QFileSystemEngine/Win: Use GetTempPath2 when available

Because the documentation for GetTempPath nows says apps should call
GetTempPath2.[0]

Starting with Windows 11[1], and recently Windows 10[2],
GetTempPath2 was added. The difference being that elevated
processes are returned a different directory. Usually
'C:\Windows\SystemTemp'.

Currently temporary files of an elevated process may be placed in a
world write-able location. GetTempPath2, by default, but can be
overridden, places it in a directory that's only accessible by SYSTEM
and administrators.

[0] https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-gettemppathw#remarks
[1] https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-gettemppath2w
(Minimum supported client - Windows 11 Build 22000)
[2] https://blogs.windows.com/windows-insider/2025/03/13/releasing-windows-10-build-19045-5674-to-the-release-preview-channel/
(This update enables system processes to store temporary files ...)

[ChangeLog][QtCore][Important Behavior Changes] On
Windows, generating temporary directories for processes with elevated
privileges may now return a different path with a stricter
set of permissions. Please consult Microsoft's documentation from when
they made the same change for the .NET framework:
https://support.microsoft.com/en-us/topic/gettemppath-changes-in-windows-february-cumulative-update-preview-4cc631fb-9d97-4118-ab6d-f643cd0a7259

Change-Id: I5caf11151fb2f711bbc5599231f140598b3c9d03
Reviewed-by: Marc Mutz <[email protected]>
(cherry picked from commit 69633bcb58e681bac5bff3744e5a2352788dc36c)
Reviewed-by: Qt Cherry-pick Bot <[email protected]>
(cherry picked from commit 6a684a53b371ec483b27bf243af24819be63f85f)
(cherry picked from commit bbeccc0c22e520f46f0b33e281fa5ac85ac9c727)
Reviewed-by: Jani Heikkinen <[email protected]>

1 files changed, 9 insertions, 1 deletions

diff --git a/src/corelib/io/qfilesystemengine_win.cpp b/src/corelib/io/qfilesystemengine_win.cpp
index 3ec32e31a12..f4c9d656646 100644
--- a/src/corelib/io/qfilesystemengine_win.cpp
+++ b/src/corelib/io/qfilesystemengine_win.cpp
@@ -1633,7 +1633,15 @@ QString QFileSystemEngine::tempPath()
 {
     QString ret;
     wchar_t tempPath[MAX_PATH];
-    const DWORD len = GetTempPath(MAX_PATH, tempPath);
+    using GetTempPathPrototype = DWORD (WINAPI *)(DWORD, LPWSTR);
+    // We try to resolve GetTempPath2 and use that, otherwise fall back to GetTempPath:
+    static GetTempPathPrototype getTempPathW = []() {
+        const HMODULE kernel32 = GetModuleHandleW(L"kernel32.dll");
+        if (auto *func = QFunctionPointer(GetProcAddress(kernel32, "GetTempPath2W")))
+            return GetTempPathPrototype(func);
+        return GetTempPath;
+    }();
+    const DWORD len = getTempPathW(MAX_PATH, tempPath);
     if (len) { // GetTempPath() can return short names, expand.
         wchar_t longTempPath[MAX_PATH];
         const DWORD longLen = GetLongPathName(tempPath, longTempPath, MAX_PATH);