Callbacks are an effective programming discipline for implementing
event-driven programming, especially in environments like Ethereum which
forbid shared global state and concurrency.
Callbacks allow a callee to delegate the execution back to the caller.
Though effective, they can lead to subtle mistakes principally in open environments where
callbacks can be added in a new code.
Indeed, several high profile bugs in smart contracts exploit callbacks.

We present the first static technique ensuring \emph{modularity} in
the presence of callbacks and apply it to verify prominent smart
contracts. Modularity ensures that external calls to other contracts
cannot affect the behavior of the contract. Importantly, modularity
is guaranteed without restricting programming.

In general, checking modularity is undecidable—even for programs without loops.
This paper describes an effective technique for soundly ensuring modularity harnessing SMT solvers.
The main idea is to define a constructive version of modularity using \emph{commutativity} and \emph{projection} operations on program segments.
We believe that this approach is also accessible to programmers, since counterexamples to modularity can be generated automatically
by the SMT solvers, allowing programmers to understand and fix the error.

We implemented our approach in order to demonstrate the precision of
the modularity analysis and applied it to real smart contracts, including a subset of the 150 most active contracts in
Ethereum.
Our implementation decompiles bytecode programs into an intermediate representation and then implements the modularity checking
using SMT queries.
Overall, we argue that our experimental results indicate that the method can be applied to many realistic contracts,
and that it is able to prove modularity where other methods fail.