I would rather pass both smd's to surfacedeformBind:

In surfacedeformBind:

1. rename smd parameter to smd_orig (it should always be clear when we are using original vs. evaluated data, as much as possible).
2. add a new smd_eval parameter, and use it for error messages.

Among other benefits of this approach vs. just passing a string pointer, you can make 'printf'-like reports easily, while with proposed solution here any besides purely static strings messages would require more changes.
And another issue with current patch is that it would break i18n, you'd need to use the N_() macro…