Data dependency: add "must depend on"

[montyly]

Right now, the data dependency is an over-approximation that merges all the dependencies coming from all the node's fathers.

As a result, in a code like:

function f(address destination) internal{
     // use of destination
}
function g() internal{
     f(msg.sender):
     f(owner); 
}

destination in f is both dependent of msg.sender and owner. We should have a must depend on mode, to detect if a variable is always dependent of another one.

We can implement it by adding a new mode in analyses/data_dependency/data_dependency.py, where only the dependencies present in all the node's fathers are kept.

[montyly]

TODO @montyly: write examples/testcases

[priyankabose]

@montyly I am planning to start implementing this in Slither. Just need a few clarifications. When you say that the "must depend on" mode should should only consider the dependencies present in all the node's fathers, do you mean by immediate fathers?

[0xalpharush]

Could this just be a constant value/ constant propagation analysis? If the variable was always msg.sender, it is a constant, and we can check if the value that must be depended on is equivalent to the constant value. Otherwise, it is not a constant/ undefined, and we can return false.

[priyankabose]

I have a clear idea now where to start after talking to @0xalpharush. I will start with the constants first.