Windows version from `sys.getwindowsversion()` is wrong in binaries that do not declare Windows 10 support

[MatejKafka]

Bug report

Bug description:

Repro:

1. Install Python for Windows (ideally create a new installation, since you'll need to patch it).
2. Using Resource Hacker, or any other PE resource editing tool, remove the <supportedOS Id="{8e0f7a12-bfb3-4fe8-b9a5-48fd50a15a9a}"/> line from the PE manifest, or just delete the whole manifest from the binary. Write back the patched binary. This enables compatibility shims for Windows 8 in the Win32 API when running the binary.
3. Run ...\python.exe -c "import sys; print(sys.getwindowsversion())"
4. Observe that the returned version indicates Windows 8 (major=6, minor=3) instead of the actual OS version (major=10, minor=0 for both Windows 10 and 11).

The root cause is very similar to https://issues.chromium.org/issues/494938698, see the full description there.

To retrieve the Windows version, Python reads the file version from kernel32.dll, because GetVersionEx will report old values due to backwards compatibility. However, GetFileVersionInfoW, which is used in the sysmodule.c implementation of sys.getwindowsversion() also has an internal compatibility shim where if the calling binary does not declare support for Windows 10, it will shim the actual DLL versions with a fake Windows 8 version for system libraries, resulting in sys.getwindowsversion() reporting a wrong OS version.

This is not an issue for python.exe directly, because it has a proper PE manifest that declares Windows 10 support, but it is an issue for apps that embed CPython and do not have a PE manifest, or do not declare Windows 10 support.

Proposed fix

Replace the whole implementation with a call to RtlGetVersion from ntdll, which is now officially documented, returns the correct version without shims and there's a substantial list of prominent projects relying on it already.

CPython versions tested on:

3.15

Operating systems tested on:

Windows

[picnixz]

but it is an issue for apps that embed CPython and do not have a PE manifest, or do not declare Windows 10 support.

Isn't it their fault then? cc @zooba should we support this (weird) setup?

[MatejKafka]

should we support this (weird) setup?

See the comment in the linked function, the current implementation explicitly tries to support apps running in compatibility mode. If CPython doesn't want to support running in compatibility mode, you could just call GetVersionEx and spare yourself the trouble. :)

Also note that the compatibility mode is on by default, you have to explicitly add a PE manifest to your binary with the right config to opt-out of it, so I expect it to be fairly common among binaries that embed CPython.

[picnixz]

Oh I see. Ok so the fix seems fairly simple then!