I hope I have made it clear that almost everything said about passives in standard books of writing advice (and most of what linguistics books say as well) is mistaken. Often wildly mistaken.
The passive is not an undesirable feature limited to bad writing, it's a useful construction often needed for clear expression, and every good writer uses it.
The passive does not always involve a use of the copula.
The passive does not always involve masking the identity of the agent — it can be used to put the spotlight on the agent.
The NP that is the subject in a passive is not always the one that would have been the direct object if the clause had been designed as an active one: it can be an NP that would have been the complement of a preposition — some passive clauses involve stranded prepositions.