dll that is identical IL.
+        internal static readonly string[] CandidateRuntimeIdentifiers = new[]
+        {
+            "win-arm64",
+            "win-x64",
+            "win-x86",
+            "linux-x64",
+            "linux-arm64",
+            "osx"
+        };
+
+        [ModuleInitializer]
+        internal static void Initialize()
+        {
+            AssemblyLoadContext.Default.Resolving += ResolveMonoPosix;
+        }
+
+        internal static Assembly? ResolveMonoPosix(AssemblyLoadContext context, AssemblyName assemblyName)
+        {
+            var path = FindAssemblyPath(AppContext.BaseDirectory, assemblyName?.Name);
+            if (path is null)
+            {
+                return null;
+            }
+
+            return context.LoadFromAssemblyPath(path);
+        }
+
+        /// <summary>
+        ///     Returns the path to a shipped managed Mono.Posix.NETStandard.dll under the given base
+        ///     directory, or null if the requested assembly is not Mono.Posix.NETStandard or no
+        ///     shipped copy could be found.
+        /// </summary>
+        internal static string? FindAssemblyPath(string baseDirectory, string? assemblyName)
+        {
+            if (string.IsNullOrEmpty(baseDirectory) || !string.Equals(assemblyName, MonoPosixAssemblyName, StringComparison.OrdinalIgnoreCase))
+            {
+                return null;
+            }
+
+            foreach (var rid in CandidateRuntimeIdentifiers)
+            {
+                var candidate = Path.Combine(baseDirectory, "runtimes", rid, "lib", "netstandard2.0", $"{MonoPosixAssemblyName}.dll");
+                if (File.Exists(candidate))
+                {
+                    return candidate;
+                }
+            }
+
+            return null;
+        }
+    }
+}

Tests/MonoPosixAssemblyResolverTests.cs

@@ -0,0 +1,79 @@
+// Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT License.
+using Microsoft.CST.AttackSurfaceAnalyzer.Utils;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System.IO;
+using System.Runtime.Loader;
+
+namespace Microsoft.CST.AttackSurfaceAnalyzer.Tests
+{
+    /// <summary>
+    ///     Tests for <see cref="MonoPosixAssemblyResolver" />, which provides the fallback used to
+    ///     load Mono.Posix.NETStandard on RIDs (such as win-arm64) for which the package ships no
+    ///     runtime asset.
+    /// </summary>
+    [TestClass]
+    public class MonoPosixAssemblyResolverTests
+    {
+        [TestMethod]
+        public void FindAssemblyPathReturnsNullForUnrelatedAssembly()
+        {
+            var dir = Directory.CreateDirectory(Path.Combine(Path.GetTempPath(), Path.GetRandomFileName())).FullName;
+            try
+            {
+                Assert.IsNull(MonoPosixAssemblyResolver.FindAssemblyPath(dir, "System.Text.Json"));
+            }
+            finally
+            {
+                Directory.Delete(dir, true);
+            }
+        }
+
+        [TestMethod]
+        public void FindAssemblyPathReturnsNullWhenNoRuntimeAssetShipped()
+        {
+            var dir = Directory.CreateDirectory(Path.Combine(Path.GetTempPath(), Path.GetRandomFileName())).FullName;
+            try
+            {
+                Assert.IsNull(MonoPosixAssemblyResolver.FindAssemblyPath(dir, MonoPosixAssemblyResolver.MonoPosixAssemblyName));
+            }
+            finally
+            {
+                Directory.Delete(dir, true);
+            }
+        }
+
+        [TestMethod]
+        public void FindAssemblyPathLocatesShippedRuntimeAsset()
+        {
+            var baseDir = Directory.CreateDirectory(Path.Combine(Path.GetTempPath(), Path.GetRandomFileName())).FullName;
+            try
+            {
+                // Simulate the shipped layout for an RID other than the (missing) win-arm64.
+                var ridDir = Path.Combine(baseDir, "runtimes", "win-x64", "lib", "netstandard2.0");
+                Directory.CreateDirectory(ridDir);
+                var expected = Path.Combine(ridDir, $"{MonoPosixAssemblyResolver.MonoPosixAssemblyName}.dll");
+                File.WriteAllText(expected, string.Empty);
+
+                var found = MonoPosixAssemblyResolver.FindAssemblyPath(baseDir, MonoPosixAssemblyResolver.MonoPosixAssemblyName);
+
+                Assert.AreEqual(expected, found);
+            }
+            finally
+            {
+                Directory.Delete(baseDir, true);
+            }
+        }
+
+        [TestMethod]
+        public void ResolverIsRegisteredAndMonoPosixLoads()
+        {
+            // The module initializer should have registered the resolver, and Mono.Posix.NETStandard
+            // should be loadable (either via the default host on supported RIDs or via the fallback).
+            var assembly = AssemblyLoadContext.Default.LoadFromAssemblyName(
+                new System.Reflection.AssemblyName(MonoPosixAssemblyResolver.MonoPosixAssemblyName));
+
+            Assert.IsNotNull(assembly);
+            Assert.IsNotNull(assembly.GetType("Mono.Unix.UnixFileInfo"));
+        }
+    }
+}