qazbnm456/sc-sentry/arithmetic-rounding

Arithmetic and rounding

An instance is arithmetic whose result is wrong in a direction that matters, at a location a fix would change. Two families, and they are found differently.

Family one: precision destroyed by order of operations

The dominant shape, and the one you can establish from the source alone:

amount / total scale // divides first: the quotient truncates amount scale / total // correct: scales first

Integer division truncates toward zero, so dividing before multiplying throws away every digit below the divisor. Look for a division whose result is then multiplied, or fed into another division. divides_then_scales finds it.

Related, same family:

• (a / b) * b reconstructed and compared to a
• a share price computed as totalAssets / totalSupply and then multiplied by

a user's shares, rather than shares * totalAssets / totalSupply

• a fee taken as amount * feeBps / 10000 where amount * feeBps can round to

zero for small amounts — a fee-free path for dust

• mixing token decimals without normalising: 6-decimal USDC against 18-decimal

WETH, scaled by a constant that is right for one of them

Family two: the rounding direction favours the wrong party

x / y rounds down. Whether that is a defect depends on who benefits:

• rounding down when computing what a user owes the protocol → the user

underpays

• rounding up when computing what the protocol owes a user → the protocol

overpays

• a deposit that mints assets * supply / totalAssets shares rounding down, and

a withdrawal that burns the same formula rounding down — the difference is a slow drain

This family usually needs the protocol's own intent to call. "Rounds down" is not a defect; "rounds down where the accounting identity requires up" is. If the source does not state which way it must go, say so rather than guessing — an unverifiable claim costs precision and measures nothing.

Not an instance

• a division by a literal power of ten used only for display or an event
• unchecked arithmetic where the bound is established immediately above
• Solidity ≥ 0.8 overflow: the compiler reverts, so an unchecked overflow is

not this class unless the block is explicitly unchecked

• a truncation of one wei where nothing accumulates it

What to cite

The expression, not the whole statement, when the statement holds several. The smallest complete expression whose rewrite fixes it.