rayDiv and rayMul in WadRayMath.sol are not inverse operations

[idotial]

rayDiv(a = 117820610000000000, b = 1018703874771532247390984066)=>(115657369052830960)
rayMul(a = 115657369052830960, b = 1018703874771532247390984066)=>(117820609999999999)

[yug49]

• The behavior is expected: rayDiv and rayMul are not mathematically perfect inverses due to rounding and fixed-point representation.
• Not a bug: Aave explicitly documents rounding in the header comment (Operations are rounded. If a value is >=.5, will be rounded up, otherwise rounded down.)
• The “issue” is only a problem if someone assumes exact reversibility, which is never possible in fixed-point math without infinite precision.

The given example

a = 117820610000000000
b = 1018703874771532247390984066

rayDiv(a, b) => 115657369052830960
rayMul(result, b) => 117820609999999999

The difference is

117820610000000000 - 117820609999999999 = 1 wei

That’s 1 unit at 18 decimals, far smaller than any meaningful financial impact.