runtime: [.NET 6.0.5] HeapValidate() returns false for the last heap returned by GetProcessHeaps(), when both WinAPI calls are made thru P/Invoke

Description

In a minimal .NET 6 console test app (running on a Win10 host OS with .NET 6.0.5 runtime and Visual Studio 2022 installed), calling the GetProcessHeaps() WinAPI function through C# P/Invoke, and then calling HeapValidate(), also through P/Invoke, on the very last Heap returned by the former, always returns false.

Additionally, when other more complex C++ based DLLs that are performing the same GetProcessHeaps() + HeapValidate() call sequence internally, are accessed through C# P/Invoke, they may also trigger a Memory Access Violation in ntdll.dll sometimes.

The same use case neither returns false, nor optionally crashes, when the test app is targeting .NET Framework 4.7.2. So the question is: Is this .NET6-specific behavior expected, or a bug?

Here is a minimal C# code sample demonstrating the HeapValidate() returning false:

/* TestApp.csproj defined as:
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <ImplicitUsings>disable</ImplicitUsings>
    <Nullable>disable</Nullable>
  </PropertyGroup>
</Project>
*/

using System;
using System.Runtime.InteropServices;

internal static class TestApp
{
    [DllImport("kernel32.dll", SetLastError = true)]
    private static extern UInt32 GetProcessHeaps(UInt32 NumberOfHeaps, IntPtr[] ProcessHeaps);

    [Flags]
    private enum HeapValidateFlags : uint
    {
        HEAP_NO_FLAGS_ZERO = 0x00000000,
        HEAP_NO_SERIALIZE = 0x00000001
    }

    [DllImport("kernel32.dll", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool HeapValidate(IntPtr hHeap, HeapValidateFlags dwFlags, IntPtr lpMem);

    private static void Main()
    {
        IntPtr[] heaps;
        {
            uint numHeaps = GetProcessHeaps(0, null);
            heaps = new IntPtr[numHeaps];
            GetProcessHeaps(numHeaps, heaps);
        }
        foreach (IntPtr heap in heaps)
        {
            bool heapValid = HeapValidate(heap, HeapValidateFlags.HEAP_NO_FLAGS_ZERO, IntPtr.Zero);
            Console.WriteLine("heapValid = {0}", heapValid);
        }
    }
}

Configuration

• .NET 6 runtime: .NET 6.0.5
• OS: Windows 10 21H2 (OS build 19044.2604)
• Arch: x64

Regression?

The same GetProcessHeaps() and HeapValidate() WinAPI P/Invoke call sequence does not fail on .NET Framework 4.7.2, nor crashing in ntdll.dll.

Other information

When the WinAPI + P/Invoke sequence mentioned above triggers a Mem Access Violation crash in ntdll.dll, the crash stack trace looks like the following:

ntdll.dll!RtlEnterCriticalSection() ntdll.dll!RtlValidateHeap() KernelBase.dll!HeapValidate()

Thank you.