Merge pull request #6425 from ethereum-optimism/inphi/op-program-output-root

op-program: Boot program using output root

op-e2e/actions/l2_verifier.go

@@ -54,6 +54,7 @@ type L2API interface {
 	InfoByHash(ctx context.Context, hash common.Hash) (eth.BlockInfo, error)
 	// GetProof returns a proof of the account, it may return a nil result without error if the address was not found.
 	GetProof(ctx context.Context, address common.Address, storage []common.Hash, blockTag string) (*eth.AccountResult, error)
+	OutputV0AtBlock(ctx context.Context, blockHash common.Hash) (*eth.OutputV0, error)
 }
 
 func NewL2Verifier(t Testing, log log.Logger, l1 derive.L1Fetcher, eng L2API, cfg *rollup.Config) *L2Verifier {

op-e2e/system_fpp_test.go

@@ -86,7 +86,10 @@ func testVerifyL2OutputRootEmptyBlock(t *testing.T, detached bool) {
 	require.NoError(t, waitForSafeHead(ctx, receipt.BlockNumber.Uint64(), rollupClient))
 
 	t.Logf("Capture current L2 head as agreed starting point. l2Head=%x l2BlockNumber=%v", receipt.BlockHash, receipt.BlockNumber)
-	l2Head := receipt.BlockHash
+	agreedL2Output, err := rollupClient.OutputAtBlock(ctx, receipt.BlockNumber.Uint64())
+	require.NoError(t, err, "could not retrieve l2 agreed block")
+	l2Head := agreedL2Output.BlockRef.Hash
+	l2OutputRoot := agreedL2Output.OutputRoot
 
 	t.Log("=====Stopping batch submitter=====")
 	err = sys.BatchSubmitter.Stop(ctx)
@@ -136,6 +139,7 @@ func testVerifyL2OutputRootEmptyBlock(t *testing.T, detached bool) {
 	testFaultProofProgramScenario(t, ctx, sys, &FaultProofProgramTestScenario{
 		L1Head:             l1Head,
 		L2Head:             l2Head,
+		L2OutputRoot:       common.Hash(l2OutputRoot),
 		L2Claim:            common.Hash(l2Claim),
 		L2ClaimBlockNumber: l2ClaimBlockNumber,
 		Detached:           detached,
@@ -181,9 +185,12 @@ func testVerifyL2OutputRoot(t *testing.T, detached bool) {
 	})
 
 	t.Log("Capture current L2 head as agreed starting point")
-	l2AgreedBlock, err := l2Seq.BlockByNumber(ctx, nil)
+	latestBlock, err := l2Seq.BlockByNumber(ctx, nil)
+	require.NoError(t, err)
+	agreedL2Output, err := rollupClient.OutputAtBlock(ctx, latestBlock.NumberU64())
 	require.NoError(t, err, "could not retrieve l2 agreed block")
-	l2Head := l2AgreedBlock.Hash()
+	l2Head := agreedL2Output.BlockRef.Hash
+	l2OutputRoot := agreedL2Output.OutputRoot
 
 	t.Log("Sending transactions to modify existing state, within challenged period")
 	SendDepositTx(t, cfg, l1Client, l2Seq, opts, func(l2Opts *DepositTxOpts) {
@@ -214,6 +221,7 @@ func testVerifyL2OutputRoot(t *testing.T, detached bool) {
 	testFaultProofProgramScenario(t, ctx, sys, &FaultProofProgramTestScenario{
 		L1Head:             l1Head,
 		L2Head:             l2Head,
+		L2OutputRoot:       common.Hash(l2OutputRoot),
 		L2Claim:            common.Hash(l2Claim),
 		L2ClaimBlockNumber: l2ClaimBlockNumber,
 		Detached:           detached,
@@ -223,6 +231,7 @@ func testVerifyL2OutputRoot(t *testing.T, detached bool) {
 type FaultProofProgramTestScenario struct {
 	L1Head             common.Hash
 	L2Head             common.Hash
+	L2OutputRoot       common.Hash
 	L2Claim            common.Hash
 	L2ClaimBlockNumber uint64
 	Detached           bool
@@ -231,7 +240,7 @@ type FaultProofProgramTestScenario struct {
 // testFaultProofProgramScenario runs the fault proof program in several contexts, given a test scenario.
 func testFaultProofProgramScenario(t *testing.T, ctx context.Context, sys *System, s *FaultProofProgramTestScenario) {
 	preimageDir := t.TempDir()
-	fppConfig := oppconf.NewConfig(sys.RollupConfig, sys.L2GenesisCfg.Config, s.L1Head, s.L2Head, common.Hash(s.L2Claim), s.L2ClaimBlockNumber)
+	fppConfig := oppconf.NewConfig(sys.RollupConfig, sys.L2GenesisCfg.Config, s.L1Head, s.L2Head, s.L2OutputRoot, common.Hash(s.L2Claim), s.L2ClaimBlockNumber)
 	fppConfig.L1URL = sys.NodeEndpoint("l1")
 	fppConfig.L2URL = sys.NodeEndpoint("sequencer")
 	fppConfig.DataDir = preimageDir

op-node/eth/output.go

 package eth
 
 import (
+	"errors"
+
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/crypto"
 )
 
 type OutputResponse struct {
@@ -12,3 +15,70 @@ type OutputResponse struct {
 	StateRoot             common.Hash `json:"stateRoot"`
 	Status                *SyncStatus `json:"syncStatus"`
 }
+
+var (
+	ErrInvalidOutput        = errors.New("invalid output")
+	ErrInvalidOutputVersion = errors.New("invalid output version")
+
+	OutputVersionV0 = Bytes32{}
+)
+
+type Output interface {
+	// Version returns the version of the L2 output
+	Version() Bytes32
+
+	// Marshal a L2 output into a byte slice for hashing
+	Marshal() []byte
+}
+
+type OutputV0 struct {
+	StateRoot                Bytes32
+	MessagePasserStorageRoot Bytes32
+	BlockHash                common.Hash
+}
+
+func (o *OutputV0) Version() Bytes32 {
+	return OutputVersionV0
+}
+
+func (o *OutputV0) Marshal() []byte {
+	var buf [128]byte
+	version := o.Version()
+	copy(buf[:32], version[:])
+	copy(buf[32:], o.StateRoot[:])
+	copy(buf[64:], o.MessagePasserStorageRoot[:])
+	copy(buf[96:], o.BlockHash[:])
+	return buf[:]
+}
+
+// OutputRoot returns the keccak256 hash of the marshaled L2 output
+func OutputRoot(output Output) Bytes32 {
+	marshaled := output.Marshal()
+	return Bytes32(crypto.Keccak256Hash(marshaled))
+}
+
+func UnmarshalOutput(data []byte) (Output, error) {
+	if len(data) < 32 {
+		return nil, ErrInvalidOutput
+	}
+	var ver Bytes32
+	copy(ver[:], data[:32])
+	switch ver {
+	case OutputVersionV0:
+		return unmarshalOutputV0(data)
+	default:
+		return nil, ErrInvalidOutputVersion
+	}
+}
+
+func unmarshalOutputV0(data []byte) (*OutputV0, error) {
+	if len(data) != 128 {
+		return nil, ErrInvalidOutput
+	}
+	var output OutputV0
+	// data[:32] is the version
+	copy(output.StateRoot[:], data[32:64])
+	copy(output.MessagePasserStorageRoot[:], data[64:96])
+	copy(output.BlockHash[:], data[96:128])
+	return &output, nil
+}

op-node/eth/output_test.go

+package eth
+
+import (
+	"testing"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/stretchr/testify/require"
+)
+
+func TestOutputV0Codec(t *testing.T) {
+	output := OutputV0{
+		StateRoot:                Bytes32{1, 2, 3},
+		MessagePasserStorageRoot: Bytes32{4, 5, 6},
+		BlockHash:                common.Hash{7, 8, 9},
+	}
+	marshaled := output.Marshal()
+	unmarshaled, err := UnmarshalOutput(marshaled)
+	require.NoError(t, err)
+	unmarshaledV0 := unmarshaled.(*OutputV0)
+	require.Equal(t, output, *unmarshaledV0)
+
+	_, err = UnmarshalOutput([]byte{0: 0xA, 32: 0xA})
+	require.ErrorIs(t, err, ErrInvalidOutputVersion)
+	_, err = UnmarshalOutput([]byte{64: 0xA})
+	require.ErrorIs(t, err, ErrInvalidOutput)
+}

op-node/node/api.go

@@ -4,12 +4,10 @@ import (
 	"context"
 	"fmt"
 
