drm/vboxvideo: Replace fake VLA at end of vbva_mouse_pointer_shape with real VLA

Replace the fake VLA at end of the vbva_mouse_pointer_shape shape with
a real VLA to fix a "memcpy: detected field-spanning write error" warning:

[   13.319813] memcpy: detected field-spanning write (size 16896) of single field "p->data" at drivers/gpu/drm/vboxvideo/hgsmi_base.c:154 (size 4)
[   13.319841] WARNING: CPU: 0 PID: 1105 at drivers/gpu/drm/vboxvideo/hgsmi_base.c:154 hgsmi_update_pointer_shape+0x192/0x1c0 [vboxvideo]
[   13.320038] Call Trace:
[   13.320173]  hgsmi_update_pointer_shape [vboxvideo]
[   13.320184]  vbox_cursor_atomic_update [vboxvideo]

Note as mentioned in the added comment it seems the original length
calculation for the allocated and send hgsmi buffer is 4 bytes too large.
Changing this is not the goal of this patch, so this behavior is kept.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240827104523.17442-1-hdegoede@redhat.com

drivers/gpu/drm/vboxvideo/hgsmi_base.c

@@ -139,7 +139,15 @@ int hgsmi_update_pointer_shape(struct gen_pool *ctx, u32 flags,
 		flags |= VBOX_MOUSE_POINTER_VISIBLE;
 	}
 
-	p = hgsmi_buffer_alloc(ctx, sizeof(*p) + pixel_len, HGSMI_CH_VBVA,
+	/*
+	 * The 4 extra bytes come from switching struct vbva_mouse_pointer_shape
+	 * from having a 4 bytes fixed array at the end to using a proper VLA
+	 * at the end. These 4 extra bytes were not subtracted from sizeof(*p)
+	 * before the switch to the VLA, so this way the behavior is unchanged.
+	 * Chances are these 4 extra bytes are not necessary but they are kept
+	 * to avoid regressions.
+	 */
+	p = hgsmi_buffer_alloc(ctx, sizeof(*p) + pixel_len + 4, HGSMI_CH_VBVA,
 			       VBVA_MOUSE_POINTER_SHAPE);
 	if (!p)
 		return -ENOMEM;

drivers/gpu/drm/vboxvideo/vboxvideo.h

@@ -351,10 +351,8 @@ struct vbva_mouse_pointer_shape {
 	 * Bytes in the gap between the AND and the XOR mask are undefined.
 	 * XOR mask scanlines have no gap between them and size of XOR mask is:
 	 * xor_len = width * 4 * height.
-	 *
-	 * Preallocate 4 bytes for accessing actual data as p->data.
 	 */
-	u8 data[4];
+	u8 data[];
 } __packed;
 
 /* pointer is visible */