Add another static

Signed-off-by: Ludvig Liljenberg <4257730+ludfjig@users.noreply.github.com>

Author: ludfjig

src/hyperlight_guest_bin/src/guest_function/call.rs

@@ -86,6 +86,16 @@ pub(crate) fn internal_dispatch_function() {
     };
 
     let handle = unsafe { GUEST_HANDLE };
+    let canary = unsafe { crate::GUEST_HANDLE_CANARY };
+
+    if handle.peb().is_none() || canary != 0xDEAD_BEEF {
+        panic!(
+            "GUEST_HANDLE is None! canary={:#x} (expected 0xDEADBEEF), peb={:#x}. \
+             If canary is also 0, snapshot memory was not restored for this page.",
+            canary,
+            handle.peb().map_or(0, |p| p as u64),
+        );
+    }
 
     let function_call = handle
         .try_pop_shared_input_data_into::<FunctionCall>()

src/hyperlight_guest_bin/src/lib.rs

@@ -116,6 +116,10 @@ pub(crate) static HEAP_ALLOCATOR: ProfiledLockedHeap<32> =
     ProfiledLockedHeap(LockedHeap::<32>::empty());
 
 pub static mut GUEST_HANDLE: GuestHandle = GuestHandle::new();
+/// Canary value set to 0xDEAD_BEEF during init. If this reads as 0
+/// after snapshot restore while GUEST_HANDLE is None, the snapshot
+/// memory for this region was not properly restored.
+pub static mut GUEST_HANDLE_CANARY: u64 = 0;
 pub(crate) static mut REGISTERED_GUEST_FUNCTIONS: GuestFunctionRegister<GuestFunc> =
     GuestFunctionRegister::new();
 
@@ -205,6 +209,7 @@ pub(crate) extern "C" fn generic_init(
 ) -> u64 {
     unsafe {
         GUEST_HANDLE = GuestHandle::init(peb_address as *mut HyperlightPEB);
+        GUEST_HANDLE_CANARY = 0xDEAD_BEEF;
         #[allow(static_mut_refs)]
         let peb_ptr = GUEST_HANDLE.peb().unwrap();