usertrap: disable syscall patching when ptraced

Author: ret2libc

pkg/sentry/platform/systrap/usertrap/usertrap_amd64.go

@@ -193,6 +193,18 @@ func (s *State) PatchSyscall(ctx context.Context, ac *arch.Context64, mm memoryM
 		return fmt.Errorf("no task found")
 	}
 
+	// Skip syscall patching when the task is being ptraced, because
+	// single-stepping and other debugger features are incompatible with
+	// the "syshandler" routine used to handle patched syscalls (see
+	// syshandler_amd64.S). This incompatibility can result in inconsistent
+	// process states and failures (e.g. SIGSEGV).
+	// TODO: for a full fix we'd need to roll back existing patched
+	//       syscalls, in case the traced program was patched before being
+	//       traced (e.g. PTRACE_ATTACH on an already running process).
+	if task.Tracer() != nil {
+		return nil
+	}
+
 	s.mu.Lock()
 	defer s.mu.Unlock()