Good Contract Safety | Ethereum Basis Weblog

Solidity was began in October 2014 when neither the Ethereum community nor the digital machine had any real-world testing, the gasoline prices at the moment have been even drastically completely different from what they’re now. Moreover, a number of the early design selections have been taken over from Serpent. Over the last couple of months, examples and patterns that have been initially thought of best-practice have been uncovered to actuality and a few of them truly turned out to be anti-patterns. Resulting from that, we lately up to date a number of the Solidity documentation, however as most individuals most likely don’t comply with the stream of github commits to that repository, I wish to spotlight a number of the findings right here.

I cannot discuss concerning the minor points right here, please learn up on them within the documentation.

Sending Ether

Sending Ether is meant to be one of many easiest issues in Solidity, however it seems to have some subtleties most individuals don’t realise.

It’s important that at finest, the recipient of the ether initiates the payout. The next is a BAD instance of an public sale contract:

// THIS IS A NEGATIVE EXAMPLE! DO NOT USE!
contract public sale {
  deal with highestBidder;
  uint highestBid;
  perform bid() {
    if (msg.worth < highestBid) throw;
    if (highestBidder != 0)
      highestBidder.ship(highestBid); // refund earlier bidder
    highestBidder = msg.sender;
    highestBid = msg.worth;
  }
}

Due to the maximal stack depth of 1024 the brand new bidder can at all times enhance the stack measurement to 1023 after which name bid() which is able to trigger the ship(highestBid) name to silently fail (i.e. the earlier bidder won’t obtain the refund), however the brand new bidder will nonetheless be highest bidder. One method to examine whether or not ship was profitable is to examine its return worth:

/// THIS IS STILL A NEGATIVE EXAMPLE! DO NOT USE!
if (highestBidder != 0)
  if (!highestBidder.ship(highestBid))
    throw;
The 
throw
 assertion causes the present name to be reverted. This can be a unhealthy thought, as a result of the recipient, e.g. by implementing the fallback perform as 
perform() { throw; }
 can at all times drive the Ether switch to fail and this is able to have the impact that no one can overbid her.

The one method to forestall each conditions is to transform the sending sample right into a withdrawing sample by giving the recipient management over the switch:

/// THIS IS STILL A NEGATIVE EXAMPLE! DO NOT USE!
contract public sale {
  deal with highestBidder;
  uint highestBid;
  mapping(deal with => uint) refunds;
  perform bid() {
    if (msg.worth < highestBid) throw;
    if (highestBidder != 0)
      refunds[highestBidder] += highestBid;
    highestBidder = msg.sender;
    highestBid = msg.worth;
  }
  perform withdrawRefund() {
    if (msg.sender.ship(refunds[msg.sender]))
      refunds[msg.sender] = 0;
  }
}
 

Why does it nonetheless say “unfavourable instance” above the contract? Due to gasoline mechanics, the contract is definitely high quality, however it’s nonetheless not an excellent instance. The reason being that it’s unimaginable to stop code execution on the recipient as a part of a ship. Which means whereas the ship perform remains to be in progress, the recipient can name again into withdrawRefund. At that time, the refund quantity remains to be the identical and thus they might get the quantity once more and so forth. On this particular instance, it doesn’t work, as a result of the recipient solely will get the gasoline stipend (2100 gasoline) and it’s unimaginable to carry out one other ship with this quantity of gasoline. The next code, although, is weak to this assault: msg.sender.name.worth(refunds[msg.sender])().

Having thought of all this, the next code needs to be high quality (in fact it’s nonetheless not a whole instance of an public sale contract):

contract public sale {
  deal with highestBidder;
  uint highestBid;
  mapping(deal with => uint) refunds;
  perform bid() {
    if (msg.worth < highestBid) throw;
    if (highestBidder != 0)
      refunds[highestBidder] += highestBid;
    highestBidder = msg.sender;
    highestBid = msg.worth;
  }
  perform withdrawRefund() {
    uint refund = refunds[msg.sender];
    refunds[msg.sender] = 0;
    if (!msg.sender.ship(refund))
     refunds[msg.sender] = refund;
  }
}

Word that we didn’t use throw on a failed ship as a result of we’re in a position to revert all state adjustments manually and never utilizing throw has loads much less side-effects.

