Rent Exempt Epoch Transition Remediation
How to fix cached rent-exempt minimum balances.
Rent Exempt Epoch Transition Remediation
Overview
Detector Reference: Rent Exempt Epoch Transition
This guide explains how to fix patterns where the rent-exempt minimum is cached in account data, creating stale values across epoch transitions.
Recommended Fix
Always query the Rent sysvar dynamically instead of reading cached values:
// Instead of reading from account data:
// let cached_min = u64::from_le_bytes(data[32..40].try_into()?);
// Always query fresh:
let rent = Rent::get()?;
let min_balance = rent.minimum_balance(account.data_len());
require!(account.lamports() >= min_balance, NotRentExempt);Alternative Mitigations
- Refresh instruction: if caching is required for compute budget, add an
update_rent_minimuminstruction that refreshes the cached value. - Conservative buffer: cache the rent minimum with an additional safety margin (e.g., 2x) to account for potential increases.
- Remove caching entirely: the
Rent::get()sysvar access is inexpensive and should be called dynamically.
Common Mistakes
- Caching for performance: the Rent sysvar access costs very few compute units. Caching provides negligible savings while introducing staleness risk.
- Assuming rent parameters are immutable: while Solana has not changed rent parameters recently, the capability exists and caching assumes immutability.
- Not invalidating on realloc: if the account size changes, the cached minimum is doubly wrong (wrong size AND potentially wrong rate).