[RFC] FAST_CWD warnings on ARM64 insider preview

[Corinna Vinschen]

Hi Jeremy,

On May 18 11:56, Jeremy Drake via Cygwin-patches wrote:
> I have been trying out x86_64 msys2 (a fork of cygwin) on Windows insider
> preview builds on ARM64, which added support for x86_64 emulation.  I was
> getting the notorious FAST_CWD warnings, which were interfering with
> MINGW CMake.  Last night late, I went looking at the cause, and found some
> code that attempted to disable FAST_CWD warnings, but only on i686, and
> trying to look at GetNativeSystemInfo, which was lying.  I quickly put
> together this patch, and it seems to work.
> 
> Note I did this late at night, with no real regard to investigating or
> matching code style.  This patch is currently more in an RFC state, if the
> approach looks OK, and I'd be grateful for any pointers on getting it into
> shape for evental acceptance.

> +#ifndef IMAGE_FILE_MACHINE_ARM64
> +#define IMAGE_FILE_MACHINE_ARM64 0xAA64
> +#endif

IMAGE_FILE_MACHINE_ARM64 is already defined for some time in winnt.h
so we won't need the ifdef.

> +      typedef BOOL (WINAPI * IsWow64Process2_t)(HANDLE hProcess, USHORT *pProcessMachine, USHORT *pNativeMachine);
> +      IsWow64Process2_t pfnIsWow64Process2 = (IsWow64Process2_t)GetProcAddress(GetModuleHandle("kernel32.dll"), "IsWow64Process2");
> +      if (pfnIsWow64Process2 && pfnIsWow64Process2(GetCurrentProcess(), &procmachine, &nativemachine) && nativemachine == IMAGE_FILE_MACHINE_ARM64)

We're having the autoload mechanism for that, i. e., your patch can get
rid of the GetModuleHandle/GetProcAddress preliminaries entirely.

By using the LoadDLLfuncEx() expression, failure to load the function
will result in a return value of FALSE with GetLastError ==
ERROR_PROC_NOT_FOUND, so this is safe on older systems.

It's easier to understand with a full example, so I took the liberty to
rewrite your patch accordingly.  Since the idea and the basic work is
yours, I'd push this under your name, see below.


Thanks,
Corinna


>From ee6dfb3bb0a5d51416d788133f818df0b8c1a207 Mon Sep 17 00:00:00 2001
From: Jeremy Drake <cygwin@jdrake.com>
Date: Wed, 19 May 2021 11:43:48 +0200
Subject: [PATCH] Cygwin: suppress FAST_CWD warnings on ARM64

The old check was insufficient: new insider preview builds of Windows
allow running x86_64 process on ARM64.  The IsWow64Process2 function
seems to be the intended way to figure this situation out.
---
 winsup/cygwin/autoload.cc |  1 +
 winsup/cygwin/path.cc     | 33 +++++++++------------------------
 2 files changed, 10 insertions(+), 24 deletions(-)

diff --git a/winsup/cygwin/autoload.cc b/winsup/cygwin/autoload.cc
index b2de2c3f42ce..0ebe15332013 100644
--- a/winsup/cygwin/autoload.cc
+++ b/winsup/cygwin/autoload.cc
@@ -587,6 +587,7 @@ LoadDLLfuncEx (GetLogicalProcessorInformationEx, 12, kernel32, 1)
 LoadDLLfuncEx (GetProcessGroupAffinity, 12, kernel32, 1)
 LoadDLLfunc (GetSystemTimePreciseAsFileTime, 4, kernel32)
 LoadDLLfuncEx (GetThreadGroupAffinity, 8, kernel32, 1)
+LoadDLLfuncEx (IsWow64Process2, 12, kernel32, 1)
 LoadDLLfuncEx (PrefetchVirtualMemory, 16, kernel32, 1)
 LoadDLLfunc (SetThreadGroupAffinity, 12, kernel32)
 
diff --git a/winsup/cygwin/path.cc b/winsup/cygwin/path.cc
index dd7048486a65..9b38793aa7d9 100644
--- a/winsup/cygwin/path.cc
+++ b/winsup/cygwin/path.cc
@@ -4707,30 +4707,15 @@ find_fast_cwd ()
   if (!f_cwd_ptr)
     {
       bool warn = 1;
-
-#ifndef __x86_64__
-      #ifndef PROCESSOR_ARCHITECTURE_ARM64
-      #define PROCESSOR_ARCHITECTURE_ARM64 12
-      #endif
-
-      SYSTEM_INFO si;
-
-      /* Check if we're running in WOW64 on ARM64.  Skip the warning as long as
-	 there's no solution for finding the FAST_CWD pointer on that system.
-
-	 2018-07-12: Apparently current ARM64 WOW64 has a bug:
-	 It's GetNativeSystemInfo returns PROCESSOR_ARCHITECTURE_INTEL in
-	 wProcessorArchitecture.  Since that's an invalid value (a 32 bit
-	 host system hosting a 32 bit emulator for itself?) we can use this
-	 value as an indicator to skip the message as well. */
-      if (wincap.is_wow64 ())
-	{
-	  GetNativeSystemInfo (&si);
-	  if (si.wProcessorArchitecture == PROCESSOR_ARCHITECTURE_ARM64
-	      || si.wProcessorArchitecture == PROCESSOR_ARCHITECTURE_INTEL)
-	    warn = 0;
-	}
-#endif /* !__x86_64__ */
+      USHORT emulated, hosted;
+
+      /* Check if we're running in WOW64 on ARM64.  Check on 64 bit as well,
+	 given that ARM64 Windows 10 provides a x86_64 emulation soon.  Skip
+	 warning as long as there's no solution for finding the FAST_CWD
+	 pointer on that system. */
+      if (IsWow64Process2 (GetCurrentProcess (), &emulated, &hosted)
+	  && hosted == IMAGE_FILE_MACHINE_ARM64)
+	warn = 0;
 
       if (warn)
 	small_printf ("Cygwin WARNING:\n"
-- 
2.30.2