fix: eth: correctly decode EthGetStorageAt output (#10284)

* fix: eth: correctly decode EthGetStorageAt output

We cbor-encode it. Also:

1. Actually use the passed block param.
2. Check if the target actor is an EVM actor to avoid nonsense outputs.

fixes https://github.com/filecoin-project/ref-fvm/issues/1621

node/impl/full/eth.go

@@ -510,13 +510,18 @@ func (a *EthModule) EthGetCode(ctx context.Context, ethAddr ethtypes.EthAddress,
 }
 
 func (a *EthModule) EthGetStorageAt(ctx context.Context, ethAddr ethtypes.EthAddress, position ethtypes.EthBytes, blkParam string) (ethtypes.EthBytes, error) {
+	ts, err := a.parseBlkParam(ctx, blkParam)
+	if err != nil {
+		return nil, xerrors.Errorf("cannot parse block param: %s", blkParam)
+	}
+
 	l := len(position)
 	if l > 32 {
 		return nil, fmt.Errorf("supplied storage key is too long")
 	}
 
 	// pad with zero bytes if smaller than 32 bytes
-	position = append(make([]byte, 32-l, 32-l), position...)
+	position = append(make([]byte, 32-l, 32), position...)
 
 	to, err := ethAddr.ToFilecoinAddress()
 	if err != nil {
@@ -529,6 +534,18 @@ func (a *EthModule) EthGetStorageAt(ctx context.Context, ethAddr ethtypes.EthAdd
 		return nil, fmt.Errorf("failed to construct system sender address: %w", err)
 	}
 
+	actor, err := a.StateManager.LoadActor(ctx, to, ts)
+	if err != nil {
+		if xerrors.Is(err, types.ErrActorNotFound) {
+			return ethtypes.EthBytes(make([]byte, 32)), nil
+		}
+		return nil, xerrors.Errorf("failed to lookup contract %s: %w", ethAddr, err)
+	}
+
+	if !builtinactors.IsEvmActor(actor.Code) {
+		return ethtypes.EthBytes(make([]byte, 32)), nil
+	}
+
 	params, err := actors.SerializeParams(&evm.GetStorageAtParams{
 		StorageKey: *(*[32]byte)(position),
 	})
@@ -547,8 +564,6 @@ func (a *EthModule) EthGetStorageAt(ctx context.Context, ethAddr ethtypes.EthAdd
 		GasPremium: big.Zero(),
 	}
 
-	ts := a.Chain.GetHeaviestTipSet()
-
 	// Try calling until we find a height with no migration.
 	var res *api.InvocResult
 	for {
@@ -567,10 +582,22 @@ func (a *EthModule) EthGetStorageAt(ctx context.Context, ethAddr ethtypes.EthAdd
 	}
 
 	if res.MsgRct == nil {
-		return nil, fmt.Errorf("no message receipt")
+		return nil, xerrors.Errorf("no message receipt")
 	}
 
-	return res.MsgRct.Return, nil
+	if res.MsgRct.ExitCode.IsError() {
+		return nil, xerrors.Errorf("failed to lookup storage slot: %s", res.Error)
+	}
+
+	var ret abi.CborBytes
+	if err := ret.UnmarshalCBOR(bytes.NewReader(res.MsgRct.Return)); err != nil {
+		return nil, xerrors.Errorf("failed to unmarshal storage slot: %w", err)
+	}
+
+	// pad with zero bytes if smaller than 32 bytes
+	ret = append(make([]byte, 32-len(ret), 32), ret...)
+
+	return ethtypes.EthBytes(ret), nil
 }
 
 func (a *EthModule) EthGetBalance(ctx context.Context, address ethtypes.EthAddress, blkParam string) (ethtypes.EthBigInt, error) {