bugfix: the gas_limit has to be above or below 1/1024th of the parent

The gas_limit was improperly allowed to be 1/1024th of the parent gas limit. However, this is an exclusive limit that the new gas limit must be inside of, therefore requiring +1 or -1 from the bound as the inclusive, allowable values.

Author: fselmo

eth/_utils/headers.py

@@ -105,9 +105,12 @@ def compute_gas_limit_bounds(previous_limit: int) -> Tuple[int, int]:
     Compute the boundaries for the block gas limit based on the parent block.
     """
     boundary_range = previous_limit // GAS_LIMIT_ADJUSTMENT_FACTOR
-    upper_bound = min(GAS_LIMIT_MAXIMUM, previous_limit + boundary_range)
-    lower_bound = max(GAS_LIMIT_MINIMUM, previous_limit - boundary_range)
-    return lower_bound, upper_bound
+
+    # the boundary range is the exclusive limit, therefore the inclusive bounds are
+    # (boundary_range - 1) and (boundary_range + 1) for upper and lower bounds, respectively
+    upper_bound_inclusive = min(GAS_LIMIT_MAXIMUM, previous_limit + boundary_range - 1)
+    lower_bound_inclusive = max(GAS_LIMIT_MINIMUM, previous_limit - boundary_range + 1)
+    return lower_bound_inclusive, upper_bound_inclusive
 
 
 def compute_gas_limit(parent_header: BlockHeaderAPI, genesis_gas_limit: int) -> int:
@@ -152,12 +155,14 @@ def compute_gas_limit(parent_header: BlockHeaderAPI, genesis_gas_limit: int) ->
 
     gas_limit = max(
         GAS_LIMIT_MINIMUM,
-        parent_header.gas_limit - decay + usage_increase
+        # + 1 because the decay is an exclusive limit we have to remain inside of
+        (parent_header.gas_limit - decay + 1) + usage_increase
     )
 
     if gas_limit < GAS_LIMIT_MINIMUM:
         return GAS_LIMIT_MINIMUM
     elif gas_limit < genesis_gas_limit:
-        return parent_header.gas_limit + decay
+        # - 1 because the decay is an exclusive limit we have to remain inside of
+        return parent_header.gas_limit + decay - 1
     else:
         return gas_limit

eth/validation.py

@@ -230,11 +230,11 @@ def validate_vm_configuration(vm_configuration: Tuple[Tuple[int, Type[VirtualMac
 
 def validate_gas_limit(gas_limit: int, parent_gas_limit: int) -> None:
     low_bound, high_bound = compute_gas_limit_bounds(parent_gas_limit)
-    if gas_limit <= low_bound:
+    if gas_limit < low_bound:
         raise ValidationError(
             f"The gas limit {gas_limit} is too low. It must be at least {low_bound}"
         )
-    elif gas_limit >= high_bound:
+    elif gas_limit > high_bound:
         raise ValidationError(
             f"The gas limit {gas_limit} is too high. It must be at most {high_bound}"
         )

eth/vm/base.py

@@ -668,6 +668,7 @@ def validate_uncle(cls,
                 f"Uncle's gas usage ({uncle.gas_used}) is above "
                 f"the limit ({uncle.gas_limit})"
             )
+        validate_gas_limit(uncle.gas_limit, uncle_parent.gas_limit)
 
     #
     # State

tests/core/vm/test_london.py

@@ -222,7 +222,7 @@ def test_state_revert_on_reserved_0xEF_byte_for_create_transaction_post_london(
 
     block_import, _, computations = chain.mine_all(
         [create_successful_contract_transaction],
-        gas_limit=84082,
+        gas_limit=84081,
     )
     successful_create_computation = computations[0]
     successful_create_computation_state = successful_create_computation.state
@@ -336,7 +336,7 @@ def test_state_does_not_revert_on_reserved_0xEF_byte_for_create_transaction_pre_
 
     block_import, _, computations = chain.mine_all(
         [create_contract_txn_0xef_byte],
-        gas_limit=99903
+        gas_limit=99904
     )
 
     computation = computations[0]

tests/core/vm/test_vm.py

@@ -133,7 +133,7 @@ def test_validate_gas_limit_almost_too_low(noproof_consensus_chain):
     block1 = noproof_consensus_chain.mine_block()
     block2 = noproof_consensus_chain.mine_block()
 
-    max_reduction = block1.header.gas_limit // constants.GAS_LIMIT_ADJUSTMENT_FACTOR
+    max_reduction = block1.header.gas_limit // constants.GAS_LIMIT_ADJUSTMENT_FACTOR - 1
     barely_valid_low_gas_limit = block1.header.gas_limit - max_reduction
     barely_valid_header = block2.header.copy(gas_limit=barely_valid_low_gas_limit)
 
@@ -146,8 +146,8 @@ def test_validate_gas_limit_too_low(noproof_consensus_chain):
     block1 = noproof_consensus_chain.mine_block()
     block2 = noproof_consensus_chain.mine_block()
 
-    max_reduction = block1.header.gas_limit // constants.GAS_LIMIT_ADJUSTMENT_FACTOR
-    invalid_low_gas_limit = block1.header.gas_limit - max_reduction - 1
+    exclusive_decrease_limit = block1.header.gas_limit // constants.GAS_LIMIT_ADJUSTMENT_FACTOR
+    invalid_low_gas_limit = block1.header.gas_limit - exclusive_decrease_limit
     invalid_header = block2.header.copy(gas_limit=invalid_low_gas_limit)
 
     vm = noproof_consensus_chain.get_vm(block2.header)
@@ -160,7 +160,7 @@ def test_validate_gas_limit_almost_too_high(noproof_consensus_chain):
     block1 = noproof_consensus_chain.mine_block()
     block2 = noproof_consensus_chain.mine_block()
 
-    max_increase = block1.header.gas_limit // constants.GAS_LIMIT_ADJUSTMENT_FACTOR
+    max_increase = block1.header.gas_limit // constants.GAS_LIMIT_ADJUSTMENT_FACTOR - 1
     barely_valid_high_gas_limit = block1.header.gas_limit + max_increase
     barely_valid_header = block2.header.copy(gas_limit=barely_valid_high_gas_limit)
 
@@ -173,8 +173,8 @@ def test_validate_gas_limit_too_high(noproof_consensus_chain):
     block1 = noproof_consensus_chain.mine_block()
     block2 = noproof_consensus_chain.mine_block()
 
-    max_increase = block1.header.gas_limit // constants.GAS_LIMIT_ADJUSTMENT_FACTOR
-    invalid_high_gas_limit = block1.header.gas_limit + max_increase + 1
+    exclusive_increase_limit = block1.header.gas_limit // constants.GAS_LIMIT_ADJUSTMENT_FACTOR
+    invalid_high_gas_limit = block1.header.gas_limit + exclusive_increase_limit
     invalid_header = block2.header.copy(gas_limit=invalid_high_gas_limit)
 
     vm = noproof_consensus_chain.get_vm(block2.header)