Allow DAC fallback for non-CoreCLR runtimes

src/Microsoft.Diagnostics.DebugServices.Implementation/Runtime.cs

@@ -143,6 +143,12 @@ public string GetDbiFilePath()
         private ClrRuntime CreateRuntime()
         {
             CDacLoadPolicy policy = _settingsService.CDacLoadPolicy;
+            if (policy == CDacLoadPolicy.OnlyUseCDacForCoreClr)
+            {
+                policy = RuntimeType is RuntimeType.NetCore or RuntimeType.SingleFile
+                    ? CDacLoadPolicy.OnlyUseCDac
+                    : CDacLoadPolicy.PreferCDac;
+            }
             Trace.TraceInformation($"Runtime #{Id} data-access: begin (cDAC policy={policy})");
 
             if (policy != CDacLoadPolicy.UseLegacyDac)

src/Microsoft.Diagnostics.DebugServices/CDacLoadPolicy.cs

@@ -23,4 +23,9 @@ public enum CDacLoadPolicy
     /// </summary>
     UseLegacyDac = 2,
 
+    /// <summary>
+    /// Require cDAC activation for CoreCLR runtimes and allow DAC fallback for other runtimes.
+    /// </summary>
+    OnlyUseCDacForCoreClr = 3,
+
 }

src/Microsoft.Diagnostics.ExtensionCommands/Host/CommandFormatHelpers.cs

@@ -20,7 +20,7 @@ public static void DisplaySettingService(this CommandBase command)
         {
             ISettingsService settingsService = command.Services.GetService<ISettingsService>() ?? throw new DiagnosticsException("Settings service required");
             command.Console.WriteLine("Settings:");
-            command.Console.WriteLine($"-> Use cDAC: {settingsService.CDacLoadPolicy switch { CDacLoadPolicy.OnlyUseCDac => "true", CDacLoadPolicy.UseLegacyDac => "false", _ => "prefer" }}");
+            command.Console.WriteLine($"-> Use cDAC: {settingsService.CDacLoadPolicy switch { CDacLoadPolicy.OnlyUseCDac => "true", CDacLoadPolicy.UseLegacyDac => "false", CDacLoadPolicy.OnlyUseCDacForCoreClr => "coreclr", _ => "prefer" }}");
             command.Console.WriteLine($"-> DAC signature verification check enabled: {settingsService.DacSignatureVerificationEnabled}");
         }
 

src/Microsoft.Diagnostics.ExtensionCommands/Host/RuntimesCommand.cs

@@ -36,7 +36,7 @@ public class RuntimesCommand : CommandBase
         [Option(Name = "--all", Aliases = new string[] { "-a" }, Help = "Forces all runtimes to be enumerated.")]
         public bool All { get; set; }
 
-        [Option(Name = "--usecdac", Help = "Controls cDAC usage: true (required), false (disabled), prefer (allow DAC fallback).")]
+        [Option(Name = "--usecdac", Help = "Controls cDAC usage: true (required), false (disabled), prefer (allow DAC fallback), coreclr (required for CoreCLR, allow DAC fallback otherwise).")]
         public string UseCDac { get; set; }
 
         [Option(Name = "--DacSignatureVerification", Aliases = new string[] { "-v" }, Help = "Enforce the proper DAC certificate signing when loaded (true/false).")]
@@ -57,7 +57,8 @@ public override void Invoke()
                     "true" => CDacLoadPolicy.OnlyUseCDac,
                     "false" => CDacLoadPolicy.UseLegacyDac,
                     "prefer" => CDacLoadPolicy.PreferCDac,
-                    _ => throw new DiagnosticsException($"Invalid --usecdac value '{UseCDac}'. Expected true, false, or prefer."),
+                    "coreclr" => CDacLoadPolicy.OnlyUseCDacForCoreClr,
+                    _ => throw new DiagnosticsException($"Invalid --usecdac value '{UseCDac}'. Expected true, false, prefer, or coreclr."),
                 };
                 flush = true;
             }

src/SOS/SOS.Hosting/RuntimeWrapper.cs

@@ -264,7 +264,7 @@ private int GetClrDataProcess(
 
         private CDacLoadPolicy GetCDacLoadPolicy(IntPtr self)
         {
-            return _services.GetService<ISettingsService>()?.CDacLoadPolicy ?? CDacLoadPolicy.PreferCDac;
+            return EffectiveCDacLoadPolicy;
         }
 
