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fix(l1): fix exponential overflow in fake exponential #5093

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Merged
jrchatruc merged 4 commits into from
Oct 29, 2025
Merged

fix(l1): fix exponential overflow in fake exponential #5093

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d3c5215
Failing test
fedacking Oct 29, 2025
f876d03
Fixed overflow
fedacking Oct 29, 2025
7b7ec23
Update block.rs
fedacking Oct 29, 2025
dca2776
Update block.rs
fedacking Oct 29, 2025
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18 changes: 14 additions & 4 deletions crates/common/types/block.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -398,14 +398,18 @@ pub fn calculate_base_fee_per_blob_gas(parent_excess_blob_gas: u64, update_fract
// Defined in [EIP-4844](https://eips.ethereum.org/EIPS/eip-4844)
pub fn fake_exponential(factor: u64, numerator: u64, denominator: u64) -> u64 {
let mut i = 1;
let mut output = 0;
let mut numerator_accum = factor * denominator;
while numerator_accum > 0 {
let mut output = U256::zero();
let mut numerator_accum = U256::from(factor) * denominator;
while !numerator_accum.is_zero() {
output += numerator_accum;
numerator_accum = numerator_accum * numerator / (denominator * i);
i += 1;
}
output / denominator
if (output / denominator) > U256::from(u64::MAX) {
u64::MAX
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@MegaRedHand MegaRedHand Oct 29, 2025

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We should panic here, since I'm sure this is unreachable. In a later PR we can update this with an explanation.

} else {
(output / denominator).as_u64()
}
}

#[derive(Debug, thiserror::Error)]
Expand Down Expand Up @@ -983,4 +987,10 @@ mod test {
let res = calc_excess_blob_gas(&parent, schedule, fork);
assert_eq!(res, 3538944)
}

#[test]
fn test_fake_exponential_overflow() {
// With u64 this overflows
fake_exponential(57532635, 3145728, 3338477);
}
Comment on lines +991 to +995
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@MegaRedHand MegaRedHand Oct 29, 2025

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Let's add a test with the maximum expected inputs here, for good measure:

fake_exponential(MIN_BASE_FEE_PER_BLOB_GAS, u64::MAX, BLOB_BASE_FEE_UPDATE_FRACTION);

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@MegaRedHand MegaRedHand Oct 29, 2025

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This test panics.

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@fedacking fedacking Oct 29, 2025

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Created issue to define a bound #5096

}