-	"github.com/ethereum/go-ethereum"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/common/hexutil"
 	"github.com/ethereum/go-ethereum/log"
 
-	"github.com/ethereum-optimism/optimism/op-bindings/predeploys"
 	"github.com/ethereum-optimism/optimism/op-node/eth"
 	"github.com/ethereum-optimism/optimism/op-node/rollup"
 	"github.com/ethereum-optimism/optimism/op-node/version"
@@ -20,6 +18,7 @@ type l2EthClient interface {
 	// GetProof returns a proof of the account, it may return a nil result without error if the address was not found.
 	// Optionally keys of the account storage trie can be specified to include with corresponding values in the proof.
 	GetProof(ctx context.Context, address common.Address, storage []common.Hash, blockTag string) (*eth.AccountResult, error)
+	OutputV0AtBlock(ctx context.Context, blockHash common.Hash) (*eth.OutputV0, error)
 }
 
 type driverClient interface {
@@ -99,40 +98,16 @@ func (n *nodeAPI) OutputAtBlock(ctx context.Context, number hexutil.Uint64) (*et
 		return nil, fmt.Errorf("failed to get L2 block ref with sync status: %w", err)
 	}
 
-	head, err := n.client.InfoByHash(ctx, ref.Hash)
+	output, err := n.client.OutputV0AtBlock(ctx, ref.Hash)
 	if err != nil {
-		return nil, fmt.Errorf("failed to get L2 block by hash %s: %w", ref, err)
+		return nil, fmt.Errorf("failed to get L2 output at block %s: %w", ref, err)
 	}
-	if head == nil {
-		return nil, ethereum.NotFound
-	}
-
-	proof, err := n.client.GetProof(ctx, predeploys.L2ToL1MessagePasserAddr, []common.Hash{}, ref.Hash.String())
-	if err != nil {
-		return nil, fmt.Errorf("failed to get contract proof at block %s: %w", ref, err)
-	}
-	if proof == nil {
-		return nil, fmt.Errorf("proof %w", ethereum.NotFound)
-	}
-	// make sure that the proof (including storage hash) that we retrieved is correct by verifying it against the state-root
-	if err := proof.Verify(head.Root()); err != nil {
-		n.log.Error("invalid withdrawal root detected in block", "stateRoot", head.Root(), "blocknum", number, "msg", err)
-		return nil, fmt.Errorf("invalid withdrawal root hash, state root was %s: %w", head.Root(), err)
-	}
-
-	var l2OutputRootVersion eth.Bytes32 // it's zero for now
-	l2OutputRoot, err := rollup.ComputeL2OutputRootV0(head, proof.StorageHash)
-	if err != nil {
-		n.log.Error("Error computing L2 output root, nil ptr passed to hashing function")
-		return nil, err
-	}
-
 	return &eth.OutputResponse{
-		Version:               l2OutputRootVersion,
-		OutputRoot:            l2OutputRoot,
+		Version:               output.Version(),
+		OutputRoot:            eth.OutputRoot(output),
 		BlockRef:              ref,
-		WithdrawalStorageRoot: proof.StorageHash,
-		StateRoot:             head.Root(),
+		WithdrawalStorageRoot: common.Hash(output.MessagePasserStorageRoot),
+		StateRoot:             common.Hash(output.StateRoot),
 		Status:                status,
 	}, nil
 }

op-node/node/server_test.go

@@ -16,7 +16,6 @@ import (
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/log"
 
-	"github.com/ethereum-optimism/optimism/op-bindings/predeploys"
 	"github.com/ethereum-optimism/optimism/op-node/eth"
 	"github.com/ethereum-optimism/optimism/op-node/metrics"
 	"github.com/ethereum-optimism/optimism/op-node/rollup"
@@ -84,17 +83,6 @@ func TestOutputAtBlock(t *testing.T) {
 	}
 
 	l2Client := &testutils.MockL2Client{}
-	info := &testutils.MockBlockInfo{
-		InfoHash:        header.Hash(),
-		InfoParentHash:  header.ParentHash,
-		InfoCoinbase:    header.Coinbase,
-		InfoRoot:        header.Root,
-		InfoNum:         header.Number.Uint64(),
-		InfoTime:        header.Time,
-		InfoMixDigest:   header.MixDigest,
-		InfoBaseFee:     header.BaseFee,
-		InfoReceiptRoot: header.ReceiptHash,
-	}
 	ref := eth.L2BlockRef{
 		Hash:           header.Hash(),
 		Number:         header.Number.Uint64(),
@@ -103,8 +91,12 @@ func TestOutputAtBlock(t *testing.T) {
 		L1Origin:       eth.BlockID{},
 		SequenceNumber: 0,
 	}
-	l2Client.ExpectInfoByHash(common.HexToHash("0x8512bee03061475e4b069171f7b406097184f16b22c3f5c97c0abfc49591c524"), info, nil)
-	l2Client.ExpectGetProof(predeploys.L2ToL1MessagePasserAddr, []common.Hash{}, "0x8512bee03061475e4b069171f7b406097184f16b22c3f5c97c0abfc49591c524", &result, nil)
+	output := &eth.OutputV0{
+		StateRoot:                eth.Bytes32(header.Root),
+		BlockHash:                ref.Hash,
+		MessagePasserStorageRoot: eth.Bytes32(result.StorageHash),
+	}
+	l2Client.ExpectOutputV0AtBlock(common.HexToHash("0x8512bee03061475e4b069171f7b406097184f16b22c3f5c97c0abfc49591c524"), output, nil)
 
 	drClient := &mockDriverClient{}
 	status := randomSyncStatus(rand.New(rand.NewSource(123)))

op-node/rollup/output_root.go

@@ -5,32 +5,33 @@ import (
 
 	"github.com/ethereum-optimism/optimism/op-bindings/bindings"
 	"github.com/ethereum-optimism/optimism/op-node/eth"
-	"github.com/ethereum/go-ethereum/crypto"
 )
 
-var NilProof = errors.New("Output root proof is nil")
+var ErrNilProof = errors.New("output root proof is nil")
 
 // ComputeL2OutputRoot computes the L2 output root by hashing an output root proof.
 func ComputeL2OutputRoot(proofElements *bindings.TypesOutputRootProof) (eth.Bytes32, error) {
 	if proofElements == nil {
-		return eth.Bytes32{}, NilProof
+		return eth.Bytes32{}, ErrNilProof
 	}
 