         private int GetCorDebugInterface(
@@ -377,8 +377,7 @@ private IntPtr CreateClrDataProcessFromDac(IntPtr dacHandle)
         private int CreateCorDebugProcess(out IntPtr corDebugProcess)
         {
             corDebugProcess = IntPtr.Zero;
-            CDacLoadPolicy policy =
-                _services.GetService<ISettingsService>()?.CDacLoadPolicy ?? CDacLoadPolicy.PreferCDac;
+            CDacLoadPolicy policy = EffectiveCDacLoadPolicy;
             if (_runtime.RuntimeType == RuntimeType.Desktop)
             {
                 return policy == CDacLoadPolicy.OnlyUseCDac
@@ -404,6 +403,14 @@ private int CreateCorDebugProcess(out IntPtr corDebugProcess)
                 out corDebugProcess);
         }
 
+        private CDacLoadPolicy EffectiveCDacLoadPolicy =>
+            (_services.GetService<ISettingsService>()?.CDacLoadPolicy ?? CDacLoadPolicy.PreferCDac) switch
+            {
+                CDacLoadPolicy.OnlyUseCDacForCoreClr when _runtime.RuntimeType is RuntimeType.NetCore or RuntimeType.SingleFile => CDacLoadPolicy.OnlyUseCDac,
+                CDacLoadPolicy.OnlyUseCDacForCoreClr => CDacLoadPolicy.PreferCDac,
+                CDacLoadPolicy policy => policy,
+            };
+
         private int CreateDesktopCorDebugProcess(out IntPtr corDebugProcess)
         {
             corDebugProcess = IntPtr.Zero;

src/SOS/Strike/platform/runtimeimpl.cpp

@@ -595,7 +595,14 @@ CDacLoadPolicy Runtime::GetConfiguredCDacLoadPolicy()
 
 CDacLoadPolicy Runtime::GetCDacLoadPolicy() const
 {
-    return GetConfiguredCDacLoadPolicy();
+    CDacLoadPolicy policy = GetConfiguredCDacLoadPolicy();
+    if (policy == CDacLoadPolicy::OnlyUseCDacForCoreClr)
+    {
+        return GetRuntimeConfiguration() == IRuntime::WindowsDesktop
+            ? CDacLoadPolicy::PreferCDac
+            : CDacLoadPolicy::OnlyUseCDac;
+    }
+    return policy;
 }
 
 void Runtime::SetCDacLoadPolicy(CDacLoadPolicy policy)

src/SOS/Strike/strike.cpp

@@ -13501,6 +13501,9 @@ static void DisplayCDacLoadPolicy()
         case CDacLoadPolicy::PreferCDac:
             cdacPolicy = "prefer";
             break;
+        case CDacLoadPolicy::OnlyUseCDacForCoreClr:
+            cdacPolicy = "coreclr";
+            break;
         default:
             break;
     }
@@ -13540,9 +13543,13 @@ DECLARE_API(runtimes)
         {
             policy = CDacLoadPolicy::PreferCDac;
         }
+        else if (_stricmp(useCDac.data, "coreclr") == 0)
+        {
+            policy = CDacLoadPolicy::OnlyUseCDacForCoreClr;
+        }
         else
         {
-            ExtErr("Invalid --usecdac value '%s'. Expected true, false, or prefer.\n", useCDac.data);
+            ExtErr("Invalid --usecdac value '%s'. Expected true, false, prefer, or coreclr.\n", useCDac.data);
             return E_INVALIDARG;
         }
         Runtime::SetCDacLoadPolicy(policy);

src/SOS/inc/runtime.h

@@ -43,6 +43,7 @@ IRuntime : public IUnknown
         PreferCDac = 0,
         OnlyUseCDac = 1,
         UseLegacyDac = 2,
+        OnlyUseCDacForCoreClr = 3,
     };
 
     /// <summary>

src/tests/SOS.UnitTests/SOSRunner.cs

@@ -1159,7 +1159,7 @@ public async Task LoadSosExtension()
         // selects the legacy DAC so DOTNET_ENABLE_CDAC affects only the DAC-hosted contract reader.
         string cdacPolicyCommand = _config.DacMode switch
         {
-            DacMode.CDac => "runtimes --usecdac true",    // Force the standalone cDAC next to sos.dll.
+            DacMode.CDac => "runtimes --usecdac coreclr", // Require the standalone cDAC for CoreCLR and allow DAC fallback for other runtimes.
             DacMode.CDacVerify or DacMode.Dac => "runtimes --usecdac false", // Force the legacy in-box DAC.
             _ => null,
         };