Rounding errors
Application developers must take care of correct rounding of numbers, especially if the rounding impacts tokens amounts.
Cosmossdk offers two custom types for dealing with numbers:
sdk.Int
(sdk.UInt
) type for integral numberssdk.Dec
type for decimal arithmetic
The sdk.Dec
type has problems with precision and does not guarantee associativity, so it must be used carefully. But even if a more robust library for decimal numbers is deployed in the cosmossdk, rounding may be unavoidable.
Example
Below we see a simple example demonstrating sdk.Dec
type's precision problems.
func TestDec() {
a := sdk.MustNewDecFromStr("10")
b := sdk.MustNewDecFromStr("1000000010")
x := a.Quo(b).Mul(b)
fmt.Println(x) // 9.999999999999999000
q := float32(10)
w := float32(1000000010)
y := (q / w) * w
fmt.Println(y) // 10
}
Mitigations

Ensure that all tokens operations that must round results always benefit the system (application) and not users. In other words, always decide on the correct rounding direction. See Appendix G in the Umee audit report

Apply "multiplication before division" pattern. That is, instead of computing
(x / y) * z
do(x * z) / y

Observe issue #11783 for a replacement of the
sdk.Dec
type