-	digest := crypto.Keccak256Hash(
-		proofElements.Version[:],
-		proofElements.StateRoot[:],
-		proofElements.MessagePasserStorageRoot[:],
-		proofElements.LatestBlockhash[:],
-	)
-	return eth.Bytes32(digest), nil
+	if proofElements.Version != [32]byte{} {
+		return eth.Bytes32{}, errors.New("unsupported output root version")
+	}
+	l2Output := eth.OutputV0{
+		StateRoot:                eth.Bytes32(proofElements.StateRoot),
+		MessagePasserStorageRoot: proofElements.MessagePasserStorageRoot,
+		BlockHash:                proofElements.LatestBlockhash,
+	}
+	return eth.OutputRoot(&l2Output), nil
 }
 
 func ComputeL2OutputRootV0(block eth.BlockInfo, storageRoot [32]byte) (eth.Bytes32, error) {
-	var l2OutputRootVersion eth.Bytes32 // it's zero for now
-	return ComputeL2OutputRoot(&bindings.TypesOutputRootProof{
-		Version:                  l2OutputRootVersion,
-		StateRoot:                block.Root(),
+	stateRoot := block.Root()
+	l2Output := eth.OutputV0{
+		StateRoot:                eth.Bytes32(stateRoot),
 		MessagePasserStorageRoot: storageRoot,
-		LatestBlockhash:          block.Hash(),
-	})
+		BlockHash:                block.Hash(),
+	}
+	return eth.OutputRoot(&l2Output), nil
 }

op-node/sources/l2_client.go

@@ -10,6 +10,7 @@ import (
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/log"
 
+	"github.com/ethereum-optimism/optimism/op-bindings/predeploys"
 	"github.com/ethereum-optimism/optimism/op-node/client"
 	"github.com/ethereum-optimism/optimism/op-node/eth"
 	"github.com/ethereum-optimism/optimism/op-node/rollup"
@@ -165,3 +166,31 @@ func (s *L2Client) SystemConfigByL2Hash(ctx context.Context, hash common.Hash) (
 	s.systemConfigsCache.Add(hash, cfg)
 	return cfg, nil
 }
+
+func (s *L2Client) OutputV0AtBlock(ctx context.Context, blockHash common.Hash) (*eth.OutputV0, error) {
+	head, err := s.InfoByHash(ctx, blockHash)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get L2 block by hash: %w", err)
+	}
+	if head == nil {
+		return nil, ethereum.NotFound
+	}
+
+	proof, err := s.GetProof(ctx, predeploys.L2ToL1MessagePasserAddr, []common.Hash{}, blockHash.String())
+	if err != nil {
+		return nil, fmt.Errorf("failed to get contract proof at block %s: %w", blockHash, err)
+	}
+	if proof == nil {
+		return nil, fmt.Errorf("proof %w", ethereum.NotFound)
+	}
+	// make sure that the proof (including storage hash) that we retrieved is correct by verifying it against the state-root
+	if err := proof.Verify(head.Root()); err != nil {
+		return nil, fmt.Errorf("invalid withdrawal root hash, state root was %s: %w", head.Root(), err)
+	}
+	stateRoot := head.Root()
+	return &eth.OutputV0{
+		StateRoot:                eth.Bytes32(stateRoot),
+		MessagePasserStorageRoot: eth.Bytes32(proof.StorageHash),
+		BlockHash:                blockHash,
+	}, nil
+}

op-node/testutils/mock_l2.go

@@ -43,3 +43,11 @@ func (m *MockL2Client) SystemConfigByL2Hash(ctx context.Context, hash common.Has
 func (m *MockL2Client) ExpectSystemConfigByL2Hash(hash common.Hash, cfg eth.SystemConfig, err error) {
 	m.Mock.On("SystemConfigByL2Hash", hash).Once().Return(cfg, &err)
 }
+
+func (m *MockL2Client) OutputV0AtBlock(ctx context.Context, blockHash common.Hash) (*eth.OutputV0, error) {
+	return m.Mock.MethodCalled("OutputV0AtBlock", blockHash).Get(0).(*eth.OutputV0), nil
+}
+
+func (m *MockL2Client) ExpectOutputV0AtBlock(blockHash common.Hash, output *eth.OutputV0, err error) {
+	m.Mock.On("OutputV0AtBlock", blockHash).Once().Return(output, &err)
+}

op-node/testutils/random.go

@@ -269,3 +269,11 @@ func RandomOutputResponse(rng *rand.Rand) *eth.OutputResponse {
 		},
 	}
 }
+
+func RandomOutputV0(rng *rand.Rand) *eth.OutputV0 {
+	return &eth.OutputV0{
+		StateRoot:                eth.Bytes32(RandomHash(rng)),
+		MessagePasserStorageRoot: eth.Bytes32(RandomHash(rng)),
+		BlockHash:                RandomHash(rng),
+	}
+}

op-program/client/boot.go

@@ -12,7 +12,7 @@ import (
 
 const (
 	L1HeadLocalIndex preimage.LocalIndexKey = iota + 1
-	L2HeadLocalIndex
+	L2OutputRootLocalIndex
 	L2ClaimLocalIndex
 	L2ClaimBlockNumberLocalIndex
 	L2ChainConfigLocalIndex
@@ -21,7 +21,7 @@ const (
 
 type BootInfo struct {
 	L1Head             common.Hash
-	L2Head             common.Hash
+	L2OutputRoot       common.Hash
 	L2Claim            common.Hash
 	L2ClaimBlockNumber uint64
 	L2ChainConfig      *params.ChainConfig
@@ -42,7 +42,7 @@ func NewBootstrapClient(r oracleClient) *BootstrapClient {
 
 func (br *BootstrapClient) BootInfo() *BootInfo {
 	l1Head := common.BytesToHash(br.r.Get(L1HeadLocalIndex))
-	l2Head := common.BytesToHash(br.r.Get(L2HeadLocalIndex))
+	l2OutputRoot := common.BytesToHash(br.r.Get(L2OutputRootLocalIndex))
 	l2Claim := common.BytesToHash(br.r.Get(L2ClaimLocalIndex))
 	l2ClaimBlockNumber := binary.BigEndian.Uint64(br.r.Get(L2ClaimBlockNumberLocalIndex))
 	l2ChainConfig := new(params.ChainConfig)
@@ -58,7 +58,7 @@ func (br *BootstrapClient) BootInfo() *BootInfo {
 
 	return &BootInfo{
 		L1Head:             l1Head,
-		L2Head:             l2Head,
+		L2OutputRoot:       l2OutputRoot,
 		L2Claim:            l2Claim,
 		L2ClaimBlockNumber: l2ClaimBlockNumber,
 		L2ChainConfig:      l2ChainConfig,

op-program/client/boot_test.go

@@ -15,7 +15,7 @@ import (
 func TestBootstrapClient(t *testing.T) {
 	bootInfo := &BootInfo{
 		L1Head:             common.HexToHash("0x1111"),
-		L2Head:             common.HexToHash("0x2222"),
+		L2OutputRoot:       common.HexToHash("0x2222"),
 		L2Claim:            common.HexToHash("0x3333"),
 		L2ClaimBlockNumber: 1,
 		L2ChainConfig:      params.GoerliChainConfig,
@@ -34,8 +34,8 @@ func (o *mockBoostrapOracle) Get(key preimage.Key) []byte {
 	switch key.PreimageKey() {
 	case L1HeadLocalIndex.PreimageKey():
 		return o.b.L1Head[:]
-	case L2HeadLocalIndex.PreimageKey():
-		return o.b.L2Head[:]
+	case L2OutputRootLocalIndex.PreimageKey():
+		return o.b.L2OutputRoot[:]
 	case L2ClaimLocalIndex.PreimageKey():
 		return o.b.L2Claim[:]
 	case L2ClaimBlockNumberLocalIndex.PreimageKey():

