runtime.rs

// This file is part of Substrate.

// Copyright (C) Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// 	http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! Environment definition of the wasm smart-contract runtime.

use crate::{
	exec::{ExecError, ExecResult, Ext, Key, TopicOf},
	gas::{ChargedAmount, Token},
	primitives::ExecReturnValue,
	BalanceOf, CodeHash, Config, DebugBufferVec, Error, SENTINEL,
};
use codec::{Decode, DecodeLimit, Encode, MaxEncodedLen};
use frame_support::{
	dispatch::DispatchInfo,
	ensure,
	pallet_prelude::{DispatchResult, DispatchResultWithPostInfo},
	parameter_types,
	traits::Get,
	weights::Weight,
};
use pallet_contracts_proc_macro::define_env;
use pallet_contracts_uapi::{CallFlags, ReturnFlags};
use sp_io::hashing::{blake2_128, blake2_256, keccak_256, sha2_256};
use sp_runtime::{
	traits::{Bounded, Zero},
	DispatchError, RuntimeDebug,
};
use sp_std::{fmt, prelude::*};
use wasmi::{core::HostError, errors::LinkerError, Linker, Memory, Store};
use xcm::VersionedXcm;

type CallOf<T> = <T as frame_system::Config>::RuntimeCall;

/// The maximum nesting depth a contract can use when encoding types.
const MAX_DECODE_NESTING: u32 = 256;

/// Passed to [`Environment`] to determine whether it should expose deprecated interfaces.
pub enum AllowDeprecatedInterface {
	/// No deprecated interfaces are exposed.
	No,
	/// Deprecated interfaces are exposed.
	Yes,
}

/// Passed to [`Environment`] to determine whether it should expose unstable interfaces.
pub enum AllowUnstableInterface {
	/// No unstable interfaces are exposed.
	No,
	/// Unstable interfaces are exposed.
	Yes,
}

/// Trait implemented by the [`define_env`](pallet_contracts_proc_macro::define_env) macro for the
/// emitted `Env` struct.
pub trait Environment<HostState> {
	/// Adds all declared functions to the supplied [`Linker`](wasmi::Linker) and
	/// [`Store`](wasmi::Store).
	fn define(
		store: &mut Store<HostState>,
		linker: &mut Linker<HostState>,
		allow_unstable: AllowUnstableInterface,
		allow_deprecated: AllowDeprecatedInterface,
	) -> Result<(), LinkerError>;
}

/// Type of a storage key.
enum KeyType {
	/// Legacy fix sized key `[u8;32]`.
	Fix,
	/// Variable sized key used in transparent hashing,
	/// cannot be larger than MaxStorageKeyLen.
	Var(u32),
}

pub use pallet_contracts_uapi::ReturnErrorCode;

parameter_types! {
	/// Getter types used by [`crate::api_doc::Current::call_runtime`]
	const CallRuntimeFailed: ReturnErrorCode = ReturnErrorCode::CallRuntimeFailed;
	/// Getter types used by [`crate::api_doc::Current::xcm_execute`]
	const XcmExecutionFailed: ReturnErrorCode = ReturnErrorCode::XcmExecutionFailed;
}

impl From<ExecReturnValue> for ReturnErrorCode {
	fn from(from: ExecReturnValue) -> Self {
		if from.flags.contains(ReturnFlags::REVERT) {
			Self::CalleeReverted
		} else {
			Self::Success
		}
	}
}

/// The data passed through when a contract uses `seal_return`.
#[derive(RuntimeDebug)]
pub struct ReturnData {
	/// The flags as passed through by the contract. They are still unchecked and
	/// will later be parsed into a `ReturnFlags` bitflags struct.
	flags: u32,
	/// The output buffer passed by the contract as return data.
	data: Vec<u8>,
}

