fix(l1): unifiy fake_exponential to a single U256 function (#5107)

**Motivation**

There were multiple functions implementing the eip4844 helper function
`fake_exponential`. These functions should be unified to a single U256
implementation that properly takes into account the possible overflow.

**Description**

- Removed `fake_exponential` function in LEVM crate
- Removed `fake_exponential_checked` function in `block.rs`
- Changed `fake_exponential` function signature to be U256
- Changed `base_fee_per_blob_gas` to be U256 where is it used
- Added a test for the 400M limit where the function still operates

---------

Co-authored-by: Javier Rodríguez Chatruc <[email protected]>

Author: fedacking

crates/blockchain/payload.rs

@@ -250,7 +250,7 @@ impl PayloadBuildContext {
                 .is_prague_activated(payload.header.timestamp)
                 .then_some(Vec::new()),
             block_value: U256::zero(),
-            base_fee_per_blob_gas: U256::from(base_fee_per_blob_gas),
+            base_fee_per_blob_gas,
             payload,
             blobs_bundle: BlobsBundle::default(),
             store: storage.clone(),

crates/common/serde_utils.rs

@@ -137,6 +137,31 @@ pub mod u256 {
                 .collect()
         }
     }
+    pub mod hex_str_opt {
+        use serde::Serialize;
+
+        use super::*;
+
+        pub fn serialize<S>(value: &Option<U256>, serializer: S) -> Result<S::Ok, S::Error>
+        where
+            S: Serializer,
+        {
+            Option::<String>::serialize(&value.map(|v| format!("{v:#x}")), serializer)
+        }
+
+        pub fn deserialize<'de, D>(d: D) -> Result<Option<U256>, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            let value = Option::<String>::deserialize(d)?;
+            match value {
+                Some(s) if !s.is_empty() => U256::from_str_radix(s.trim_start_matches("0x"), 16)
+                    .map_err(|_| D::Error::custom("Failed to deserialize U256 value"))
+                    .map(Some),
+                _ => Ok(None),
+            }
+        }
+    }
 }
 
 pub mod u32 {

crates/common/types/block.rs

@@ -384,74 +384,82 @@ fn check_gas_limit(gas_limit: u64, parent_gas_limit: u64) -> bool {
 
 /// Calculates the base fee per blob gas for the current block based on
 /// it's parent excess blob gas and the update fraction, which depends on the fork.
-pub fn calculate_base_fee_per_blob_gas(parent_excess_blob_gas: u64, update_fraction: u64) -> u64 {
+pub fn calculate_base_fee_per_blob_gas(parent_excess_blob_gas: u64, update_fraction: u64) -> U256 {
     if update_fraction == 0 {
-        return 0;
+        return U256::zero();
     }
     fake_exponential(
-        MIN_BASE_FEE_PER_BLOB_GAS,
-        parent_excess_blob_gas,
+        U256::from(MIN_BASE_FEE_PER_BLOB_GAS),
+        U256::from(parent_excess_blob_gas),
         update_fraction,
     )
+    .unwrap_or_default()
 }
 
-// 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 = 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;
-    }
-    if (output / denominator) > U256::from(u64::MAX) {
-        u64::MAX
-    } else {
-        (output / denominator).as_u64()
-    }
-}
-
-#[derive(Debug, thiserror::Error)]
-pub enum FakeExponentialError {
-    #[error("Denominator cannot be zero.")]
-    DenominatorIsZero,
-    #[error("Checked div failed is None.")]
-    CheckedDiv,
-    #[error("Checked mul failed is None.")]
-    CheckedMul,
-}
-
-pub fn fake_exponential_checked(
-    factor: u64,
-    numerator: u64,
+/// Approximates factor * e ** (numerator / denominator) using Taylor expansion
+/// https://eips.ethereum.org/EIPS/eip-4844#helpers
+/// 400_000_000 numerator is the limit for this operation to work with U256,
+/// it will overflow with a larger numerator
+pub fn fake_exponential(
+    factor: U256,
+    numerator: U256,
     denominator: u64,
-) -> Result<u64, FakeExponentialError> {
-    let mut i = 1_u64;
-    let mut output = 0_u64;
-    let mut numerator_accum = factor * denominator;
+) -> Result<U256, FakeExponentialError> {
     if denominator == 0 {
         return Err(FakeExponentialError::DenominatorIsZero);
     }
 
-    while numerator_accum > 0 {
-        output = output.saturating_add(numerator_accum);
+    if numerator.is_zero() {
+        return Ok(factor);
+    }
 
-        let denominator_i = denominator
-            .checked_mul(i)
-            .ok_or(FakeExponentialError::CheckedMul)?;
+    let mut output: U256 = U256::zero();
+    let denominator_u256: U256 = denominator.into();
 
