That is not the correct way to test for integer overflow.

The correct sequence of instructions is given in the RISC-V documentation and it needs more instructions.

"Integer overflow" means "overflow in operations with signed integers". It does not mean "overflow in operations with non-negative integers". The latter is normally referred as "carry".

The 2 instructions given above detect carry, not overflow.

Carry is needed for multi-word operations, and these are also painful on RISC-V, but overflow detection is required much more frequently, i.e. it is needed at any arithmetic operation, unless it can be proven by static program analysis that overflow is impossible at that operation.