Fix EXTCODECOPY with empty account (#846)

* Fix EXTCODECOPY with empty account (#1429)

We treat empty accounts by storing their code_hash in the RwTable as 0.
EXTCODECOPY was obtaining the bytecode length by querying the bytecode
table with code_hash=0 on existing accounts, but that entry should be
invalid (there's no bytecode with code_hash=0). Skip the bytecode table
length lookup when code_hash=0.

I've also reintroduced the `Block::debug_print_txs_steps_rw_ops`
function, updated to use the new `Block::get_rws` API. This function is
not used in the code, but it's very convenient to call it when
debugging.

Resolve
privacy-ethereum#1190

- [x] Bug fix (non-breaking change which fixes an issue)

* fix

---------

Co-authored-by: Eduard S <eduardsanou@posteo.net>

Author: lispc

bus-mapping/src/evm/opcodes/extcodecopy.rs

@@ -11,9 +11,6 @@ use eth_types::{Bytecode, GethExecStep, ToAddress, ToWord, H256, U256};
 #[derive(Clone, Copy, Debug)]
 pub(crate) struct Extcodecopy;
 
-// TODO: Update to treat code_hash == 0 as account not_exists once the circuit
-// is implemented https://github.com/privacy-scaling-explorations/zkevm-circuits/pull/720
-
 impl Opcode for Extcodecopy {
     fn gen_associated_ops(
         state: &mut CircuitInputStateRef,

bus-mapping/src/state_db.rs

@@ -11,6 +11,7 @@ use std::collections::{BTreeSet, HashMap, HashSet};
 
 lazy_static! {
     static ref ACCOUNT_ZERO: Account = Account::zero();
+    /// Hash value for empty code hash.
     static ref EMPTY_CODE_HASH: Hash = CodeDB::hash(&[]);
     /// bytes of empty code hash, in little endian order.
     pub static ref EMPTY_CODE_HASH_LE: [u8; 32] = {

zkevm-circuits/src/evm_circuit/execution/extcodecopy.rs

@@ -9,6 +9,7 @@ use crate::{
                 Transition,
             },
             from_bytes,
+            math_gadget::IsZeroGadget,
             memory_gadget::{
                 CommonMemoryAddressGadget, MemoryAddressGadget, MemoryCopierGasGadget,
                 MemoryExpansionGadget,
@@ -36,6 +37,7 @@ pub(crate) struct ExtcodecopyGadget<F> {
     reversion_info: ReversionInfo<F>,
     is_warm: Cell<F>,
     code_hash: Cell<F>,
+    not_exists: IsZeroGadget<F>,
     code_size: Cell<F>,
     copy_rwc_inc: Cell<F>,
     memory_expansion: MemoryExpansionGadget<F, 1, N_BYTES_MEMORY_WORD_SIZE>,
@@ -82,11 +84,14 @@ impl<F: Field> ExecutionGadget<F> for ExtcodecopyGadget<F> {
             AccountFieldTag::CodeHash,
             code_hash.expr(),
         );
-        // TODO: If external_address doesn't exist, we will get code_hash = 0.  With
-        // this value, the bytecode_length lookup will not work, and the copy
-        // from code_hash = 0 will not work. We should use EMPTY_HASH when
-        // code_hash = 0.
-        cb.bytecode_length(code_hash.expr(), code_size.expr());
+        let not_exists = IsZeroGadget::construct(cb, code_hash.expr());
+        let exists = not::expr(not_exists.expr());
+        cb.condition(exists.expr(), |cb| {
+            cb.bytecode_length(code_hash.expr(), code_size.expr());
+        });
+        cb.condition(not_exists.expr(), |cb| {
+            cb.require_zero("code_size is zero when non_exists", code_size.expr());
+        });
 
         let memory_address = MemoryAddressGadget::construct(cb, memory_offset, memory_length);
         let memory_expansion = MemoryExpansionGadget::construct(cb, [memory_address.end_offset()]);
@@ -151,6 +156,7 @@ impl<F: Field> ExecutionGadget<F> for ExtcodecopyGadget<F> {
             is_warm,
             reversion_info,
             code_hash,
+            not_exists,
             code_size,
             copy_rwc_inc,
             memory_expansion,
@@ -193,6 +199,8 @@ impl<F: Field> ExecutionGadget<F> for ExtcodecopyGadget<F> {
         let code_hash = block.rws[step.rw_indices[8]].account_value_pair().0;
         self.code_hash
             .assign(region, offset, region.code_hash(code_hash))?;
+        self.not_exists
+            .assign_value(region, offset, region.code_hash(code_hash))?;
 
         let code_size = if code_hash.is_zero() {
             0

zkevm-circuits/src/table.rs

@@ -1108,23 +1108,6 @@ impl BytecodeTable {
             || "bytecode table",
             |mut region| {
                 let mut offset = 0;
-
-                region.assign_fixed(
-                    || "bytecode table all-zero row",
-                    self.q_enable,
-                    offset,
-                    || Value::known(F::one()),
-                )?;
-                for column in <BytecodeTable as LookupTable<F>>::advice_columns(self) {
-                    region.assign_advice(
-                        || "bytecode table all-zero row",
-                        column,
-                        offset,
-                        || Value::known(F::zero()),
-                    )?;
-                }
-                offset += 1;
-
                 let bytecode_table_columns =
                     <BytecodeTable as LookupTable<F>>::advice_columns(self);
                 for bytecode in bytecodes.clone() {