Update exception struct to match current version of rustc

src/panic.rs

@@ -7,6 +7,8 @@ use crate::abi::*;
 pub use crate::panic_handler::*;
 use crate::panicking::Exception;
 
+static CANARY: u8 = 0;
+
 #[repr(transparent)]
 struct RustPanic(Box<dyn Any + Send>, DropGuard);
 
@@ -25,6 +27,8 @@ impl Drop for DropGuard {
 #[repr(C)]
 struct ExceptionWithPayload {
     exception: MaybeUninit<UnwindException>,
+    // See rust/library/panic_unwind/src/gcc.rs for the canary values
+    canary: *const u8,
     payload: RustPanic,
 }
 
@@ -34,12 +38,23 @@ unsafe impl Exception for RustPanic {
     fn wrap(this: Self) -> *mut UnwindException {
         Box::into_raw(Box::new(ExceptionWithPayload {
             exception: MaybeUninit::uninit(),
+            canary: &CANARY,
             payload: this,
         })) as *mut UnwindException
     }
 
     unsafe fn unwrap(ex: *mut UnwindException) -> Self {
-        let ex = unsafe { Box::from_raw(ex as *mut ExceptionWithPayload) };
+        let ex = ex as *mut ExceptionWithPayload;
+        let canary = unsafe { core::ptr::addr_of!((*ex).canary).read() };
+        if !core::ptr::eq(canary, &CANARY) {
+            // This is a Rust exception but not generated by us.
+            #[cfg(feature = "panic-handler")]
+            {
+                foreign_exception();
+            }
+            crate::util::abort();
+        }
+        let ex = unsafe { Box::from_raw(ex) };
         ex.payload
     }
 }