Csdrhrt

Could someone explain the use of performing a multiplication such as this? I mean the logic is fine, but what is the importance from a security point of view?

function mul(uint x, uint y) internal pure returns (uint z) {

        require(y == 0 || (z = x * y) / y == x, "ds-math-mul-overflow");

    }

This is from the dapphub library.
I have included the link herewith:
https://github.com/dapphub/ds-math/blob/master/src/math.sol

From security point of view, it is ensuring that after multiplication, returned value is not getting overflowed.

Suppose we have a 8-bit unsigned integer which store value from 0 to 255. So the multiplication of 130*2 will return 260 and when it will be time to store it in variable it will be get overflowed and will store the value 5. So this require in question checks that if we perform the reverse operation, we should get the initial value which will not be possible in overflow bug.

y == 0 will be an exception in the detection of above case so code is considering it separately.

For more: https://consensys.github.io/smart-contract-best-practices/known_attacks/#integer-overflow-and-underflow

function sub(uint x, uint y) internal pure returns (uint z) {

        require((z = x - y) <= x, "ds-math-sub-underflow");

    }

checks x-y <= x and if condition(x-y <= x) is true, return z(x-y)

in the case x-y > x , print log "ds-math-sub-underflow" and throws