op-program/client/l2/cache.go

 package l2
 
 import (
+	"github.com/ethereum-optimism/optimism/op-node/eth"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/hashicorp/golang-lru/v2/simplelru"
@@ -13,21 +14,24 @@ const nodeCacheSize = 100_000
 const codeCacheSize = 10_000
 
 type CachingOracle struct {
-	oracle Oracle
-	blocks *simplelru.LRU[common.Hash, *types.Block]
-	nodes  *simplelru.LRU[common.Hash, []byte]
-	codes  *simplelru.LRU[common.Hash, []byte]
+	oracle  Oracle
+	blocks  *simplelru.LRU[common.Hash, *types.Block]
+	nodes   *simplelru.LRU[common.Hash, []byte]
+	codes   *simplelru.LRU[common.Hash, []byte]
+	outputs *simplelru.LRU[common.Hash, eth.Output]
 }
 
 func NewCachingOracle(oracle Oracle) *CachingOracle {
 	blockLRU, _ := simplelru.NewLRU[common.Hash, *types.Block](blockCacheSize, nil)
 	nodeLRU, _ := simplelru.NewLRU[common.Hash, []byte](nodeCacheSize, nil)
 	codeLRU, _ := simplelru.NewLRU[common.Hash, []byte](codeCacheSize, nil)
+	outputLRU, _ := simplelru.NewLRU[common.Hash, eth.Output](codeCacheSize, nil)
 	return &CachingOracle{
-		oracle: oracle,
-		blocks: blockLRU,
-		nodes:  nodeLRU,
-		codes:  codeLRU,
+		oracle:  oracle,
+		blocks:  blockLRU,
+		nodes:   nodeLRU,
+		codes:   codeLRU,
+		outputs: outputLRU,
 	}
 }
 
@@ -60,3 +64,13 @@ func (o *CachingOracle) BlockByHash(blockHash common.Hash) *types.Block {
 	o.blocks.Add(blockHash, block)
 	return block
 }
+
+func (o *CachingOracle) OutputByRoot(root common.Hash) eth.Output {
+	output, ok := o.outputs.Get(root)
+	if ok {
+		return output
+	}
+	output = o.oracle.OutputByRoot(root)
+	o.outputs.Add(root, output)
+	return output
+}

op-program/client/l2/cache_test.go

@@ -4,6 +4,7 @@ import (
 	"math/rand"
 	"testing"
 
+	"github.com/ethereum-optimism/optimism/op-node/eth"
 	"github.com/ethereum-optimism/optimism/op-node/testutils"
 	"github.com/ethereum-optimism/optimism/op-program/client/l2/test"
 	"github.com/ethereum/go-ethereum/common"
@@ -66,3 +67,22 @@ func TestCodeByHash(t *testing.T) {
 	actual = oracle.CodeByHash(hash)
 	require.Equal(t, node, actual)
 }
+
+func TestOutputByRoot(t *testing.T) {
+	stub, _ := test.NewStubOracle(t)
+	oracle := NewCachingOracle(stub)
+
+	rng := rand.New(rand.NewSource(1))
+	output := testutils.RandomOutputV0(rng)
+
+	// Initial call retrieves from the stub
+	root := common.Hash(eth.OutputRoot(output))
+	stub.Outputs[root] = output
+	actual := oracle.OutputByRoot(root)
+	require.Equal(t, output, actual)
+
+	// Later calls should retrieve from cache
+	delete(stub.Outputs, root)
+	actual = oracle.OutputByRoot(root)
+	require.Equal(t, output, actual)
+}

op-program/client/l2/engine_backend.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"math/big"
 
+	"github.com/ethereum-optimism/optimism/op-node/eth"
 	"github.com/ethereum-optimism/optimism/op-program/client/l2/engineapi"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/consensus"
@@ -39,8 +40,13 @@ type OracleBackedL2Chain struct {
 
 var _ engineapi.EngineBackend = (*OracleBackedL2Chain)(nil)
 
-func NewOracleBackedL2Chain(logger log.Logger, oracle Oracle, chainCfg *params.ChainConfig, l2Head common.Hash) (*OracleBackedL2Chain, error) {
-	head := oracle.BlockByHash(l2Head)
+func NewOracleBackedL2Chain(logger log.Logger, oracle Oracle, chainCfg *params.ChainConfig, l2OutputRoot common.Hash) (*OracleBackedL2Chain, error) {
+	output := oracle.OutputByRoot(l2OutputRoot)
+	outputV0, ok := output.(*eth.OutputV0)
+	if !ok {
+		return nil, fmt.Errorf("unsupported L2 output version: %d", output.Version())
+	}
+	head := oracle.BlockByHash(outputV0.BlockHash)
 	logger.Info("Loaded L2 head", "hash", head.Hash(), "number", head.Number())
 	return &OracleBackedL2Chain{
 		log:      logger,

op-program/client/l2/engine_backend_test.go

@@ -5,6 +5,7 @@ import (
 	"testing"
 
 	"github.com/ethereum-optimism/optimism/op-chain-ops/genesis"
+	"github.com/ethereum-optimism/optimism/op-node/eth"
 	"github.com/ethereum-optimism/optimism/op-node/testlog"
 	"github.com/ethereum-optimism/optimism/op-program/client/l2/engineapi"
 	"github.com/ethereum-optimism/optimism/op-program/client/l2/engineapi/test"
@@ -199,7 +200,8 @@ func setupOracleBackedChainWithLowerHead(t *testing.T, blockCount int, headBlock
 	logger := testlog.Logger(t, log.LvlDebug)
 	chainCfg, blocks, oracle := setupOracle(t, blockCount, headBlockNumber)
 	head := blocks[headBlockNumber].Hash()
-	chain, err := NewOracleBackedL2Chain(logger, oracle, chainCfg, head)
+	stubOutput := eth.OutputV0{BlockHash: head}
+	chain, err := NewOracleBackedL2Chain(logger, oracle, chainCfg, common.Hash(eth.OutputRoot(&stubOutput)))
 	require.NoError(t, err)
 	return blocks, chain
 }
@@ -232,7 +234,12 @@ func setupOracle(t *testing.T, blockCount int, headBlockNumber int) (*params.Cha
 	genesisBlock := l2Genesis.MustCommit(db)
 	blocks, _ := core.GenerateChain(chainCfg, genesisBlock, consensus, db, blockCount, func(i int, gen *core.BlockGen) {})
 	blocks = append([]*types.Block{genesisBlock}, blocks...)
-	oracle := l2test.NewStubOracleWithBlocks(t, blocks[:headBlockNumber+1], db)
+
+	var outputs []eth.Output
+	for _, block := range blocks {
+		outputs = append(outputs, &eth.OutputV0{BlockHash: block.Hash()})
+	}
+	oracle := l2test.NewStubOracleWithBlocks(t, blocks[:headBlockNumber+1], outputs, db)
 	return chainCfg, blocks, oracle
 }
 

op-program/client/l2/hints.go

@@ -11,6 +11,7 @@ const (
 	HintL2Transactions = "l2-transactions"
 	HintL2Code         = "l2-code"
 	HintL2StateNode    = "l2-state-node"
+	HintL2Output       = "l2-output"
 )
 
 type BlockHeaderHint common.Hash
@@ -44,3 +45,11 @@ var _ preimage.Hint = StateNodeHint{}
 func (l StateNodeHint) Hint() string {
 	return HintL2StateNode + " " + (common.Hash)(l).String()
 }
+
+type L2OutputHint common.Hash
+
+var _ preimage.Hint = L2OutputHint{}
+
+func (l L2OutputHint) Hint() string {
+	return HintL2Output + " " + (common.Hash)(l).String()
+}

op-program/client/l2/oracle.go

@@ -32,6 +32,8 @@ type Oracle interface {
 
 	// BlockByHash retrieves the block with the given hash.
 	BlockByHash(blockHash common.Hash) *types.Block
+
+	OutputByRoot(root common.Hash) eth.Output
 }
 
