feat: enforce gas limits from private kernels (#6105)

Now we assert that we stayed within user limits during private execution.

docs/docs/protocol-specs/gas-and-fees/kernel-tracking.md

@@ -206,6 +206,7 @@ It must:
 - check that there are enqueued public functions or a public teardown function
 - partition the side effects produced during private execution into revertible and non-revertible sets of `PublicAccumulatedData`
 - compute gas used for the revertible and non-revertible. Both sets can have a DA component, but the revertible set will also include the teardown gas allocations the user specified (if any). This ensures that the user effectively pre-pays for the gas consumed in teardown.
+- ensure the gas used (across revertible and non-revertible) is less than the gas limits
 - ensure that `fee_payer` is set, and set it in the `PublicKernelCircuitPublicInputs`
 - copy the constants from the `PrivateKernelData` to the `PublicKernelCircuitPublicInputs.constants`
 

noir-projects/noir-protocol-circuits/crates/private-kernel-lib/src/kernel_circuit_public_inputs_composer.nr

@@ -113,13 +113,20 @@ impl KernelCircuitPublicInputsComposer {
 
     pub fn finish(self) -> KernelCircuitPublicInputs {
         let teardown_gas = self.previous_kernel.public_inputs.constants.tx_context.gas_settings.teardown_gas_limits;
-        self.public_inputs.finish_tail(teardown_gas)
+        let inputs = self.public_inputs.finish_tail(teardown_gas);
+        let limits = self.previous_kernel.public_inputs.constants.tx_context.gas_settings.gas_limits;
+        assert(inputs.end.gas_used.within(limits), "The gas used exceeds the gas limits");
+        inputs
     }
 
     pub fn finish_to_public(self) -> PublicKernelCircuitPublicInputs {
         let min_revertible_side_effect_counter = self.previous_kernel.public_inputs.min_revertible_side_effect_counter;
         let teardown_gas = self.previous_kernel.public_inputs.constants.tx_context.gas_settings.teardown_gas_limits;
-        self.public_inputs.finish_to_public(teardown_gas, min_revertible_side_effect_counter)
+        let inputs = self.public_inputs.finish_to_public(teardown_gas, min_revertible_side_effect_counter);
+        let limits = self.previous_kernel.public_inputs.constants.tx_context.gas_settings.gas_limits;
+        let total_gas_used = inputs.end.gas_used + inputs.end_non_revertible.gas_used;
+        assert(total_gas_used.within(limits), "The gas used exceeds the gas limits");
+        inputs
     }
 
     fn silo_values(&mut self) {

noir-projects/noir-protocol-circuits/crates/private-kernel-lib/src/private_kernel_tail.nr

@@ -115,6 +115,8 @@ mod tests {
     impl PrivateKernelTailInputsBuilder {
         pub fn new() -> Self {
             let mut previous_kernel = FixtureBuilder::new();
+            previous_kernel.tx_context.gas_settings.gas_limits = Gas::new(1_000_000, 1_000_000);
+
             previous_kernel.append_new_nullifiers(1);
 
             PrivateKernelTailInputsBuilder {
@@ -572,4 +574,12 @@ mod tests {
             + Gas::new(DA_GAS_PER_BYTE * DA_BYTES_PER_FIELD * 1, 0); // tx nullifier
         assert_eq(public_inputs.end.gas_used, expected_gas_consumed);
     }
+
+    #[test(should_fail_with="The gas used exceeds the gas limits")]
+    unconstrained fn gas_limits_are_enforced() {
+        let mut builder = PrivateKernelTailInputsBuilder::new();
+        builder.previous_kernel.tx_context.gas_settings.teardown_gas_limits = Gas::new(300, 300);
+        builder.previous_kernel.tx_context.gas_settings.gas_limits = Gas::new(1, 1);
+        builder.failed();
+    }
 }

noir-projects/noir-protocol-circuits/crates/private-kernel-lib/src/private_kernel_tail_to_public.nr

@@ -118,6 +118,7 @@ mod tests {
     impl PrivateKernelTailToPublicInputsBuilder {
         pub fn new() -> Self {
             let mut previous_kernel = FixtureBuilder::new();
+            previous_kernel.tx_context.gas_settings.gas_limits = Gas::new(1_000_000, 1_000_000);
             previous_kernel.append_new_nullifiers(1);
             previous_kernel.push_public_call_request(1, false);
 
@@ -600,4 +601,12 @@ mod tests {
         let expected_gas_consumed = Gas::new(300, 300) + Gas::tx_overhead();
         assert_eq(public_inputs.end.gas_used, expected_gas_consumed);
     }
+
+    #[test(should_fail_with="The gas used exceeds the gas limits")]
+    unconstrained fn gas_limits_are_enforced() {
+        let mut builder = PrivateKernelTailToPublicInputsBuilder::new();
+        builder.previous_kernel.tx_context.gas_settings.teardown_gas_limits = Gas::new(300, 300);
+        builder.previous_kernel.tx_context.gas_settings.gas_limits = Gas::new(1, 1);
+        builder.failed();
+    }
 }

noir-projects/noir-protocol-circuits/crates/types/src/abis/accumulated_data/private_accumulated_data_builder.nr

@@ -39,15 +39,10 @@ struct PrivateAccumulatedDataBuilder {
     private_call_stack: BoundedVec<CallRequest, MAX_PRIVATE_CALL_STACK_LENGTH_PER_TX>,
     public_call_stack: BoundedVec<CallRequest, MAX_PUBLIC_CALL_STACK_LENGTH_PER_TX>,
 
-    gas_used: Gas,
-    non_revertible_gas_used: Gas,
 }
 
 impl PrivateAccumulatedDataBuilder {
     pub fn finish(self) -> PrivateAccumulatedData {
-        assert(self.gas_used.is_empty());
-        assert(self.non_revertible_gas_used.is_empty());
-
         PrivateAccumulatedData {
             new_note_hashes: self.new_note_hashes.storage,
             new_nullifiers: self.new_nullifiers.storage,
@@ -318,8 +313,6 @@ impl Empty for PrivateAccumulatedDataBuilder {
             unencrypted_log_preimages_length: 0,
             private_call_stack: BoundedVec::new(),
             public_call_stack: BoundedVec::new(),
-            gas_used: Gas::empty(),
-            non_revertible_gas_used: Gas::empty(),
         }
     }
 }

noir-projects/noir-protocol-circuits/crates/types/src/abis/gas.nr

@@ -27,6 +27,10 @@ impl Gas {
     pub fn is_empty(self) -> bool {
         (self.da_gas == 0) & (self.l2_gas == 0)
     }
+
+    pub fn within(self, limits: Gas) -> bool {
+        (self.da_gas <= limits.da_gas) & (self.l2_gas <= limits.l2_gas)
+    }
 }
 
 impl Add for Gas {

noir-projects/noir-protocol-circuits/crates/types/src/tests/fixture_builder.nr

@@ -133,9 +133,7 @@ impl FixtureBuilder {
             encrypted_log_preimages_length: self.encrypted_log_preimages_length,
             unencrypted_log_preimages_length: self.unencrypted_log_preimages_length,
             private_call_stack: self.private_call_stack,
-            public_call_stack: self.public_call_stack,
-            gas_used: self.gas_used,
-            non_revertible_gas_used: self.non_revertible_gas_used
+            public_call_stack: self.public_call_stack
         };
         public_inputs.finish()
     }