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Bytecode Symbolic Execution Result

addLiquidity-1.txt

Symbolic exploration of addLiquidity bytecode when total_liquidity > 0.

  1. if assert deadline > block.timestamp and max_tokens > 0 fails:
    • REVERT
  2. else if assert min_liquidity > 0 fails:
    • REVERT
  3. else if self.token.balanceOf(self) throws/reverts:
    • REVERT
  4. else if msg.value == 0:
    • REVERT (since it leads to the failure of assert liquidity_minted >= min_liquidity afterwards)
  5. else if msg.value * token_reserve overflows:
    • REVERT
  6. else if msg.value * token_reserve / eth_reserve + 1 overflows (in addition):
    • REVERT
  7. else if msg.value * total_liquidity overflows:
    • REVERT
  8. else if assert max_tokens >= token_amount and liquidity_minted >= min_liquidity fails:
    • REVERT
  9. else if self.balances[msg.sender] + liquidity_minted overflows:
    • REVERT
  10. else if total_liquidity + liquidity_minted overflows:
    • REVERT
  11. else if self.token.transferFrom(msg.sender, self, token_amount) throws/reverts:
    • REVERT
  12. else if assert self.token.transferFrom(msg.sender, self, token_amount) fails:
    • REVERT
  13. else:
    • UPDATE self.balances[msg.sender] := self.balances[msg.sender] + liquidity_minted
    • UPDATE self.totalSupply := total_liquidity + liquidity_minted
    • RETURN liquidity_minted

We have 13 symbolic paths. Here, "else if" implicitly means the negation of all previous conditions, as in the usual programming language semantics. There are many reverting paths. Most of them are trivial, falling into the following categories:

  • assertion failure
  • external call failure (i.e., throwing, reverting, or returning false)
  • arithmetic overflows

A non-trivial case is the path #4, where msg.value == 0 leads to the failure of the assertion liquidity_minted >= min_liquidity later. This case is not trivial to catch without the symbolic execution result. Indeed, the latest version add assert msg.value > 0 in the beginning to save the unnecessary gas consumption in that case.

Note that here we assume that the existing balance before calling this function is non-zero, that is, self.balance > msg.value, thus eth_reserve is not zero and no division-by-zero failure occurs. Let us show that total_liquidity > 0 implies eth_reserve > 0. We show it by induction:

  • Base case. In the beginning, once the contract is created, total_liquidity = 0 and eth_reserve >= 0.
  • Inductive case: We have four (families) of functions that can update self.balance, that is, addLiquidity, removeLiquidity, ethToToken*, and tokenToEth*:
    • addLiquidity:
      • When total_liquidity = 0, we have total_liquidity > 0 and eth_reserve > 0 at the end of the function since msg.value > 0.
      • When total_liquidity > 0, it strictly increases both total_liquidity and eth_reserve since msg.value > 0.
    • removeLiquidity: We have eth_amount < self.balance if amount < total_liquidity. That is, the remaining balance is greater than 0 at the end of the function, as long as it does not burn the whole liquidity.
    • ethToToken* family: They always increase the balance.
    • tokenToEth* family: The remaining balance is always greater than 0 at the end of the functions. They do not allow to buy the whole amount of the reserved Ether, since getOutputPrice reverts when output_amount >= output_reserve, and the return value of getInputPrice is always less than the balance (i.e., input_amount_with_fee * output_reserve / ((input_reserve * 1000) + input_amount_with_fee) < output_reserve because input_reserve > 0). In other word, you need the infinite amount of tokens to buy the whole Ether, which is not possible in reality.

addLiquidity-2.txt

Symbolic exploration of addLiquidity bytecode when total_liquidity = 0.

  1. if assert deadline > block.timestamp and max_tokens > 0 fails:
    • REVERT
  2. else if assert (self.factory != ZERO_ADDRESS and self.token != ZERO_ADDRESS) and msg.value >= 1000000000 fails:
    • REVERT
  3. else if self.factory.getExchange(self.token) throws/reverts:
    • REVERT
  4. else if assert self.factory.getExchange(self.token) == self fails:
    • REVERT
  5. else if self.token.transferFrom(msg.sender, self, token_amount) throws/reverts:
    • REVERT
  6. else if assert self.token.transferFrom(msg.sender, self, token_amount) fails:
    • REVERT
  7. else:
    • UPDATE self.totalSupply := initial_liquidity
    • UPDATE self.balances[msg.sender] := initial_liquidity
    • RETURN initial_liquidity

removeLiquidity.txt

Symbolic exploration of removeLiquidity bytecode.

