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Security additions + edits (#2281)

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* Edits with a focus on security risks

* Modified example

* Flip sign - example of the risk of Solidity code when published to an append only ledger

* Fixed formatting
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Nemil Dalal 2016-06-20 14:56:44 -04:00 committed by ven
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@ -8,7 +8,7 @@ contributors:
Solidity lets you program on [Ethereum](https://www.ethereum.org/), a
blockchain-based virtual machine that allows the creation and
execution of smart contracts, without needing centralized or trusted parties.
execution of smart contracts, without requiring centralized or trusted parties.
Solidity is a statically typed, contract programming language that has
similarities to Javascript and C. Like objects in OOP, each contract contains
@ -18,8 +18,17 @@ global variables.
Some Ethereum contract examples include crowdfunding, voting, and blind auctions.
There is a high risk and high cost of errors in Solidity code, so you must be very careful to test
and slowly rollout. WITH THE RAPID CHANGES IN ETHEREUM, THIS DOCUMENT IS UNLIKELY TO STAY UP TO
DATE, SO YOU SHOULD FOLLOW THE SOLIDITY CHAT ROOM AND ETHEREUM BLOG FOR THE LATEST. ALL CODE HERE IS
PROVIDED AS IS, WITH SUBSTANTIAL RISK OF ERRORS OR DEPRECATED CODE PATTERNS.
Unlike other code, you may also need to add in design patterns like pausing, deprecation, and
throttling usage to reduce risk. This document primarily discusses syntax, and so excludes many
popular design patterns.
As Solidity and Ethereum are under active development, experimental or beta
features are explicitly marked, and subject to change. Pull requests welcome.
features are typically marked, and subject to change. Pull requests welcome.
```javascript
// First, a simple Bank contract
@ -40,6 +49,7 @@ contract SimpleBank { // CamelCase
// Declare state variables outside function, persist through life of contract
// dictionary that maps addresses to balances
// always be careful about overflow attacks with numbers
mapping (address => uint) private balances;
// "private" means that other contracts can't directly query balances
@ -49,7 +59,7 @@ contract SimpleBank { // CamelCase
// 'public' makes externally readable (not writeable) by users or contracts
// Events - publicize actions to external listeners
event DepositMade(address accountAddress, uint amount);
event LogDepositMade(address accountAddress, uint amount);
// Constructor, can receive one or many variables here; only one allowed
function AcmeBank() {
@ -65,7 +75,7 @@ contract SimpleBank { // CamelCase
// no "this." or "self." required with state variable
// all values set to data type's initial value by default
DepositMade(msg.sender, msg.value); // fire event
LogDepositMade(msg.sender, msg.value); // fire event
return balances[msg.sender];
}
@ -76,11 +86,14 @@ contract SimpleBank { // CamelCase
/// @return The balance remaining for the user
function withdraw(uint withdrawAmount) public returns (uint remainingBal) {
if(balances[msg.sender] >= withdrawAmount) {
// Note the way we deduct the balance right away, before sending - due to
// the risk of a recursive call that allows the caller to request an amount greater
// than their balance
balances[msg.sender] -= withdrawAmount;
if (!msg.sender.send(withdrawAmount)) {
// to be safe, may be sending to contract that
// has overridden 'send' which may then fail
// increment back only on fail, as may be sending to contract that
// has overridden 'send' on the receipt end
balances[msg.sender] += withdrawAmount;
}
}
@ -150,8 +163,10 @@ address public owner;
// All addresses can be sent ether
owner.send(SOME_BALANCE); // returns false on failure