 // PreimageOracle implements Oracle using by interfacing with the pure preimage.Oracle
@@ -85,3 +87,13 @@ func (p *PreimageOracle) CodeByHash(codeHash common.Hash) []byte {
 	p.hint.Hint(CodeHint(codeHash))
 	return p.oracle.Get(preimage.Keccak256Key(codeHash))
 }
+
+func (p *PreimageOracle) OutputByRoot(l2OutputRoot common.Hash) eth.Output {
+	p.hint.Hint(L2OutputHint(l2OutputRoot))
+	data := p.oracle.Get(preimage.Keccak256Key(l2OutputRoot))
+	output, err := eth.UnmarshalOutput(data)
+	if err != nil {
+		panic(fmt.Errorf("invalid L2 output data for root %s: %w", l2OutputRoot, err))
+	}
+	return output
+}

op-program/client/l2/oracle_test.go

@@ -122,3 +122,21 @@ func TestPreimageOracleCodeByHash(t *testing.T) {
 		})
 	}
 }
+
+func TestPreimageOracleOutputByRoot(t *testing.T) {
+	rng := rand.New(rand.NewSource(123))
+
+	for i := 0; i < 10; i++ {
+		t.Run(fmt.Sprintf("output_%d", i), func(t *testing.T) {
+			po, hints, preimages := mockPreimageOracle(t)
+			output := testutils.RandomOutputV0(rng)
+
+			h := common.Hash(eth.OutputRoot(output))
+			preimages[preimage.Keccak256Key(h).PreimageKey()] = output.Marshal()
+			hints.On("hint", L2OutputHint(h).Hint()).Once().Return()
+			gotOutput := po.OutputByRoot(h)
+			hints.AssertExpectations(t)
+			require.Equal(t, hexutil.Bytes(output.Marshal()), hexutil.Bytes(gotOutput.Marshal()), "output matches")
+		})
+	}
+}

op-program/client/l2/test/stub_oracle.go

@@ -3,6 +3,7 @@ package test
 import (
 	"testing"
 
+	"github.com/ethereum-optimism/optimism/op-node/eth"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/rawdb"
 	"github.com/ethereum/go-ethereum/core/types"
@@ -16,8 +17,9 @@ type stateOracle interface {
 }
 
 type StubBlockOracle struct {
-	t      *testing.T
-	Blocks map[common.Hash]*types.Block
+	t       *testing.T
+	Blocks  map[common.Hash]*types.Block
+	Outputs map[common.Hash]eth.Output
 	stateOracle
 }
 
@@ -26,18 +28,25 @@ func NewStubOracle(t *testing.T) (*StubBlockOracle, *StubStateOracle) {
 	blockOracle := StubBlockOracle{
 		t:           t,
 		Blocks:      make(map[common.Hash]*types.Block),
+		Outputs:     make(map[common.Hash]eth.Output),
 		stateOracle: stateOracle,
 	}
 	return &blockOracle, stateOracle
 }
 
-func NewStubOracleWithBlocks(t *testing.T, chain []*types.Block, db ethdb.Database) *StubBlockOracle {
+func NewStubOracleWithBlocks(t *testing.T, chain []*types.Block, outputs []eth.Output, db ethdb.Database) *StubBlockOracle {
 	blocks := make(map[common.Hash]*types.Block, len(chain))
 	for _, block := range chain {
 		blocks[block.Hash()] = block
 	}
+	o := make(map[common.Hash]eth.Output, len(outputs))
+	for _, output := range outputs {
+		o[common.Hash(eth.OutputRoot(output))] = output
+	}
 	return &StubBlockOracle{
+		t:           t,
 		Blocks:      blocks,
+		Outputs:     o,
 		stateOracle: &KvStateOracle{t: t, Source: db},
 	}
 }
@@ -50,6 +59,14 @@ func (o StubBlockOracle) BlockByHash(blockHash common.Hash) *types.Block {
 	return block
 }
 
+func (o StubBlockOracle) OutputByRoot(root common.Hash) eth.Output {
+	output, ok := o.Outputs[root]
+	if !ok {
+		o.t.Fatalf("requested unknown output root %s", root)
+	}
+	return output
+}
+
 // KvStateOracle loads data from a source ethdb.KeyValueStore
 type KvStateOracle struct {
 	t      *testing.T

op-program/client/program.go

@@ -53,7 +53,7 @@ func RunProgram(logger log.Logger, preimageOracle io.ReadWriter, preimageHinter
 		bootInfo.RollupConfig,
 		bootInfo.L2ChainConfig,
 		bootInfo.L1Head,
-		bootInfo.L2Head,
+		bootInfo.L2OutputRoot,
 		bootInfo.L2Claim,
 		bootInfo.L2ClaimBlockNumber,
 		l1PreimageOracle,
@@ -62,9 +62,9 @@ func RunProgram(logger log.Logger, preimageOracle io.ReadWriter, preimageHinter
 }
 
 // runDerivation executes the L2 state transition, given a minimal interface to retrieve data.
-func runDerivation(logger log.Logger, cfg *rollup.Config, l2Cfg *params.ChainConfig, l1Head common.Hash, l2Head common.Hash, l2Claim common.Hash, l2ClaimBlockNum uint64, l1Oracle l1.Oracle, l2Oracle l2.Oracle) error {
+func runDerivation(logger log.Logger, cfg *rollup.Config, l2Cfg *params.ChainConfig, l1Head common.Hash, l2OutputRoot common.Hash, l2Claim common.Hash, l2ClaimBlockNum uint64, l1Oracle l1.Oracle, l2Oracle l2.Oracle) error {
 	l1Source := l1.NewOracleL1Client(logger, l1Oracle, l1Head)
-	engineBackend, err := l2.NewOracleBackedL2Chain(logger, l2Oracle, l2Cfg, l2Head)
+	engineBackend, err := l2.NewOracleBackedL2Chain(logger, l2Oracle, l2Cfg, l2OutputRoot)
 	if err != nil {
 		return fmt.Errorf("failed to create oracle-backed L2 chain: %w", err)
 	}

op-program/host/cmd/main_test.go

@@ -20,6 +20,7 @@ var (
 	l1HeadValue        = common.HexToHash("0x111111").Hex()
 	l2HeadValue        = common.HexToHash("0x222222").Hex()
 	l2ClaimValue       = common.HexToHash("0x333333").Hex()
+	l2OutputRoot       = common.HexToHash("0x444444").Hex()
 	l2ClaimBlockNumber = uint64(1203)
 	// Note: This is actually the L1 goerli genesis config. Just using it as an arbitrary, valid genesis config
 	l2Genesis       = core.DefaultGoerliGenesisBlock()
@@ -48,6 +49,7 @@ func TestDefaultCLIOptionsMatchDefaultConfig(t *testing.T) {
 		config.OPGoerliChainConfig,
 		common.HexToHash(l1HeadValue),
 		common.HexToHash(l2HeadValue),
+		common.HexToHash(l2OutputRoot),
 		common.HexToHash(l2ClaimValue),
 		l2ClaimBlockNumber)
 	require.Equal(t, defaultCfg, cfg)
@@ -136,6 +138,21 @@ func TestL2Head(t *testing.T) {
 	})
 }
 
