Read a 32-bit process's command line from a 64-bit host

src/DiffEngine.Tests/Wow64CommandLineTests.cs

@@ -0,0 +1,83 @@
+#if NET10_0
+/// <summary>
+/// Reading the command line of a 32-bit process from this 64-bit one.
+/// <para>
+/// NtQueryInformationProcess with ProcessBasicInformation answers a 64-bit caller with the 64-bit
+/// PEB, even when the target is running under WOW64. Reading that with 32-bit offsets produced
+/// nothing, so every 32-bit diff tool - which is most of the %ProgramFiles(x86)% installs the
+/// resolver goes out of its way to find - had no command line: never seen as already running, and
+/// never killed. The 32-bit PEB has to be asked for by name.
+/// </para>
+/// </summary>
+[NotInParallel]
+[RunOn(TUnit.Core.Enums.OS.Windows)]
+public class Wow64CommandLineTests
+{
+    [Test]
+    public async Task AThirtyTwoBitProcessHasAReadableCommandLine()
+    {
+        var wow = Path.Combine(
+            Environment.GetFolderPath(Environment.SpecialFolder.Windows),
+            "SysWOW64",
+            "cmd.exe");
+        if (!Environment.Is64BitProcess ||
+            !File.Exists(wow))
+        {
+            // A 32-bit host, or a Windows with no WOW64 layer. Nothing to say here
+            return;
+        }
+
+        // Distinctive, so this cannot match any other cmd on the machine
+        var marker = $"DiffEngineWow64Probe{Guid.NewGuid():N}";
+        var process = Process.Start(
+            new ProcessStartInfo
+            {
+                FileName = wow,
+                // Shaped like a diff tool invocation, because FindAll only keeps command
+                // lines with two file path arguments
+                Arguments = $"/c ping -n 30 127.0.0.1 >nul & rem C:\\probe\\{marker}.received.txt C:\\probe\\{marker}.verified.txt",
+                UseShellExecute = false,
+                CreateNoWindow = true
+            })!;
+
+        try
+        {
+            await Assert.That(await WaitForCommandLine(marker)).IsTrue();
+        }
+        finally
+        {
+            try
+            {
+                if (!process.HasExited)
+                {
+                    process.Kill();
+                }
+            }
+            catch
+            {
+                // Nothing useful to do if it has already gone
+            }
+
+            process.Dispose();
+        }
+    }
+
+    static async Task<bool> WaitForCommandLine(string marker)
+    {
+        for (var attempt = 0; attempt < 40; attempt++)
+        {
+            var found = WindowsProcess
+                .FindAll([with(StringComparer.OrdinalIgnoreCase), "cmd.exe"])
+                .Any(_ => _.Command.Contains(marker, StringComparison.Ordinal));
+            if (found)
+            {
+                return true;
+            }
+
+            await Task.Delay(250);
+        }
+
+        return false;
+    }
+}
+#endif

src/DiffEngine/Process/WindowsProcess.cs

@@ -29,6 +29,18 @@ private static partial int NtQueryInformationProcess(
         int size,
         out int returnLength);
 
+    /// <summary>
+    /// The same export, asked for a pointer sized answer rather than a struct: the ProcessWow64
+    /// Information class returns the address of a WOW64 target's 32-bit PEB.
+    /// </summary>
+    [LibraryImport("ntdll.dll", EntryPoint = "NtQueryInformationProcess")]
+    private static partial int NtQueryWow64Peb(
+        SafeProcessHandle handle,
+        int processInformationClass,
+        ref IntPtr info,
+        int size,
+        out int returnLength);
+
     [LibraryImport("kernel32.dll", SetLastError = true)]
     [return: MarshalAs(UnmanagedType.Bool)]
     private static partial bool IsWow64Process(
@@ -89,6 +101,14 @@ static extern bool ReadProcessMemory(
         IntPtr size,
         out IntPtr bytesRead);
 
+    [DllImport("ntdll.dll", EntryPoint = "NtQueryInformationProcess")]
+    static extern int NtQueryWow64Peb(
+        SafeProcessHandle handle,
+        int processInformationClass,
+        ref IntPtr info,
+        int size,
+        out int returnLength);
+
     [DllImport("kernel32.dll", SetLastError = true)]
     static extern bool IsWow64Process(SafeProcessHandle handle, out bool isWow64);
 #endif
@@ -99,6 +119,12 @@ static extern bool ReadProcessMemory(
     const int processTerminate = 0x0001;
     const int processBasicInformation = 0;
 
+    /// <summary>
+    /// ProcessWow64Information. Returns the address of the 32-bit PEB for a WOW64 target, or zero
+    /// for one that is not running under WOW64.
+    /// </summary>
+    const int processWow64Information = 26;
+
     [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
     struct PROCESSENTRY32W
     {
@@ -298,16 +324,46 @@ static int FindDotSeparatedPath(CharSpan span)
                 return null;
             }
 
-            return Environment.Is64BitProcess && !isTarget32Bit
-                ? ReadCommandLine64(handle, pbi.PebBaseAddress)
-                : ReadCommandLine32(handle, pbi.PebBaseAddress);
+            if (!Environment.Is64BitProcess)
+            {
+                // A 32-bit caller cannot reach a 64-bit target's PEB, so there is nothing useful to
+                // read rather than something wrong to read
+                if (Environment.Is64BitOperatingSystem &&
+                    !isTarget32Bit)
+                {
+                    return null;
+                }
+
+                return ReadCommandLine32(handle, pbi.PebBaseAddress);
+            }
+
+            if (!isTarget32Bit)
+            {
+                return ReadCommandLine64(handle, pbi.PebBaseAddress);
+            }
+
+            // A WOW64 target seen from a 64-bit caller. ProcessBasicInformation answers with the
+            // 64-bit PEB the system keeps for it, and reading that with 32-bit offsets yields
+            // nothing usable - so every 32-bit diff tool, which is most of the %ProgramFiles(x86)%
+            // installs the resolver goes out of its way to find, had no command line at all: never
+            // detected as already running, and never killed. The 32-bit PEB has to be asked for
+            return TryGetWow64Peb(handle, out var wow64Peb)
+                ? ReadCommandLine32(handle, wow64Peb)
+                : null;
         }
         catch
         {
             return null;
         }
     }
 
+    static bool TryGetWow64Peb(SafeProcessHandle handle, out IntPtr peb)
+    {
+        peb = IntPtr.Zero;
+        return NtQueryWow64Peb(handle, processWow64Information, ref peb, IntPtr.Size, out _) == 0 &&
+               peb != IntPtr.Zero;
+    }
+
     static string? ReadCommandLine64(SafeProcessHandle handle, IntPtr pebAddress)
     {
         // In 64-bit PEB, ProcessParameters is at offset 0x20