From 74db542bf46d6238a68e5f54305250f37750d12a Mon Sep 17 00:00:00 2001
From: Riccardo Zaglia <riccardo.zaglia5@gmail.com>
Date: Sun, 15 Sep 2024 20:15:38 +0200
Subject: [PATCH] Fix UB in `Instance::supports_<prop>()`

---
 openxr/src/instance.rs | 43 +++++++++++++++++++++++++-----------------
 1 file changed, 26 insertions(+), 17 deletions(-)

diff --git a/openxr/src/instance.rs b/openxr/src/instance.rs
index e5f3417..bee4344 100644
--- a/openxr/src/instance.rs
+++ b/openxr/src/instance.rs
@@ -149,35 +149,44 @@ impl Instance {
         }
     }
 
+    // Note: to avoid undefined behaviour, `ext_props` should be initialized filling in all fields
+    // manually. The OpenXR spec does not guarantee that `get_system_properties()` will return an
+    // error when pushing `ext_props` to `SystemProperties::next` if the corresponding extension is
+    // unsupported. Using `T::out()` and then `assume_init()` could lead to undefined behaviour
+    // because not all fields may be initialized.
     #[inline]
-    pub fn supports_render_model_loading(&self, system: SystemId) -> Result<bool> {
+    pub fn get_system_props_ext<T>(&self, system: SystemId, ext_props: &mut T) -> Result<()> {
         unsafe {
-            let mut render_model = sys::SystemRenderModelPropertiesFB::out(ptr::null_mut());
-            let mut p = sys::SystemProperties::out(&mut render_model as *mut _ as _);
+            let mut p = sys::SystemProperties::out(ext_props as *mut _ as _);
             cvt((self.fp().get_system_properties)(
                 self.as_raw(),
                 system,
                 p.as_mut_ptr(),
             ))?;
-            Ok(render_model
-                .assume_init()
-                .supports_render_model_loading
-                .into())
         }
+        Ok(())
+    }
+
+    #[inline]
+    pub fn supports_render_model_loading(&self, system: SystemId) -> Result<bool> {
+        let mut props = sys::SystemRenderModelPropertiesFB {
+            ty: sys::SystemRenderModelPropertiesFB::TYPE,
+            next: ptr::null_mut(),
+            supports_render_model_loading: sys::FALSE,
+        };
+        self.get_system_props_ext(system, &mut props)?;
+        Ok(props.supports_render_model_loading.into())
     }
 
     #[inline]
     pub fn supports_hand_tracking(&self, system: SystemId) -> Result<bool> {
-        unsafe {
-            let mut hand = sys::SystemHandTrackingPropertiesEXT::out(ptr::null_mut());
-            let mut p = sys::SystemProperties::out(&mut hand as *mut _ as _);
-            cvt((self.fp().get_system_properties)(
-                self.as_raw(),
-                system,
-                p.as_mut_ptr(),
-            ))?;
-            Ok(hand.assume_init().supports_hand_tracking.into())
-        }
+        let mut props = sys::SystemHandTrackingPropertiesEXT {
+            ty: sys::SystemHandTrackingPropertiesEXT::TYPE,
+            next: ptr::null_mut(),
+            supports_hand_tracking: sys::FALSE,
+        };
+        self.get_system_props_ext(system, &mut props)?;
+        Ok(props.supports_hand_tracking.into())
     }
 
     /// Construct a `Path` from a string