+func TestL2OutputRoot(t *testing.T) {
+	t.Run("Required", func(t *testing.T) {
+		verifyArgsInvalid(t, "flag l2.outputroot is required", addRequiredArgsExcept("--l2.outputroot"))
+	})
+
+	t.Run("Valid", func(t *testing.T) {
+		cfg := configForArgs(t, replaceRequiredArg("--l2.outputroot", l2OutputRoot))
+		require.Equal(t, common.HexToHash(l2OutputRoot), cfg.L2OutputRoot)
+	})
+
+	t.Run("Invalid", func(t *testing.T) {
+		verifyArgsInvalid(t, config.ErrInvalidL2OutputRoot.Error(), replaceRequiredArg("--l2.outputroot", "something"))
+	})
+}
+
 func TestL1Head(t *testing.T) {
 	t.Run("Required", func(t *testing.T) {
 		verifyArgsInvalid(t, "flag l1.head is required", addRequiredArgsExcept("--l1.head"))
@@ -307,6 +324,7 @@ func requiredArgs() map[string]string {
 		"--network":        "goerli",
 		"--l1.head":        l1HeadValue,
 		"--l2.head":        l2HeadValue,
+		"--l2.outputroot":  l2OutputRoot,
 		"--l2.claim":       l2ClaimValue,
 		"--l2.blocknumber": strconv.FormatUint(l2ClaimBlockNumber, 10),
 	}

op-program/host/config/config.go

@@ -22,6 +22,7 @@ var (
 	ErrMissingL2Genesis    = errors.New("missing l2 genesis")
 	ErrInvalidL1Head       = errors.New("invalid l1 head")
 	ErrInvalidL2Head       = errors.New("invalid l2 head")
+	ErrInvalidL2OutputRoot = errors.New("invalid l2 output root")
 	ErrL1AndL2Inconsistent = errors.New("l1 and l2 options must be specified together or both omitted")
 	ErrInvalidL2Claim      = errors.New("invalid l2 claim")
 	ErrInvalidL2ClaimBlock = errors.New("invalid l2 claim block number")
@@ -41,9 +42,12 @@ type Config struct {
 	L1TrustRPC bool
 	L1RPCKind  sources.RPCProviderKind
 
-	// L2Head is the agreed L2 block to start derivation from
+	// L2Head is the l2 block hash contained in the L2 Output referenced by the L2OutputRoot
+	// TODO(inphi): This can be made optional with hardcoded rollup configs and output oracle addresses by searching the oracle for the l2 output root
 	L2Head common.Hash
-	L2URL  string
+	// L2OutputRoot is the agreed L2 output root to start derivation from
+	L2OutputRoot common.Hash
+	L2URL        string
 	// L2Claim is the claimed L2 output root to verify
 	L2Claim common.Hash
 	// L2ClaimBlockNumber is the block number the claimed L2 output root is from
@@ -73,6 +77,9 @@ func (c *Config) Check() error {
 	if c.L2Head == (common.Hash{}) {
 		return ErrInvalidL2Head
 	}
+	if c.L2OutputRoot == (common.Hash{}) {
+		return ErrInvalidL2OutputRoot
+	}
 	if c.L2Claim == (common.Hash{}) {
 		return ErrInvalidL2Claim
 	}
@@ -99,12 +106,21 @@ func (c *Config) FetchingEnabled() bool {
 }
 
 // NewConfig creates a Config with all optional values set to the CLI default value
-func NewConfig(rollupCfg *rollup.Config, l2Genesis *params.ChainConfig, l1Head common.Hash, l2Head common.Hash, l2Claim common.Hash, l2ClaimBlockNum uint64) *Config {
+func NewConfig(
+	rollupCfg *rollup.Config,
+	l2Genesis *params.ChainConfig,
+	l1Head common.Hash,
+	l2Head common.Hash,
+	l2OutputRoot common.Hash,
+	l2Claim common.Hash,
+	l2ClaimBlockNum uint64,
+) *Config {
 	return &Config{
 		Rollup:             rollupCfg,
 		L2ChainConfig:      l2Genesis,
 		L1Head:             l1Head,
 		L2Head:             l2Head,
+		L2OutputRoot:       l2OutputRoot,
 		L2Claim:            l2Claim,
 		L2ClaimBlockNumber: l2ClaimBlockNum,
 		L1RPCKind:          sources.RPCKindBasic,
@@ -123,6 +139,10 @@ func NewConfigFromCLI(log log.Logger, ctx *cli.Context) (*Config, error) {
 	if l2Head == (common.Hash{}) {
 		return nil, ErrInvalidL2Head
 	}
+	l2OutputRoot := common.HexToHash(ctx.String(flags.L2OutputRoot.Name))
+	if l2OutputRoot == (common.Hash{}) {
+		return nil, ErrInvalidL2OutputRoot
+	}
 	l2Claim := common.HexToHash(ctx.String(flags.L2Claim.Name))
 	if l2Claim == (common.Hash{}) {
 		return nil, ErrInvalidL2Claim
@@ -152,6 +172,7 @@ func NewConfigFromCLI(log log.Logger, ctx *cli.Context) (*Config, error) {
 		L2URL:              ctx.String(flags.L2NodeAddr.Name),
 		L2ChainConfig:      l2ChainConfig,
 		L2Head:             l2Head,
+		L2OutputRoot:       l2OutputRoot,
 		L2Claim:            l2Claim,
 		L2ClaimBlockNumber: l2ClaimBlockNum,
 		L1Head:             l1Head,

op-program/host/config/config_test.go

@@ -16,6 +16,7 @@ var (
 	validL1Head          = common.Hash{0xaa}
 	validL2Head          = common.Hash{0xbb}
 	validL2Claim         = common.Hash{0xcc}
+	validL2OutputRoot    = common.Hash{0xdd}
 	validL2ClaimBlockNum = uint64(15)
 )
 
@@ -55,6 +56,13 @@ func TestL2HeadRequired(t *testing.T) {
 	require.ErrorIs(t, err, ErrInvalidL2Head)
 }
 
+func TestL2OutputRootRequired(t *testing.T) {
+	config := validConfig()
+	config.L2OutputRoot = common.Hash{}
+	err := config.Check()
+	require.ErrorIs(t, err, ErrInvalidL2OutputRoot)
+}
+
 func TestL2ClaimRequired(t *testing.T) {
 	config := validConfig()
 	config.L2Claim = common.Hash{}
@@ -151,7 +159,7 @@ func TestRejectExecAndServerMode(t *testing.T) {
 }
 
 func validConfig() *Config {
-	cfg := NewConfig(validRollupConfig, validL2Genesis, validL1Head, validL2Head, validL2Claim, validL2ClaimBlockNum)
+	cfg := NewConfig(validRollupConfig, validL2Genesis, validL1Head, validL2Head, validL2OutputRoot, validL2Claim, validL2ClaimBlockNum)
 	cfg.DataDir = "/tmp/configTest"
 	return cfg
 }