/// Enumerates all possible reasons why a trap was generated.
///
/// This is either used to supply the caller with more information about why an error
/// occurred (the SupervisorError variant).
/// The other case is where the trap does not constitute an error but rather was invoked
/// as a quick way to terminate the application (all other variants).
#[derive(RuntimeDebug)]
pub enum TrapReason {
	/// The supervisor trapped the contract because of an error condition occurred during
	/// execution in privileged code.
	SupervisorError(DispatchError),
	/// Signals that trap was generated in response to call `seal_return` host function.
	Return(ReturnData),
	/// Signals that a trap was generated in response to a successful call to the
	/// `seal_terminate` host function.
	Termination,
}

impl<T: Into<DispatchError>> From<T> for TrapReason {
	fn from(from: T) -> Self {
		Self::SupervisorError(from.into())
	}
}

impl fmt::Display for TrapReason {
	fn fmt(&self, _f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
		Ok(())
	}
}

impl HostError for TrapReason {}

#[cfg_attr(test, derive(Debug, PartialEq, Eq))]
#[derive(Copy, Clone)]
pub enum RuntimeCosts {
	/// Weight charged for copying data from the sandbox.
	CopyFromContract(u32),
	/// Weight charged for copying data to the sandbox.
	CopyToContract(u32),
	/// Weight of calling `seal_caller`.
	Caller,
	/// Weight of calling `seal_is_contract`.
	IsContract,
	/// Weight of calling `seal_code_hash`.
	CodeHash,
	/// Weight of calling `seal_own_code_hash`.
	OwnCodeHash,
	/// Weight of calling `seal_caller_is_origin`.
	CallerIsOrigin,
	/// Weight of calling `caller_is_root`.
	CallerIsRoot,
	/// Weight of calling `seal_address`.
	Address,
	/// Weight of calling `seal_gas_left`.
	GasLeft,
	/// Weight of calling `seal_balance`.
	Balance,
	/// Weight of calling `seal_value_transferred`.
	ValueTransferred,
	/// Weight of calling `seal_minimum_balance`.
	MinimumBalance,
	/// Weight of calling `seal_block_number`.
	BlockNumber,
	/// Weight of calling `seal_now`.
	Now,
	/// Weight of calling `seal_weight_to_fee`.
	WeightToFee,
	/// Weight of calling `seal_input` without the weight of copying the input.
	InputBase,
	/// Weight of calling `seal_return` for the given output size.
	Return(u32),
	/// Weight of calling `seal_terminate`.
	Terminate,
	/// Weight of calling `seal_random`. It includes the weight for copying the subject.
	Random,
	/// Weight of calling `seal_deposit_event` with the given number of topics and event size.
	DepositEvent { num_topic: u32, len: u32 },
	/// Weight of calling `seal_debug_message` per byte of passed message.
	DebugMessage(u32),
	/// Weight of calling `seal_set_storage` for the given storage item sizes.
	SetStorage { old_bytes: u32, new_bytes: u32 },
	/// Weight of calling `seal_clear_storage` per cleared byte.
	ClearStorage(u32),
	/// Weight of calling `seal_contains_storage` per byte of the checked item.
	ContainsStorage(u32),
	/// Weight of calling `seal_get_storage` with the specified size in storage.
	GetStorage(u32),
	/// Weight of calling `seal_take_storage` for the given size.
	TakeStorage(u32),
	/// Weight of calling `seal_transfer`.
	Transfer,
	/// Base weight of calling `seal_call`.
	CallBase,
	/// Weight of calling `seal_delegate_call` for the given input size.
	DelegateCallBase,
	/// Weight of the transfer performed during a call.
	CallSurchargeTransfer,
	/// Weight per byte that is cloned by supplying the `CLONE_INPUT` flag.
	CallInputCloned(u32),
	/// Weight of calling `seal_instantiate` for the given input length and salt.
	InstantiateBase { input_data_len: u32, salt_len: u32 },
	/// Weight of the transfer performed during an instantiate.
	InstantiateSurchargeTransfer,
	/// Weight of calling `seal_hash_sha_256` for the given input size.
	HashSha256(u32),
	/// Weight of calling `seal_hash_keccak_256` for the given input size.
	HashKeccak256(u32),
	/// Weight of calling `seal_hash_blake2_256` for the given input size.
	HashBlake256(u32),
	/// Weight of calling `seal_hash_blake2_128` for the given input size.
	HashBlake128(u32),
	/// Weight of calling `seal_ecdsa_recover`.
	EcdsaRecovery,
	/// Weight of calling `seal_sr25519_verify` for the given input size.
	Sr25519Verify(u32),
	/// Weight charged by a chain extension through `seal_call_chain_extension`.
	ChainExtension(Weight),
	/// Weight charged for calling into the runtime.
	CallRuntime(Weight),
	/// Weight charged for calling xcm_execute.
	CallXcmExecute(Weight),
	/// Weight of calling `seal_set_code_hash`
	SetCodeHash,
	/// Weight of calling `ecdsa_to_eth_address`
	EcdsaToEthAddress,
	/// Weight of calling `reentrance_count`
	ReentrantCount,
	/// Weight of calling `account_reentrance_count`
	AccountEntranceCount,
	/// Weight of calling `instantiation_nonce`
	InstantiationNonce,
	/// Weight of calling `lock_delegate_dependency`
	LockDelegateDependency,
	/// Weight of calling `unlock_delegate_dependency`
	UnlockDelegateDependency,
}