if (owner.send) {} // typically wrap in 'if', as contract addresses have
// functions have executed on send and can fail
if (owner.send) {} // REMEMBER: wrap in 'if', as contract addresses have
// functions executed on send and these can fail
// Also, make sure to deduct balances BEFORE attempting a send, as there is a risk of a recursive
// call that can drain the contract
// can override send by defining your own
@ -351,8 +366,11 @@ function b() {
// access events from outside blockchain (with lightweight clients)
// typically declare after contract parameters
// Typically, capitalized - and add Log in front to be explicit and prevent confusion
// with a function call
// Declare
event Sent(address from, address to, uint amount); // note capital first letter
event LogSent(address indexed from, address indexed to, uint amount); // note capital first letter
// Call
Sent(from, to, amount);
@ -396,7 +414,10 @@ onlyIfState(State.A)
modifier checkValue(uint amount) {
_
if (msg.value > amount) {
msg.sender.send(amount - msg.value);
uint amountToRefund = amount - msg.value;
if (!msg.sender.send(amountToRefund)) {
throw;
}
}
}
@ -409,6 +430,21 @@ modifier checkValue(uint amount) {
// Syntax same as javascript, but no type conversion from non-boolean
// to boolean (comparison operators must be used to get the boolean val)
// For loops that are determined by user behavior, be careful - as contracts have a maximal
// amount of gas for a block of code - and will fail if that is exceeded
// For example:
for(uint x = 0; x < refundAddressList.length; x++) {
if (!refundAddressList[x].send(SOME_AMOUNT)) {
throw;
}
}
// Two errors above:
// 1. A failure on send stops the loop from completing, tying up money
// 2. This loop could be arbitrarily long (based on the amount of users who need refunds), and
// therefore may always fail as it exceeds the max gas for a block
// Instead, you should let people withdraw individually from their subaccount, and mark withdrawn
// 7. OBJECTS/CONTRACTS
@ -587,13 +623,13 @@ contract CrowdFunder {
address public fundRecipient; // creator may be different than recipient
uint public minimumToRaise; // required to tip, else everyone gets refund
string campaignUrl;
byte constant version = 1;
// Data structures
enum State {
Fundraising,
ExpiredRefundPending,
Successful,
ExpiredRefundComplete
ExpiredRefund,
Successful
}
struct Contribution {
uint amount;
@ -604,11 +640,11 @@ contract CrowdFunder {
State public state = State.Fundraising; // initialize on create
uint public totalRaised;
uint public raiseBy;
uint public completeAt;
Contribution[] contributions;
event fundingReceived(address addr, uint amount, uint currentTotal);
event allRefundsSent();
event winnerPaid(address winnerAddress);
event LogFundingReceived(address addr, uint amount, uint currentTotal);
event LogWinnerPaid(address winnerAddress);
modifier inState(State _state) {
if (state != _state) throw;
@ -620,10 +656,13 @@ contract CrowdFunder {
_
}
// Wait 6 months after final contract state before allowing contract destruction
modifier atEndOfLifecycle() {
if(state != State.ExpiredRefundComplete && state != State.Successful) {
if(!((state == State.ExpiredRefund || state == State.Successful) &&
completeAt + 6 months < now)) {
throw;
}
_
}
function CrowdFunder(
@ -651,9 +690,10 @@ contract CrowdFunder {
);
totalRaised += msg.value;
fundingReceived(msg.sender, msg.value, totalRaised);
LogFundingReceived(msg.sender, msg.value, totalRaised);
checkIfFundingCompleteOrExpired();
return contributions.length - 1; // return id
}
function checkIfFundingCompleteOrExpired() {
@ -663,9 +703,9 @@ contract CrowdFunder {
// could incentivize sender who initiated state change here
} else if ( now > raiseBy ) {
state = State.ExpiredRefundPending;
refundAll();
state = State.ExpiredRefund; // backers can now collect refunds by calling getRefund(id)
