Precision in the trapezoidal borrow-rate curve, and the Yearn post-mortem behind it

Mentorship Series #23 · 15 January 2026 · @0xfirefist · Panoptic · Code4rena

We continued the Panoptic contest on Code4rena.

Lesson Summary:

There seems to be enough precision when the utilisation error is small, since it only rounds down to 0 when the error is tiny.
The trapezoidal rule in the borrowRate function: check it holds for all values and conditions.
Check Yearn exploit post-mortem
What if every rate is 0? Look for edge cases in the trapezoidal rule — this one cannot happen, because of line 1841 in _borrowRate.
How is elapsed == 0 handled in _borrowRate? Verdict: the precision is handled well.
What if _borrowRate is spammed? Verdict: fine.
Call it often, call it rarely, skip a day, make a huge deposit, borrow a lot.