Potential null pointer dereference in Win32Window::WndProc #289

[rly09]

Summary by Sourcery

Bug Fixes:

• Prevent a potential null pointer dereference in Win32Window::WndProc when handling window messages after creation.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Refactors Win32Window::WndProc to avoid a potential null pointer dereference by always returning DefWindowProc immediately after handling WM_NCCREATE and by guarding subsequent use of the window instance pointer with a null check.

Sequence diagram for updated Win32Window::WndProc message handling

sequenceDiagram
    participant OS
    participant Win32Window_WndProc as Win32Window_WndProc
    participant GetThisFromHandle
    participant Win32Window_MessageHandler as Win32Window_MessageHandler
    participant DefWindowProc

    OS->>Win32Window_WndProc: WndProc(window, message, wparam, lparam)
    alt message is WM_NCCREATE
        Win32Window_WndProc->>OS: Get CREATESTRUCT and lpCreateParams
        OS-->>Win32Window_WndProc: CREATESTRUCT with Win32Window pointer
        Win32Window_WndProc->>Win32Window_WndProc: EnableFullDpiSupportIfAvailable(window)
        Win32Window_WndProc->>Win32Window_WndProc: set window_handle_
        Win32Window_WndProc->>DefWindowProc: DefWindowProc(window, message, wparam, lparam)
        DefWindowProc-->>OS: LRESULT
    else any other message
        Win32Window_WndProc->>GetThisFromHandle: GetThisFromHandle(window)
        GetThisFromHandle-->>Win32Window_WndProc: Win32Window pointer or null
        alt Win32Window pointer not null
            Win32Window_WndProc->>Win32Window_MessageHandler: MessageHandler(window, message, wparam, lparam)
            Win32Window_MessageHandler-->>OS: LRESULT
        else Win32Window pointer is null
            Win32Window_WndProc-->>OS: 0 or default LRESULT
        end
    end

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Class diagram for Win32Window with updated WndProc behavior

classDiagram
    class Win32Window {
        HWND window_handle_
        +static LRESULT CALLBACK WndProc(HWND window, UINT message, WPARAM wparam, LPARAM lparam)
        +LRESULT MessageHandler(HWND window, UINT message, WPARAM wparam, LPARAM lparam)
        +static Win32Window* GetThisFromHandle(HWND window)
        +static void EnableFullDpiSupportIfAvailable(HWND window)
    }

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File-Level Changes

Change	Details	Files
Harden Win32Window::WndProc against null pointer dereference when retrieving the Win32Window instance pointer.	Return DefWindowProc immediately after processing WM_NCCREATE, instead of falling through to further message handling Move retrieval of the Win32Window instance pointer (GetThisFromHandle) outside the else-if chain for clarity Add an explicit null check on the Win32Window instance pointer before calling MessageHandler	windows/runner/win32_window.cpp

Possibly linked issues

• #(not specified): PR adds a null check around GetThisFromHandle and adjusts WM_NCCREATE flow, directly fixing the reported crash risk.

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[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• The potential null dereference still exists in the WM_NCCREATE path: that is used without checking whether window_struct->lpCreateParams (and thus that) is non-null before assigning that->window_handle_, so consider adding a null check or early return before dereferencing.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The potential null dereference still exists in the WM_NCCREATE path: `that` is used without checking whether `window_struct->lpCreateParams` (and thus `that`) is non-null before assigning `that->window_handle_`, so consider adding a null check or early return before dereferencing.

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