found some sharp bits

Author: awortman-fastly

lucet-runtime/lucet-runtime-internals/src/instance/signals.rs

@@ -229,6 +229,20 @@ extern "C" fn handle_signal(signum: c_int, siginfo_ptr: *mut siginfo_t, ucontext
 
             // set up the faulting instruction pointer as the return address for `initiate_unwind`;
             // extremely unsafe, doesn't handle any edge cases yet
+            //
+            // TODO(Andy) can we avoid pushing onto the guest stack until knowing we want to force
+            // an unwind? maybe a "last address" field on ctx. This can be populated on context
+            // swap out (return address of lucet_context_swap) as well as here in the signal
+            // handler.
+            //
+            // TODO(Andy) if the last address is obtained through the signal handler, for a signal
+            // received exactly when we have just executed a `call` to a guest function, we
+            // actually want to not push it (or push it +1?) lest we try to unwind with a return
+            // address == start of function, where the system unwinder will unwind for the function
+            // at address-1, (probably) fail to find the function, and `abort()`.
+            //
+            // if `rip` == the start of some guest function, we can probably just discard it and
+            // use the return address instead.
             inst.push(rip as u64);
         }
         switch_to_host