op-program/host/flags/flags.go

@@ -47,9 +47,14 @@ var (
 	}
 	L2Head = &cli.StringFlag{
 		Name:    "l2.head",
-		Usage:   "Hash of the agreed L2 block to start derivation from",
+		Usage:   "Hash of the L2 block at l2.outputroot",
 		EnvVars: prefixEnvVars("L2_HEAD"),
 	}
+	L2OutputRoot = &cli.StringFlag{
+		Name:    "l2.outputroot",
+		Usage:   "Agreed L2 Output Root to start derivation from",
+		EnvVars: prefixEnvVars("L2_OUTPUT_ROOT"),
+	}
 	L2Claim = &cli.StringFlag{
 		Name:    "l2.claim",
 		Usage:   "Claimed L2 output root to validate",
@@ -103,6 +108,7 @@ var Flags []cli.Flag
 var requiredFlags = []cli.Flag{
 	L1Head,
 	L2Head,
+	L2OutputRoot,
 	L2Claim,
 	L2BlockNumber,
 }

op-program/host/host.go

@@ -26,7 +26,7 @@ import (
 )
 
 type L2Source struct {
-	*sources.L2Client
+	*L2Client
 	*sources.DebugClient
 }
 
@@ -205,7 +205,7 @@ func makePrefetcher(ctx context.Context, logger log.Logger, kv kvstore.KV, cfg *
 	if err != nil {
 		return nil, fmt.Errorf("failed to create L1 client: %w", err)
 	}
-	l2Cl, err := sources.NewL2Client(l2RPC, logger, nil, l2ClCfg)
+	l2Cl, err := NewL2Client(l2RPC, logger, nil, &L2ClientConfig{L2ClientConfig: l2ClCfg, L2Head: cfg.L2Head})
 	if err != nil {
 		return nil, fmt.Errorf("failed to create L2 client: %w", err)
 	}

op-program/host/host_test.go

@@ -23,7 +23,8 @@ func TestServerMode(t *testing.T) {
 	dir := t.TempDir()
 
 	l1Head := common.Hash{0x11}
-	cfg := config.NewConfig(&chaincfg.Goerli, config.OPGoerliChainConfig, l1Head, common.Hash{0x22}, common.Hash{0x33}, 1000)
+	l2OutputRoot := common.Hash{0x33}
+	cfg := config.NewConfig(&chaincfg.Goerli, config.OPGoerliChainConfig, l1Head, common.Hash{0x22}, l2OutputRoot, common.Hash{0x44}, 1000)
 	cfg.DataDir = dir
 	cfg.ServerMode = true
 
@@ -43,7 +44,8 @@ func TestServerMode(t *testing.T) {
 	hClient := preimage.NewHintWriter(hintClient)
 	l1PreimageOracle := l1.NewPreimageOracle(pClient, hClient)
 
-	require.Equal(t, l1Head.Bytes(), pClient.Get(client.L1HeadLocalIndex), "Should get preimages")
+	require.Equal(t, l1Head.Bytes(), pClient.Get(client.L1HeadLocalIndex), "Should get l1 head preimages")
+	require.Equal(t, l2OutputRoot.Bytes(), pClient.Get(client.L2OutputRootLocalIndex), "Should get l2 output root preimages")
 
 	// Should exit when a preimage is unavailable
 	require.Panics(t, func() {

op-program/host/kvstore/local.go

@@ -19,7 +19,7 @@ func NewLocalPreimageSource(config *config.Config) *LocalPreimageSource {
 
 var (
 	l1HeadKey             = client.L1HeadLocalIndex.PreimageKey()
-	l2HeadKey             = client.L2HeadLocalIndex.PreimageKey()
+	l2OutputRootKey       = client.L2OutputRootLocalIndex.PreimageKey()
 	l2ClaimKey            = client.L2ClaimLocalIndex.PreimageKey()
 	l2ClaimBlockNumberKey = client.L2ClaimBlockNumberLocalIndex.PreimageKey()
 	l2ChainConfigKey      = client.L2ChainConfigLocalIndex.PreimageKey()
@@ -30,8 +30,8 @@ func (s *LocalPreimageSource) Get(key common.Hash) ([]byte, error) {
 	switch [32]byte(key) {
 	case l1HeadKey:
 		return s.config.L1Head.Bytes(), nil
-	case l2HeadKey:
-		return s.config.L2Head.Bytes(), nil
+	case l2OutputRootKey:
+		return s.config.L2OutputRoot.Bytes(), nil
 	case l2ClaimKey:
 		return s.config.L2Claim.Bytes(), nil
 	case l2ClaimBlockNumberKey:

op-program/host/kvstore/local_test.go

@@ -17,7 +17,7 @@ func TestLocalPreimageSource(t *testing.T) {
 	cfg := &config.Config{
 		Rollup:             &chaincfg.Goerli,
 		L1Head:             common.HexToHash("0x1111"),
-		L2Head:             common.HexToHash("0x2222"),
+		L2OutputRoot:       common.HexToHash("0x2222"),
 		L2Claim:            common.HexToHash("0x3333"),
 		L2ClaimBlockNumber: 1234,
 		L2ChainConfig:      params.GoerliChainConfig,
@@ -29,7 +29,7 @@ func TestLocalPreimageSource(t *testing.T) {
 		expected []byte
 	}{
 		{"L1Head", l1HeadKey, cfg.L1Head.Bytes()},
-		{"L2Head", l2HeadKey, cfg.L2Head.Bytes()},
+		{"L2OutputRoot", l2OutputRootKey, cfg.L2OutputRoot.Bytes()},
 		{"L2Claim", l2ClaimKey, cfg.L2Claim.Bytes()},
 		{"L2ClaimBlockNumber", l2ClaimBlockNumberKey, binary.BigEndian.AppendUint64(nil, cfg.L2ClaimBlockNumber)},
 		{"Rollup", rollupKey, asJson(t, cfg.Rollup)},

op-program/host/l2_client.go

+package host
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/ethereum-optimism/optimism/op-node/client"
+	"github.com/ethereum-optimism/optimism/op-node/eth"
+	"github.com/ethereum-optimism/optimism/op-node/sources"
+	"github.com/ethereum-optimism/optimism/op-node/sources/caching"
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/log"
+)
+
+type L2Client struct {
+	*sources.L2Client
+
+	// l2Head is the L2 block hash that we use to fetch L2 output
+	l2Head common.Hash
+}
+
+type L2ClientConfig struct {
+	*sources.L2ClientConfig
+	L2Head common.Hash
+}
+
+func NewL2Client(client client.RPC, log log.Logger, metrics caching.Metrics, config *L2ClientConfig) (*L2Client, error) {
+	l2Client, err := sources.NewL2Client(client, log, metrics, config.L2ClientConfig)
+	if err != nil {
+		return nil, err
+	}
+	return &L2Client{
+		L2Client: l2Client,
+		l2Head:   config.L2Head,
+	}, nil
+}
+
+func (s *L2Client) OutputByRoot(ctx context.Context, l2OutputRoot common.Hash) (eth.Output, error) {
+	output, err := s.OutputV0AtBlock(ctx, s.l2Head)
+	if err != nil {
+		return nil, err
+	}
+	if eth.OutputRoot(output) != eth.Bytes32(l2OutputRoot) {
+		// For fault proofs, we only reference outputs at the l2 head at boot time
+		// The caller shouldn't be requesting outputs at any other block
+		return nil, fmt.Errorf("unknown output root")
+	}
+	return output, nil
+}

op-program/host/prefetcher/prefetcher.go

@@ -29,6 +29,7 @@ type L2Source interface {
 	InfoAndTxsByHash(ctx context.Context, blockHash common.Hash) (eth.BlockInfo, types.Transactions, error)
 	NodeByHash(ctx context.Context, hash common.Hash) ([]byte, error)
 	CodeByHash(ctx context.Context, hash common.Hash) ([]byte, error)
+	OutputByRoot(ctx context.Context, root common.Hash) (eth.Output, error)
 }
 
