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 fuel prices at the moment had been even drastically completely different from what they’re now. Moreover, a number of the early design selections had been taken over from Serpent. Over the past couple of months, examples and patterns that had been initially thought-about best-practice had been uncovered to actuality and a few of them really turned out to be anti-patterns. As a result of that, we not too long ago up to date a number of the Solidity documentation, however as most individuals in all probability don’t observe the stream of github commits to that repository, I wish to spotlight a number of the findings right here.

I can’t 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 can be crucial that at greatest, 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 improve the stack dimension 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 verify whether or not ship was profitable is to verify 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. It is a dangerous thought, as a result of the recipient, e.g. by implementing the fallback perform as 
perform() { throw; }
 can at all times power the Ether switch to fail and this might have the impact that no person 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 “destructive instance” above the contract? Due to fuel mechanics, the contract is definitely high quality, however it’s nonetheless not a great instance. The reason being that it’s unattainable to forestall code execution on the recipient as a part of a ship. Which means whereas the ship perform continues to be in progress, the recipient can name again into withdrawRefund. At that time, the refund quantity continues to be the identical and thus they’d 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 fuel stipend (2100 fuel) and it’s unattainable to carry out one other ship with this quantity of fuel. The next code, although, is susceptible to this assault: msg.sender.name.worth(refunds[msg.sender])().

Having thought-about all this, the next code must be high quality (after all it’s nonetheless not an entire 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;
  }
}

Be aware that we didn’t use throw on a failed ship as a result of we’re capable of revert all state adjustments manually and never utilizing throw has lots much less side-effects.

Utilizing Throw

The throw assertion is commonly fairly handy to revert any adjustments made to the state as a part of the decision (or entire transaction relying on how the perform known as). It’s a must to remember, although, that it additionally causes all fuel to be spent and is thus costly and can probably stall calls into the present perform. Due to that, I wish to suggest to make use of it solely within the following conditions:

1. Revert Ether switch to the present perform

If a perform just isn’t meant to obtain Ether or not within the present state or with the present arguments, you must use throw to reject the Ether. Utilizing throw is the one method to reliably ship again Ether due to fuel and stack depth points: The recipient may need an error within the fallback perform that takes an excessive amount of fuel and thus can’t obtain the Ether or the perform may need been referred to as in a malicious context with too excessive stack depth (maybe even previous the calling perform).

Be aware that by chance sending Ether to a contract just isn’t at all times a UX failure: You may 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 referred to as features

In case you name features on different contracts, you’ll be able to by no means understand how they’re applied. Which means the consequences of those calls are additionally not know and thus the one method to revert these results is to make use of throw. After all you must at all times write your contract to not name these features within the first place, if you realize you’ll have to revert the consequences, 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 fuel 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 fuel in all (cheap) 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 really 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 instance). This implies you can create an arbitrary variety of clones of your self. Creating such clones will improve the size of the loop within the tallyVotes perform till it takes extra fuel than is obtainable inside a single block.

This is applicable to something that makes use of loops, additionally the place loops usually are not explicitly seen within the contract, for instance whenever 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 in case 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 may not be the one one affected by its failure.

As a facet word, this was one motive 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 solid, to forestall 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 can solid 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 could solely use 2300, fuel which neither permits any storage write nor perform calls that ship alongside Ether. Mainly the one factor you must 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 fuel restriction. Capabilities really should reject Ether despatched to them if they don’t need to obtain any, however we’re serious about probably inverting this behaviour in some future launch.