}
completeAt = now;
}
function payOut()
@ -676,22 +716,27 @@ contract CrowdFunder {
throw;
}
winnerPaid(fundRecipient);
LogWinnerPaid(fundRecipient);
}
function refundAll()
function getRefund(id)
public
inState(State.ExpiredRefundPending)
inState(State.ExpiredRefund)
{
uint length = contributions.length;
for (uint i = 0; i < length; i++) {
if(!contributions[i].contributor.send(contributions[i].amount)) {
throw;
}
if (contributions.length <= id || id < 0 || contributions[id].amount == 0 ) {
throw;
}
allRefundsSent();
state = State.ExpiredRefundComplete;
uint amountToRefund = contributions[id].amount;
contributions[id].amount = 0;
if(!contributions[id].contributor.send(amountToSend)) {
contributions[id].amount = amountToSend;
return false;
}
return true;
}
function removeContract()
@ -700,13 +745,13 @@ contract CrowdFunder {
atEndOfLifecycle()
{
selfdestruct(msg.sender);
// creator gets all money that hasn't be claimed
}
function () { throw; }
}
// ** END EXAMPLE **
// 10. OTHER NATIVE FUNCTIONS
// Currency units
@ -732,8 +777,14 @@ sha3("ab", "cd");
ripemd160("abc");
sha256("def");
// 11. SECURITY
// 11. LOW LEVEL FUNCTIONS
// Bugs can be disastrous in Ethereum contracts - and even popular patterns in Solidity,
// may be found to be antipatterns
// See security links at the end of this doc
// 12. LOW LEVEL FUNCTIONS
// call - low level, not often used, does not provide type safety
successBoolean = someContractAddress.call('function_name', 'arg1', 'arg2');
@ -742,7 +793,7 @@ successBoolean = someContractAddress.call('function_name', 'arg1', 'arg2');
someContractAddress.callcode('function_name');
// 12. STYLE NOTES
// 13. STYLE NOTES
// Based on Python's PEP8 style guide
// Quick summary:
@ -753,7 +804,7 @@ someContractAddress.callcode('function_name');
// else should be placed on own line
// 13. NATSPEC COMENTS
// 14. NATSPEC COMENTS
// used for documentation, commenting, and external UIs
// Contract natspec - always above contract definition
@ -773,9 +824,8 @@ someContractAddress.callcode('function_name');
- [Solidity Docs](https://solidity.readthedocs.org/en/latest/)
- [Solidity Style Guide](https://ethereum.github.io/solidity//docs/style-guide/): Ethereum's style guide is heavily derived from Python's [pep8](https://www.python.org/dev/peps/pep-0008/) style guide.
- [Browser-based Solidity Editor](http://chriseth.github.io/browser-solidity/)
- [Gitter Chat room](https://gitter.im/ethereum/solidity)
- [Gitter Solidity Chat room](https://gitter.im/ethereum/solidity)
- [Modular design strategies for Ethereum Contracts](https://docs.erisindustries.com/tutorials/solidity/)
- Editor Snippets ([Ultisnips format](https://gist.github.com/nemild/98343ce6b16b747788bc))
## Sample contracts
- [Dapp Bin](https://github.com/ethereum/dapp-bin)
@ -783,13 +833,24 @@ someContractAddress.callcode('function_name');
- [ConsenSys Contracts](https://github.com/ConsenSys/dapp-store-contracts)
- [State of Dapps](http://dapps.ethercasts.com/)
## Security
- [Thinking About Smart Contract Security](https://blog.ethereum.org/2016/06/19/thinking-smart-contract-security/)
- [Smart Contract Security](https://blog.ethereum.org/2016/06/10/smart-contract-security/)
- [Hacking Distributed Blog](http://hackingdistributed.com/)
## Information purposefully excluded
- Libraries
## Style
- Python's [PEP8](https://www.python.org/dev/peps/pep-0008/) is used as the baseline style guide, including its general philosophy
## Editors
- [Vim Solidity](https://github.com/tomlion/vim-solidity)
- Editor Snippets ([Ultisnips format](https://gist.github.com/nemild/98343ce6b16b747788bc))
## Future To Dos
- New keywords: protected, inheritable
- List of common design patterns (throttling, RNG, version upgrade)
- Common security anti patterns
Feel free to send a pull request with any edits - or email nemild -/at-/ gmail