[cDAC][wasm] MethodDesc validation rejects every MethodDesc: PrecodeStubs lookup needs PrecodeMachineDescriptor (absent with portable entry points) #135035

Description

On WebAssembly targets, IRuntimeTypeSystem.GetMethodDescHandle(ptr) throws ArgumentException: Invalid method desc pointer for valid MethodDescs. In practice that means no MethodDesc can be named on WASM, including every MethodDesc that IStackWalk.GetMethodDescPtr / IExecutionManager.GetMethodDesc returns.

Root cause

MethodValidation.ValidateMethodDescPointer checks the temporary entry point:

TargetCodePointer temporaryEntryPoint = GetTemporaryEntryPointIfExists(umd);
if (temporaryEntryPoint != TargetCodePointer.Null)
{
    Contracts.IPrecodeStubs precode = _target.Contracts.PrecodeStubs;
    TargetPointer methodDesc = precode.GetMethodDescFromStubAddress(temporaryEntryPoint);
    ...

PrecodeStubs_1.GetMethodDescFromStubAddress needs the PrecodeMachineDescriptor data type, which the descriptor compiles out under FEATURE_PORTABLE_ENTRYPOINTS:

// src/coreclr/vm/datadescriptor/datadescriptor.inc
#ifndef FEATURE_PORTABLE_ENTRYPOINTS
CDAC_TYPE_BEGIN(PrecodeMachineDescriptor)

The lookup throws InvalidOperationException: Field not found in any layout (names=[PrecodeMachineDescriptor]). The blanket catch (System.Exception) in ValidateMethodDescPointer swallows that and returns false, so every MethodDesc that has a temporary entry point is rejected.

I confirmed this by instrumenting each return false / catch in a local build. Every frame MethodDesc on a live browser target failed with exactly this exception at the temporary-entry-point check.

Expected

With portable entry points, the temporary entry point is a PortableEntryPoint whose MethodDesc field is the owner, matching the native FEATURE_PORTABLE_ENTRYPOINTS path. IExecutionManager already reads Data.PortableEntryPoint for GetDiagnosticCodeStartFromEntryPoint (#134827). Validation should take the same path when portable entry points are in use, instead of going through precode stubs.

A secondary concern: the blanket catch converts a reader/descriptor mismatch into "invalid pointer", which hid the root cause here.

Repro

• Target: CoreCLR browser-wasm, nightly 12.0.0-alpha.1.26480.103 (runtime cd42bb5).
• Reader: cDAC built from main @ bb1b237b389.
• At any CDP pause, walk a thread with IStackWalk, then call RuntimeTypeSystem.GetMethodDescHandle(GetMethodDescPtr(frame)) on any frameless frame. It throws Invalid method desc pointer.

Notes

As a workaround, Blazor-Playground/nesm overrides PrecodeStubs c1 on WASM targets. The override resolves GetMethodDescFromStubAddress via PortableEntryPoint.MethodDesc. With it, R2R and interpreted frames on a live target resolve to names that match an independent static ReadyToRun (MethodDefEntryPoints) name oracle.

Found while validating a live IStackWalk consumer in Blazor-Playground/nesm; related to #130988 / #134827.