From 7f2121205aed5d0a5dadd11e09e2970f57e56254 Mon Sep 17 00:00:00 2001
From: Paul Hauner <paul@paulhauner.com>
Date: Wed, 29 Apr 2020 11:37:14 +1000
Subject: [PATCH] Ensure genesis is not triggered too early (#1052)

---
 beacon_node/genesis/src/eth1_genesis_service.rs | 14 ++++++++++----
 eth2/state_processing/src/genesis.rs            | 17 +++++++++++++----
 eth2/state_processing/src/lib.rs                |  5 ++++-
 3 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/beacon_node/genesis/src/eth1_genesis_service.rs b/beacon_node/genesis/src/eth1_genesis_service.rs
index 14df35fa1..ffefb58b2 100644
--- a/beacon_node/genesis/src/eth1_genesis_service.rs
+++ b/beacon_node/genesis/src/eth1_genesis_service.rs
@@ -10,7 +10,7 @@ use futures::{
 use parking_lot::Mutex;
 use slog::{debug, error, info, trace, Logger};
 use state_processing::{
-    initialize_beacon_state_from_eth1, is_valid_genesis_state,
+    eth2_genesis_time, initialize_beacon_state_from_eth1, is_valid_genesis_state,
     per_block_processing::process_deposit, process_activations,
 };
 use std::sync::Arc;
@@ -223,9 +223,15 @@ impl Eth1GenesisService {
             .blocks()
             .read()
             .iter()
-            // It's only worth scanning blocks that have timestamps _after_ genesis time. It's
-            // impossible for any other block to trigger genesis.
-            .filter(|block| block.timestamp >= spec.min_genesis_time)
+            // Filter out any blocks that would result in a genesis time that is earlier than
+            // `MIN_GENESIS_TIME`.
+            //
+            // Note: any `SafeArith` errors are suppressed here; we simply skip blocks that cause
+            // overflow/div-by-zero.
+            .filter(|block| {
+                eth2_genesis_time(block.timestamp, spec)
+                    .map_or(false, |t| t >= spec.min_genesis_time)
+            })
             // The block cache might be more recently updated than deposit cache. Restrict any
             // block numbers that are not known by all caches.
             .filter(|block| {
diff --git a/eth2/state_processing/src/genesis.rs b/eth2/state_processing/src/genesis.rs
index 9ae9fabdc..d219b9090 100644
--- a/eth2/state_processing/src/genesis.rs
+++ b/eth2/state_processing/src/genesis.rs
@@ -1,6 +1,6 @@
 use super::per_block_processing::{errors::BlockProcessingError, process_deposit};
 use crate::common::DepositDataTree;
-use safe_arith::SafeArith;
+use safe_arith::{ArithError, SafeArith};
 use tree_hash::TreeHash;
 use types::DEPOSIT_TREE_DEPTH;
 use types::*;
@@ -15,9 +15,7 @@ pub fn initialize_beacon_state_from_eth1<T: EthSpec>(
     deposits: Vec<Deposit>,
     spec: &ChainSpec,
 ) -> Result<BeaconState<T>, BlockProcessingError> {
-    let genesis_time = eth1_timestamp
-        .safe_sub(eth1_timestamp.safe_rem(spec.min_genesis_delay)?)?
-        .safe_add(2.safe_mul(spec.min_genesis_delay)?)?;
+    let genesis_time = eth2_genesis_time(eth1_timestamp, spec)?;
     let eth1_data = Eth1Data {
         // Temporary deposit root
         deposit_root: Hash256::zero(),
@@ -79,3 +77,14 @@ pub fn process_activations<T: EthSpec>(
     }
     Ok(())
 }
+
+/// Returns the `state.genesis_time` for the corresponding `eth1_timestamp`.
+///
+/// Does _not_ ensure that the time is greater than `MIN_GENESIS_TIME`.
+///
+/// Spec v0.11.1
+pub fn eth2_genesis_time(eth1_timestamp: u64, spec: &ChainSpec) -> Result<u64, ArithError> {
+    eth1_timestamp
+        .safe_sub(eth1_timestamp.safe_rem(spec.min_genesis_delay)?)?
+        .safe_add(2.safe_mul(spec.min_genesis_delay)?)
+}
diff --git a/eth2/state_processing/src/lib.rs b/eth2/state_processing/src/lib.rs
index 86dc2294f..7b871c1db 100644
--- a/eth2/state_processing/src/lib.rs
+++ b/eth2/state_processing/src/lib.rs
@@ -10,7 +10,10 @@ pub mod per_epoch_processing;
 pub mod per_slot_processing;
 pub mod test_utils;
 
-pub use genesis::{initialize_beacon_state_from_eth1, is_valid_genesis_state, process_activations};
+pub use genesis::{
+    eth2_genesis_time, initialize_beacon_state_from_eth1, is_valid_genesis_state,
+    process_activations,
+};
 pub use per_block_processing::{
     block_signature_verifier, errors::BlockProcessingError, per_block_processing, signature_sets,
     BlockSignatureStrategy, BlockSignatureVerifier, VerifySignatures,