Align manifest assembly MVID table to 4 bytes in composite R2R images

src/coreclr/tools/aot/ILCompiler.ReadyToRun.Tests/TestCases/R2RTestSuites.cs

@@ -320,6 +320,86 @@ static void Validate(ReadyToRunReader reader)
         }
     }
 
+    [Fact]
+    public void CompositeManifestAssemblyMvidsAreAligned()
+    {
+        var compositeLib = new CompiledAssembly
+        {
+            AssemblyName = "MvidCompositeLib",
+            SourceResourceNames = ["CrossModuleInlining/Dependencies/CompositeLib.cs"],
+        };
+        var compositeMain = new CompiledAssembly
+        {
+            AssemblyName = nameof(CompositeManifestAssemblyMvidsAreAligned),
+            SourceResourceNames = ["CrossModuleInlining/CompositeBasic.cs"],
+            References = [compositeLib]
+        };
+
+        new R2RTestRunner(_output).Run(new R2RTestCase(
+            nameof(CompositeManifestAssemblyMvidsAreAligned),
+            [
+                new(nameof(CompositeManifestAssemblyMvidsAreAligned),
+                [
+                    new CrossgenAssembly(compositeLib),
+                    new CrossgenAssembly(compositeMain),
+                ])
+                {
+                    Options = [Crossgen2Option.Composite, Crossgen2Option.Optimize],
+                    Validate = Validate,
+                },
+            ]));
+
+        static void Validate(ReadyToRunReader reader)
+        {
+            string diag;
+            Assert.True(R2RAssert.ManifestAssemblyMvidsTableIsAligned(reader, out diag), diag);
+        }
+    }
+
+    public static bool IsWindows => System.OperatingSystem.IsWindows();
+
+    [ConditionalFact(nameof(IsWindows))]
+    public void CompositeManifestAssemblyMvidsArePaddedWhenPdbPresent()
+    {
+        var compositeLib = new CompiledAssembly
+        {
+            AssemblyName = "MvidCompositeLib",
+            SourceResourceNames = ["CrossModuleInlining/Dependencies/CompositeLib.cs"],
+        };
+        var compositeMain = new CompiledAssembly
+        {
+            AssemblyName = nameof(CompositeManifestAssemblyMvidsArePaddedWhenPdbPresent),
+            SourceResourceNames = ["CrossModuleInlining/CompositeBasic.cs"],
+            References = [compositeLib]
+        };
+
+        new R2RTestRunner(_output).Run(new R2RTestCase(
+            nameof(CompositeManifestAssemblyMvidsArePaddedWhenPdbPresent),
+            [
+                new(nameof(CompositeManifestAssemblyMvidsArePaddedWhenPdbPresent),
+                [
+                    new CrossgenAssembly(compositeLib),
+                    new CrossgenAssembly(compositeMain),
+                ])
+                {
+                    // --pdb creates an odd-sized debug directory section that exposes the MVID table
+                    // misalignment bug. The odd size derives from the composite output name length, so
+                    // renaming this test can shift the table back onto a 4-byte boundary and silently
+                    // neutralize the regression coverage; verify it still misaligns without the fix if
+                    // the name changes.
+                    Options = [Crossgen2Option.Composite, Crossgen2Option.Optimize],
+                    AdditionalArgs = ["--pdb"],
+                    Validate = Validate,
+                },
+            ]));
+
+        static void Validate(ReadyToRunReader reader)
+        {
+            string diag;
+            Assert.True(R2RAssert.ManifestAssemblyMvidsTableIsAligned(reader, out diag), diag);
+        }
+    }
+
     [Fact]
     public void RuntimeAsyncMethodEmission()
     {

src/coreclr/tools/aot/ILCompiler.ReadyToRun.Tests/TestCasesRunner/R2RResultChecker.cs

@@ -116,6 +116,40 @@ public static bool HasExpectedArmHotColdRuntimeFunctionTargets(ReadyToRunReader
         return result;
     }
 
+    /// <summary>
+    /// Returns true if the manifest assembly MVID table in a composite image is present, holds a
+    /// whole number of 16-byte GUID entries, and starts on a 4-byte aligned RVA. The runtime reads
+    /// each entry as a GUID by value, so the table must be 4-byte aligned to avoid alignment faults
+    /// (SIGBUS) on architectures such as 32-bit ARM that do not permit unaligned multi-word loads.
+    /// </summary>
+    public static bool ManifestAssemblyMvidsTableIsAligned(ReadyToRunReader reader, out string diagnostic)
+    {
+        const int GuidByteSize = 16;
+        const int RequiredAlignment = 4;
+
+        if (!reader.ReadyToRunHeader.Sections.TryGetValue(ReadyToRunSectionType.ManifestAssemblyMvids, out ReadyToRunSection section))
+        {
+            diagnostic = "Expected ManifestAssemblyMvids section not found.";
+            return false;
+        }
+
+        var failures = new List<string>();
+
+        if (section.Size <= 0)
+            failures.Add("Expected ManifestAssemblyMvids section to be non-empty.");
+
+        if (section.Size % GuidByteSize != 0)
+            failures.Add($"ManifestAssemblyMvids section size {section.Size} should be a multiple of {GuidByteSize} (a table of GUIDs).");
+
+        if ((section.RelativeVirtualAddress % RequiredAlignment) != 0)
+            failures.Add($"ManifestAssemblyMvids section RVA 0x{section.RelativeVirtualAddress:X8} should be aligned to {RequiredAlignment} bytes.");
+
+        diagnostic = failures.Count == 0
+            ? $"ManifestAssemblyMvids table is {RequiredAlignment}-byte aligned ({section.Size / GuidByteSize} entries)."
+            : string.Join(Environment.NewLine, failures);
+        return failures.Count == 0;
+    }
+
     private static bool SectionRVAIsEven(ReadyToRunReader reader, ReadyToRunSectionType sectionType, List<string> failures)
     {
         if (!reader.ReadyToRunHeader.Sections.TryGetValue(sectionType, out ReadyToRunSection section))

src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/ManifestAssemblyMvidHeaderNode.cs

@@ -56,7 +56,9 @@ public override ObjectData GetData(NodeFactory factory, bool relocsOnly = false)
             }
 
             byte[] manifestAssemblyMvidTable = _manifestNode.GetManifestAssemblyMvidTableData();
-            return new ObjectData(manifestAssemblyMvidTable, Array.Empty<Relocation>(), alignment: 1, new ISymbolDefinitionNode[] { this });
+
+            // GUID has a natural alignment of 4, so the table must be at least 4-byte aligned.
+            return new ObjectData(manifestAssemblyMvidTable, Array.Empty<Relocation>(), alignment: 4, new ISymbolDefinitionNode[] { this });
         }
     }
 }