Utilizing Throw

The throw assertion is usually fairly handy to revert any adjustments made to the state as a part of the decision (or entire transaction relying on how the perform is named). You must bear in mind, although, that it additionally causes all gasoline to be spent and is thus costly and can doubtlessly stall calls into the present perform. Due to that, I wish to advocate to make use of it solely within the following conditions:

1. Revert Ether switch to the present perform

If a perform is just not meant to obtain Ether or not within the present state or with the present arguments, it’s best to use throw to reject the Ether. Utilizing throw is the one method to reliably ship again Ether due to gasoline and stack depth points: The recipient might need an error within the fallback perform that takes an excessive amount of gasoline and thus can not obtain the Ether or the perform might need been known as in a malicious context with too excessive stack depth (even perhaps previous the calling perform).

Word that by accident sending Ether to a contract is just not at all times a UX failure: You’ll be able to by no means predict wherein order or at which era transactions are added to a block. If the contract is written to solely settle for the primary transaction, the Ether included within the different transactions needs to be rejected.

2. Revert results of known as features

For those who name features on different contracts, you possibly can by no means understand how they’re applied. Which means the results of those calls are additionally not know and thus the one method to revert these results is to make use of throw. After all it’s best to at all times write your contract to not name these features within the first place, if you already know you’ll have to revert the results, however there are some use-cases the place you solely know that after the very fact.

Loops and the Block Fuel Restrict

There’s a restrict of how a lot gasoline might be spent in a single block. This restrict is versatile, however it’s fairly exhausting to extend it. Which means each single perform in your contract ought to keep beneath a specific amount of gasoline in all (affordable) conditions. The next is a BAD instance of a voting contract:

/// THIS IS STILL A NEGATIVE EXAMPLE! DO NOT USE!
contract Voting {
  mapping(deal with => uint) voteWeight;
  deal with[] yesVotes;
  uint requiredWeight;
  deal with beneficiary;
  uint quantity;
  perform voteYes() { yesVotes.push(msg.sender); }
  perform tallyVotes() {
    uint yesVotes;
    for (uint i = 0; i < yesVotes.size; ++i)
      yesVotes += voteWeight[yesVotes[i]];
    if (yesVotes > requiredWeight)
      beneficiary.ship(quantity);
  }
}

The contract truly has a number of points, however the one I wish to spotlight right here is the issue of the loop: Assume that vote weights are transferrable and splittable like tokens (consider the DAO tokens for example). This implies that you could create an arbitrary variety of clones of your self. Creating such clones will enhance the size of the loop within the tallyVotes perform till it takes extra gasoline than is obtainable inside a single block.

This is applicable to something that makes use of loops, additionally the place loops are usually not explicitly seen within the contract, for instance once you copy arrays or strings inside storage. Once more, it’s high quality to have arbitrary-length loops if the size of the loop is managed by the caller, for instance should you iterate over an array that was handed as a perform argument. However by no means create a state of affairs the place the loop size is managed by a celebration that might not be the one one affected by its failure.

As a aspect notice, this was one purpose why we now have the idea of blocked accounts contained in the DAO contract: Vote weight is counted on the level the place the vote is forged, to stop the truth that the loop will get caught, and if the vote weight wouldn’t be mounted till the tip of the voting interval, you may forged a second vote by simply transferring your tokens after which voting once more.

Receiving Ether / the fallback perform

If you’d like your contract to obtain Ether by way of the common ship() name, it’s a must to make its fallback perform low cost. It will probably solely use 2300, gasoline which neither permits any storage write nor perform calls that ship alongside Ether. Principally the one factor it’s best to do contained in the fallback perform is log an occasion in order that exterior processes can react on the very fact. After all any perform of a contract can obtain ether and isn’t tied to that gasoline restriction. Features truly must reject Ether despatched to them if they don’t need to obtain any, however we’re serious about doubtlessly inverting this behaviour in some future launch.