 type Prefetcher struct {
@@ -124,6 +125,12 @@ func (p *Prefetcher) prefetch(ctx context.Context, hint string) error {
 			return fmt.Errorf("failed to fetch L2 contract code %s: %w", hash, err)
 		}
 		return p.kvStore.Put(preimage.Keccak256Key(hash).PreimageKey(), code)
+	case l2.HintL2Output:
+		output, err := p.l2Fetcher.OutputByRoot(ctx, hash)
+		if err != nil {
+			return fmt.Errorf("failed to fetch L2 output root %s: %w", hash, err)
+		}
+		return p.kvStore.Put(preimage.Keccak256Key(hash).PreimageKey(), output.Marshal())
 	}
 	return fmt.Errorf("unknown hint type: %v", hintType)
 }

op-program/host/prefetcher/prefetcher_test.go

@@ -286,6 +286,15 @@ type l2Client struct {
 	*testutils.MockDebugClient
 }
 
+func (m *l2Client) OutputByRoot(ctx context.Context, root common.Hash) (eth.Output, error) {
+	out := m.Mock.MethodCalled("OutputByRoot", root)
+	return out[0].(eth.Output), *out[1].(*error)
+}
+
+func (m *l2Client) ExpectOutputByRoot(root common.Hash, output eth.Output, err error) {
+	m.Mock.On("OutputByRoot", root).Once().Return(output, &err)
+}
+
 func createPrefetcher(t *testing.T) (*Prefetcher, *testutils.MockL1Source, *l2Client, kvstore.KV) {
 	logger := testlog.Logger(t, log.LvlDebug)
 	kv := kvstore.NewMemKV()

op-program/host/prefetcher/retry.go

@@ -125,6 +125,20 @@ func (s *RetryingL2Source) CodeByHash(ctx context.Context, hash common.Hash) ([]
 	return code, err
 }
 
+func (s *RetryingL2Source) OutputByRoot(ctx context.Context, root common.Hash) (eth.Output, error) {
+	var output eth.Output
+	err := backoff.DoCtx(ctx, maxAttempts, s.strategy, func() error {
+		o, err := s.source.OutputByRoot(ctx, root)
+		if err != nil {
+			s.logger.Warn("Failed to fetch l2 output", "root", root, "err", err)
+			return err
+		}
+		output = o
+		return nil
+	})
+	return output, err
+}
+
 func NewRetryingL2Source(logger log.Logger, source L2Source) *RetryingL2Source {
 	return &RetryingL2Source{
 		logger:   logger,

op-program/host/prefetcher/retry_test.go

@@ -119,6 +119,8 @@ func TestRetryingL2Source(t *testing.T) {
 		&types.Transaction{},
 	}
 	data := []byte{1, 2, 3, 4, 5}
+	output := &eth.OutputV0{}
+	wrongOutput := &eth.OutputV0{BlockHash: common.Hash{0x99}}
 
 	t.Run("InfoAndTxsByHash Success", func(t *testing.T) {
 		source, mock := createL2Source(t)
@@ -187,6 +189,28 @@ func TestRetryingL2Source(t *testing.T) {
 		require.NoError(t, err)
 		require.Equal(t, data, actual)
 	})
+
+	t.Run("OutputByRoot Success", func(t *testing.T) {
+		source, mock := createL2Source(t)
+		defer mock.AssertExpectations(t)
+		mock.ExpectOutputByRoot(hash, output, nil)
+
+		actualOutput, err := source.OutputByRoot(ctx, hash)
+		require.NoError(t, err)
+		require.Equal(t, output, actualOutput)
+	})
+
+	t.Run("OutputByRoot Error", func(t *testing.T) {
+		source, mock := createL2Source(t)
+		defer mock.AssertExpectations(t)
+		expectedErr := errors.New("boom")
+		mock.ExpectOutputByRoot(hash, wrongOutput, expectedErr)
+		mock.ExpectOutputByRoot(hash, output, nil)
+
+		actualOutput, err := source.OutputByRoot(ctx, hash)
+		require.NoError(t, err)
+		require.Equal(t, output, actualOutput)
+	})
 }
 
 func createL2Source(t *testing.T) (*RetryingL2Source, *MockL2Source) {
@@ -217,6 +241,11 @@ func (m *MockL2Source) CodeByHash(ctx context.Context, hash common.Hash) ([]byte
 	return out[0].([]byte), *out[1].(*error)
 }
 
+func (m *MockL2Source) OutputByRoot(ctx context.Context, root common.Hash) (eth.Output, error) {
+	out := m.Mock.MethodCalled("OutputByRoot", root)
+	return out[0].(eth.Output), *out[1].(*error)
+}
+
 func (m *MockL2Source) ExpectInfoAndTxsByHash(blockHash common.Hash, info eth.BlockInfo, txs types.Transactions, err error) {
 	m.Mock.On("InfoAndTxsByHash", blockHash).Once().Return(info, txs, &err)
 }
@@ -229,4 +258,8 @@ func (m *MockL2Source) ExpectCodeByHash(hash common.Hash, code []byte, err error
 	m.Mock.On("CodeByHash", hash).Once().Return(code, &err)
 }
 
+func (m *MockL2Source) ExpectOutputByRoot(root common.Hash, output eth.Output, err error) {
+	m.Mock.On("OutputByRoot", root).Once().Return(output, &err)
+}
+
 var _ L2Source = (*MockL2Source)(nil)

op-program/verify/cmd/goerli.go

@@ -100,7 +100,31 @@ func Run(l1RpcUrl string, l2RpcUrl string, l2OracleAddr common.Address) error {
 	if err != nil {
 		return fmt.Errorf("retrieve agreed l2 block: %w", err)
 	}
-	l2Head := l2AgreedBlock.Hash()
+	agreedOutputIndex, err := outputOracle.GetL2OutputIndexAfter(callOpts, l2AgreedBlock.Number())
+	if err != nil {
+		return fmt.Errorf("failed to output index after agreed block")
+	}
+	// Find an output that differs from what is being claimed
+	var agreedOutput bindings.TypesOutputProposal
+	for {
+		agreedOutput, err = outputOracle.GetL2Output(callOpts, agreedOutputIndex)
+		if err != nil {
+			return fmt.Errorf("retrieve agreed output: %w", err)
+		}
+		if agreedOutput.OutputRoot != output.OutputRoot {
+			break
+		}
+		fmt.Printf("Output at %v equals output at finalized block. Continuing search...\n", agreedOutput.L2BlockNumber)
+		agreedOutputIndex.Sub(agreedOutputIndex, big.NewInt(1))
+		if agreedOutputIndex.Int64() < 0 {
+			return fmt.Errorf("failed to find an output different from finalized block output")
+		}
+	}
+	l2BlockAtOutput, err := l2Client.BlockByNumber(ctx, agreedOutput.L2BlockNumber)
+	if err != nil {
+		return fmt.Errorf("retrieve agreed block: %w", err)
+	}
+	l2Head := l2BlockAtOutput.Hash()
 
 	temp, err := os.MkdirTemp("", "oracledata")
 	if err != nil {
@@ -120,6 +144,7 @@ func Run(l1RpcUrl string, l2RpcUrl string, l2OracleAddr common.Address) error {
 		"--datadir", temp,
 		"--l1.head", l1Head.Hex(),
 		"--l2.head", l2Head.Hex(),
+		"--l2.outputroot", common.Bytes2Hex(agreedOutput.OutputRoot[:]),
 		"--l2.claim", l2Claim.Hex(),
 		"--l2.blocknumber", l2BlockNumber.String(),
 	}