-        if denominator_i == 0 {
-            return Err(FakeExponentialError::DenominatorIsZero);
+    // Initial multiplication: factor * denominator
+    let mut numerator_accum = factor
+        .checked_mul(denominator_u256)
+        .ok_or(FakeExponentialError::CheckedMul)?;
+
+    let mut denominator_by_i = denominator_u256;
+
+    #[expect(
+        clippy::arithmetic_side_effects,
+        reason = "division can't overflow since denominator is not 0"
+    )]
+    {
+        while !numerator_accum.is_zero() {
+            // Safe addition to output
+            output = output
+                .checked_add(numerator_accum)
+                .ok_or(FakeExponentialError::CheckedAdd)?;
+
+            // Safe multiplication and division within loop
+            numerator_accum = numerator_accum
+                .checked_mul(numerator)
+                .ok_or(FakeExponentialError::CheckedMul)?
+                / denominator_by_i;
+
+            // denominator comes from a u64 value, will never overflow before other variables.
+            denominator_by_i += denominator_u256;
         }
 
-        numerator_accum = numerator_accum * numerator / denominator_i;
-        i += 1;
+        output
+            .checked_div(denominator.into())
+            .ok_or(FakeExponentialError::CheckedDiv)
     }
+}
 
-    output
-        .checked_div(denominator)
-        .ok_or(FakeExponentialError::CheckedDiv)
+#[derive(Debug, thiserror::Error, Serialize, Clone, PartialEq, Deserialize, Eq)]
+pub enum FakeExponentialError {
+    #[error("FakeExponentialError: Denominator cannot be zero.")]
+    DenominatorIsZero,
+    #[error("FakeExponentialError: Checked div failed is None.")]
+    CheckedDiv,
+    #[error("FakeExponentialError: Checked mul failed is None.")]
+    CheckedMul,
+    #[error("FakeExponentialError: Checked add failed is None.")]
+    CheckedAdd,
 }
 
 // Calculates the base fee for the current block based on its gas_limit and parent's gas and fee
@@ -748,8 +756,8 @@ pub fn calc_excess_blob_gas(parent: &BlockHeader, schedule: ForkBlobSchedule, fo
     }
 
     if fork >= Fork::Osaka
-        && BLOB_BASE_COST * parent_base_fee_per_gas
-            > (GAS_PER_BLOB as u64)
+        && U256::from(BLOB_BASE_COST * parent_base_fee_per_gas)
+            > (U256::from(GAS_PER_BLOB))
                 * calculate_base_fee_per_blob_gas(
                     parent_excess_blob_gas,
                     schedule.base_fee_update_fraction,
@@ -767,7 +775,7 @@ pub fn calc_excess_blob_gas(parent: &BlockHeader, schedule: ForkBlobSchedule, fo
 mod test {
     use super::*;
     use crate::constants::EMPTY_KECCACK_HASH;
-    use crate::types::ELASTICITY_MULTIPLIER;
+    use crate::types::{BLOB_BASE_FEE_UPDATE_FRACTION, ELASTICITY_MULTIPLIER};
     use ethereum_types::H160;
     use hex_literal::hex;
     use std::str::FromStr;
@@ -991,6 +999,18 @@ mod test {
     #[test]
     fn test_fake_exponential_overflow() {
         // With u64 this overflows
-        fake_exponential(57532635, 3145728, 3338477);
+        assert!(fake_exponential(U256::from(57532635), U256::from(3145728), 3338477).is_ok());
+    }
+
+    #[test]
+    fn test_fake_exponential_bounds_overflow() {
+        // Making sure the limit we state in the documentation of 400_000_000 works
+        let thing = fake_exponential(
+            MIN_BASE_FEE_PER_BLOB_GAS.into(),
+            400_000_000.into(),
+            BLOB_BASE_FEE_UPDATE_FRACTION,
+        );
+        // With u64 this overflows
+        assert!(thing.is_ok());
     }
 }

crates/l2/sequencer/l1_committer.rs

@@ -17,8 +17,7 @@ use ethrex_common::{
     Address, H256, U256,
     types::{
         AccountUpdate, BLOB_BASE_FEE_UPDATE_FRACTION, BlobsBundle, Block, BlockNumber, Fork,
-        Genesis, MIN_BASE_FEE_PER_BLOB_GAS, TxType, batch::Batch, blobs_bundle,
-        fake_exponential_checked,
+        Genesis, MIN_BASE_FEE_PER_BLOB_GAS, TxType, batch::Batch, blobs_bundle, fake_exponential,
     },
 };
 use ethrex_l2_common::{
@@ -975,7 +974,7 @@ impl L1Committer {
                 20, // 20% of headroom
             )
             .await?
-            .to_le_bytes();
+            .to_little_endian();
 
             let gas_price_per_blob = U256::from_little_endian(&le_bytes);
 
@@ -1214,7 +1213,7 @@ async fn estimate_blob_gas(
     eth_client: &EthClient,
     arbitrary_base_blob_gas_price: u64,
     headroom: u64,
-) -> Result<u64, CommitterError> {
+) -> Result<U256, CommitterError> {
     let latest_block = eth_client
         .get_block_by_number(BlockIdentifier::Tag(BlockTag::Latest), false)
         .await?;
@@ -1241,17 +1240,17 @@ async fn estimate_blob_gas(
 
