26020 Lab 5: Solidity
1 Setup
Make sure you have downloaded Remix from Blackboard:
and that you can compile and run programs. To do this, you might need to click on
the ’plug’ icon on the left hand menu, and made sure ‘Solidity compiler’ and ‘deploy
and run transactions’ are enabled. This will let you compile and run Solidity programs
in Remix as seen in the videos.
Remix is a browser based editor, and has been tested for this course on Google
Chrome on Linux and Windows. With other browsers you may get strange behaviour.
It is better to edit in a separate text editor and paste into Remix for testing, as it
can have problems with saving files and allowing text to be copied out of it in some
browsers. Make sure you always have a copy of your code in another editor so that
you don’t lose your work.
Clone the gitlab repository COMP26020-lab5-S-Solidity_ where username
is replaced by your username. This contains the files you will need for the exercise.
While doing these exercises, refer to the Solidity documentation for version 6: https:
//docs.soliditylang.org/en/v0.6.0/ .
2 Background
The exercises concern three contracts which should interact with each other, alongside
other contracts which we assume exist (but do not implement or worry about the
implementation of).
The first contract we consider is a ‘paylock’. The idea is that a supplier does some
work, which can then be collected by a customer. If the customer collects early, they
get a discount, and how much discount they get depends on how early: there are two
deadlines. If they miss the second deadline they forfeit their discount altogether.
1
The blobs indicate possible states of the paylock, and the arrows represent function
calls. The ‘Start’ arrow represents the constructor. The idea is that the functions
should only succeed if the paylock is in the state at the beginning of the arrow, and
then the resulting state should be the one at the end. Of course, there are other
conditions: collect_1_Y should only succeed if called before the first deadline, and
collect_1_N should only succeed if called once the first deadline has passed; similar
considerations apply to the other two collect functions.
Look in the file paylock.sol to see a partially finished implementation of the paylock.
The first two exercises (see next section) concern only the logic of the paylock.
They are about adding features to the implementation, though we never complete a
realistic implementation.
The subsequent exercises are about implementing a supplier which has to interact
with both the paylock contract and a rental contract which it needs to use to complete
its work. As above, we will only model certain aspects of these contracts. On the one
hand this makes the exercises manageable, but on the other hand it can be confusing
if not pointed out: you would naturally wonder when we would add the rest of the
necessary features!
3 Exercises
The implementation of the paylock which you are given does not model the passage
of time. To do this, we will add a tick function, representing the passage of one unit
of time. We shall assume for the moment that the tick function is going to be called
by a neutral third party, who we trust to call it at a regular interval. For now we also
trust all other contracts in the universe not to call this function. (And assume that
the blockchain updates quickly enough that this is a reasonable model of time! This
is not how one would deal with time in a real smart contract system.)
2
EXERCISE (2 marks)
Add an int variable clock and a tick function which models the passage of
time.
Modify the various collect functions to adhere to the deadlines, where we
consider the first deadline to happen if the clock has reached 4 units of time or
more, and the second deadline to be when the clock has increased by 4 units
of time or more from when collect_1_N was called.
We now need to make sure this tick function can only be called by the agreed third
party.
EXERCISE (2 marks)
Add an address variable timeAdd to the contract. Add an argument to the
constructor and set the value of timeAdd to that argument.
Now modify tick so that it can only be called by someone from the address
timeAdd .
Tip: when testing your code, copy one of the addresses from the ‘Account’
dropdown menu and paste it into the constructor argument. That should make
it easier to experiment.
Look in the file supplier.txt and paste its contents at the end of paylock.sol. Note
how the Supplier contract interacts with the paylock, indicating to the paylock when
it has finished its task. In the next exercise, we will make it interact with the Rental
contract too. The idea is that in order to finish its job, the Supplier must rent a
resource, then return it, before calling finish will succeed.
EXERCISE (2 marks)
Add functions aquire_resource and return_resource which must be called
in that order to the Supplier contract. To do this you will need to add new
local variables.
Add a local variable representing an instance of the Rental contract, and
allow the address of an instance of Rental to be passed as an argument to the
constructor. Modify the aquire_resource and return_resource functions so
that they call the appropriate functions of the Rental contract.
Tip: Since the constructor of Supplier requires the addresses of a Paylock
and a Rental, make sure you deploy instances of those first when testing.
We will now make our model of the Rental contract somewhat more realistic, by
requiring the payment of a deposit which is returned once the rented resource is returned.
For th purposes of the lab we assume that the deposit is 1 wei.
3
Since the Rental contract is not supposed to assume that it is being called be a
Supplier, it should assume that the contract it is connected to implements a receive
function; you can read about this in the Solidity language documentation:
https://docs.soliditylang.org/en/v0.6.0/contracts.html#receive-ether-function.
Since we are not allowed to assume the calling contract is a Supplier, it is also useful
to look at the functions which can be applied to any address:
https://docs.soliditylang.org/en/v0.6.0/types.html#members-of-addresses . In
fact, our intention is to make as few assumptions about the other contract as possible,
so we will use the low-level .call() function. Find out how to make this work and
attach a value to it.
EXERCISE (2 marks)
Modify the Rental contract in the following way. First find the commented
line //CHECK FOR PAYMENT HERE and replace it with something which prevents
the function from succeeding unless proper payment is made. You will also
have to make the functions payable.
Then find the commented line //RETURN DEPOSIT HERE and replace it with a
single use of the .call function which returns the deposit.
Modify the Supplier contract so that it has a receive function, and make
sure that Rental does not assume that the contract which calls its functions is
an instance of Supplier. Modify the external function calls made by Supplier
to Rental so that they transfer the deposit as appropriate.
At this point you should copy the file supplier.sol to supplier.sol and work
in supplier2.sol .
The rental contract as implemented has a security flaw which is described in (which
is described in the ‘Reentrancy’ section of chapter 9 of Antonopoulos’s book Mastering
Etherium (available online from the library, and also at https://github.com/
ethereumbook/ethereumbook/blob/develop/09smart-contracts-security.asciidoc).
EXERCISE (1 mark)
Modify the Supplier contract to take advantage of this security flaw to take any
Ether belonging to the Rental contract.
Make sure this work is saved in the file supplier2.sol
At this point you should copy the file supplier2.sol to suppler3.sol and work in
supplier3.sol .
EXERCISE (1 mark)
Re-order the lines of the retrieve_resource function of the Rental contract so that
the vulnerability above is fixed.
Make sure this work is saved in the file supplier2.sol
Note: You need only prevent the attack described here while preserving correct functionality;
you do not need to solve any other security flaws.
4 Submission
Submission is by gitlab, following the same procedure as the other labs for this unit. Ensure
that you have pushed a commit containing your submission (i.e. make sure you have added
all files to the repository), tagged with the tag lab5-submission , by 6pm on 26/04.
Check SPOT to make sure your submission has been received correctly, and email me
(Joe) if you notice any strange behaviour from SPOT.