  1. if assert msg.value == 0 (implicit assertion) fails:
    • REVERT
  2. else if assert (amount > 0 and deadline > block.timestamp) and (min_eth > 0 and min_tokens > 0) fails:
    • REVERT
  3. else if assert total_liquidity > 0 fails:
    • REVERT
  4. else if self.token.balanceOf(self) throws/reverts:
    • REVERT
  5. else if amount * self.balance overflows:
    • REVERT
  6. else if amount * token_reserve overflows:
    • REVERT
  7. else if assert eth_amount >= min_eth and token_amount >= min_tokens fails:
    • REVERT
  8. else if self.balances[msg.sender] - amount overflows:
    • REVERT
  9. else if total_liquidity - amount overflows:
    • REVERT
  10. else if self.token.transfer(msg.sender, token_amount) throws/reverts:
    • REVERT
  11. else if assert self.token.transfer(msg.sender, token_amount) fails:
    • REVERT
  12. else if send(msg.sender, eth_amount) throws/reverts:
    • REVERT
  13. else:
    • UPDATE self.balances[msg.sender] := self.balances[msg.sender] - amount
    • UPDATE self.totalSupply := total_liquidity - amount
    • RETURN eth_amount, token_amount

NOTE: The path #1 is due to the implicit assertion of a non @payable function.

ethToTokenSwapInput.txt

Symbolic exploration of ethToTokenSwapInput bytecode.

Inside ethToTokenInput function:

  1. else if assert deadline >= block.timestamp and (eth_sold > 0 and min_tokens > 0) fails:
    • REVERT
  2. else if self.token.balanceOf(self) throws/reverts:
    • REVERT
  3. Inside getInputPrice function:
    1. else if assert input_reserve > 0 and output_reserve > 0 fails:
      • REVERT
    2. else if input_amount * 997 overflows:
      • REVERT
    3. else if input_amount_with_fee * output_reserve overflows:
      • REVERT
    4. else if input_reserve * 1000 overflows:
      • REVERT
    5. else if (input_reserve * 1000) + input_amount_with_fee overflows (in addition):
      • REVERT
  4. else if assert tokens_bought >= min_tokens fails:
    • REVERT
  5. else if self.token.transfer(recipient, tokens_bought) throws/reverts:
    • REVERT
  6. else if assert self.token.transfer(recipient, tokens_bought) fails:
    • REVERT
  7. else:
    • RETURN msg.value * 997 * token_reserve / ((self.balance - msg.value) * 1000 + msg.value * 997)

ethToTokenSwapOutput.txt

Symbolic exploration of ethToTokenSwapOutput bytecode.

Inside ethToTokenOutput function:

  1. if assert deadline >= block.timestamp and (tokens_bought > 0 and max_eth > 0) fails:
    • REVERT
  2. else if self.token.balanceOf(self) throws/reverts:
    • REVERT
  3. Inside getOutputPrice function:
    1. else if assert input_reserve > 0 and output_reserve > 0 fails:
      • REVERT
    2. else if input_reserve * output_amount overflows:
      • REVERT
    3. else if (input_reserve * output_amount) * 1000 overflows (in the second multiplication):
      • REVERT
    4. else if output_reserve - output_amount overflows:
      • REVERT
    5. else if output_reserve == output_amount:
      • REVERT (since this leads to the division-by-zero failure of numerator / denominator afterwards)
    6. else if (output_reserve - output_amount) * 997 overflows (in multiplication):
      • REVERT
    7. else if numerator / denominator + 1 overflows (in addition):
      • REVERT
  4. else if max_eth - as_wei_value(eth_sold, 'wei') overflows (in subtraction):
    • REVERT
  5. else:
    1. if eth_refund > 0:
      1. if send(buyer, eth_refund) throws/reverts:
        • REVERT
      2. else if self.token.transfer(recipient, tokens_bought) throws/reverts:
        • REVERT
      3. else if assert self.token.transfer(recipient, tokens_bought) fails:
        • REVERT
      4. else:
        • RETURN ((self.balance - msg.value) * token_bought * 1000) / ((token_reserve - token_bought) * 997) + 1
    2. else:
      1. if self.token.transfer(recipient, tokens_bought) throws/reverts:
        • REVERT
      2. else if assert self.token.transfer(recipient, tokens_bought) fails:
        • REVERT
      3. else:
        • RETURN ((self.balance - msg.value) * token_bought * 1000) / ((token_reserve - token_bought) * 997) + 1

NOTE: The path #3.v reverts due to the division-by-zero failure of numerator / denominator, where denominator becomes zero. In other word, you are not allowed to buy the whole amount of reserved tokens.