     // If the blob's market is in high demand, the equation may give a really big number.
     // This function doesn't panic, it performs checked/saturating operations.
-    let blob_gas = fake_exponential_checked(
-        MIN_BASE_FEE_PER_BLOB_GAS,
-        total_blob_gas,
+    let blob_gas = fake_exponential(
+        U256::from(MIN_BASE_FEE_PER_BLOB_GAS),
+        U256::from(total_blob_gas),
         BLOB_BASE_FEE_UPDATE_FRACTION,
     )
     .map_err(BlobEstimationError::FakeExponentialError)?;
 
     let gas_with_headroom = (blob_gas * (100 + headroom)) / 100;
 
     // Check if we have an overflow when we take the headroom into account.
-    let blob_gas = arbitrary_base_blob_gas_price
+    let blob_gas = U256::from(arbitrary_base_blob_gas_price)
         .checked_add(gas_with_headroom)
         .ok_or(BlobEstimationError::OverflowError)?;
 

crates/networking/rpc/eth/fee_market.rs

@@ -1,5 +1,6 @@
 use ethrex_blockchain::payload::calc_gas_limit;
 use ethrex_common::{
+    U256,
     constants::GAS_PER_BLOB,
     types::{
         Block, BlockHeader, ELASTICITY_MULTIPLIER, Fork, ForkBlobSchedule, Transaction,
@@ -102,7 +103,7 @@ impl RpcHandler for FeeHistoryRequest {
         let oldest_block = start_block;
         let block_count = (end_block - start_block + 1) as usize;
         let mut base_fee_per_gas = vec![0_u64; block_count + 1];
-        let mut base_fee_per_blob_gas = vec![0_u64; block_count + 1];
+        let mut base_fee_per_blob_gas = vec![U256::zero(); block_count + 1];
         let mut gas_used_ratio = vec![0_f64; block_count];
         let mut blob_gas_used_ratio = vec![0_f64; block_count];
         let mut reward = Vec::<Vec<u64>>::with_capacity(block_count);
@@ -165,12 +166,13 @@ impl RpcHandler for FeeHistoryRequest {
         }
 
         let u64_to_hex_str = |x: u64| format!("0x{x:x}");
+        let u256_to_hex_str = |x: U256| format!("0x{x:x}");
         let response = FeeHistoryResponse {
             oldest_block: u64_to_hex_str(oldest_block),
             base_fee_per_gas: base_fee_per_gas.into_iter().map(u64_to_hex_str).collect(),
             base_fee_per_blob_gas: base_fee_per_blob_gas
                 .into_iter()
-                .map(u64_to_hex_str)
+                .map(u256_to_hex_str)
                 .collect(),
             gas_used_ratio,
             blob_gas_used_ratio,
@@ -189,7 +191,7 @@ fn project_next_block_base_fee_values(
     schedule: ForkBlobSchedule,
     fork: Fork,
     gas_ceil: u64,
-) -> (u64, u64) {
+) -> (u64, U256) {
     // NOTE: Given that this client supports the Paris fork and later versions, we are sure that the next block
     // will have the London update active, so the base fee calculation makes sense
     // Geth performs a validation for this case:

crates/networking/rpc/types/receipt.rs

@@ -1,5 +1,5 @@
 use ethrex_common::{
-    Address, Bloom, Bytes, H256,
+    Address, Bloom, Bytes, H256, U256,
     constants::GAS_PER_BLOB,
     evm::calculate_create_address,
     serde_utils,
@@ -154,10 +154,10 @@ pub struct RpcReceiptTxInfo {
     pub effective_gas_price: u64,
     #[serde(
         skip_serializing_if = "Option::is_none",
-        with = "ethrex_common::serde_utils::u64::hex_str_opt",
+        with = "serde_utils::u256::hex_str_opt",
         default = "Option::default"
     )]
-    pub blob_gas_price: Option<u64>,
+    pub blob_gas_price: Option<U256>,
     #[serde(
         skip_serializing_if = "Option::is_none",
         with = "serde_utils::u64::hex_str_opt",
@@ -171,7 +171,7 @@ impl RpcReceiptTxInfo {
         transaction: Transaction,
         index: u64,
         gas_used: u64,
-        block_blob_gas_price: u64,
+        block_blob_gas_price: U256,
         base_fee_per_gas: Option<u64>,
     ) -> Result<Self, RpcErr> {
         let nonce = transaction.nonce();

crates/vm/levm/src/errors.rs

@@ -1,6 +1,9 @@
 use bytes::Bytes;
 use derive_more::derive::Display;
-use ethrex_common::{Address, H256, U256, types::Log};
+use ethrex_common::{
+    Address, H256, U256,
+    types::{FakeExponentialError, Log},
+};
 use serde::{Deserialize, Serialize};
 use thiserror;
 
@@ -176,6 +179,8 @@ pub enum InternalError {
     /// Unexpected error when accessing the database, used in trait `Database`.
     #[error("Database access error: {0}")]
     Database(#[from] DatabaseError),
+    #[error("{0}")]
+    FakeExponentialError(#[from] FakeExponentialError),
 }
 
 impl InternalError {