impl<T: Config> Token<T> for RuntimeCosts {
	fn influence_lowest_gas_limit(&self) -> bool {
		match self {
			&Self::CallXcmExecute(_) => false,
			_ => true,
		}
	}

	fn weight(&self) -> Weight {
		let s = T::Schedule::get().host_fn_weights;
		use self::RuntimeCosts::*;
		match *self {
			CopyFromContract(len) => s.return_per_byte.saturating_mul(len.into()),
			CopyToContract(len) => s.input_per_byte.saturating_mul(len.into()),
			Caller => s.caller,
			IsContract => s.is_contract,
			CodeHash => s.code_hash,
			OwnCodeHash => s.own_code_hash,
			CallerIsOrigin => s.caller_is_origin,
			CallerIsRoot => s.caller_is_root,
			Address => s.address,
			GasLeft => s.gas_left,
			Balance => s.balance,
			ValueTransferred => s.value_transferred,
			MinimumBalance => s.minimum_balance,
			BlockNumber => s.block_number,
			Now => s.now,
			WeightToFee => s.weight_to_fee,
			InputBase => s.input,
			Return(len) => s.r#return.saturating_add(s.return_per_byte.saturating_mul(len.into())),
			Terminate => s.terminate,
			Random => s.random,
			DepositEvent { num_topic, len } => s
				.deposit_event
				.saturating_add(s.deposit_event_per_topic.saturating_mul(num_topic.into()))
				.saturating_add(s.deposit_event_per_byte.saturating_mul(len.into())),
			DebugMessage(len) => s
				.debug_message
				.saturating_add(s.deposit_event_per_byte.saturating_mul(len.into())),
			SetStorage { new_bytes, old_bytes } => s
				.set_storage
				.saturating_add(s.set_storage_per_new_byte.saturating_mul(new_bytes.into()))
				.saturating_add(s.set_storage_per_old_byte.saturating_mul(old_bytes.into())),
			ClearStorage(len) => s
				.clear_storage
				.saturating_add(s.clear_storage_per_byte.saturating_mul(len.into())),
			ContainsStorage(len) => s
				.contains_storage
				.saturating_add(s.contains_storage_per_byte.saturating_mul(len.into())),
			GetStorage(len) =>
				s.get_storage.saturating_add(s.get_storage_per_byte.saturating_mul(len.into())),
			TakeStorage(len) => s
				.take_storage
				.saturating_add(s.take_storage_per_byte.saturating_mul(len.into())),
			Transfer => s.transfer,
			CallBase => s.call,
			DelegateCallBase => s.delegate_call,
			CallSurchargeTransfer => s.call_transfer_surcharge,
			CallInputCloned(len) => s.call_per_cloned_byte.saturating_mul(len.into()),
			InstantiateBase { input_data_len, salt_len } => s
				.instantiate
				.saturating_add(s.instantiate_per_input_byte.saturating_mul(input_data_len.into()))
				.saturating_add(s.instantiate_per_salt_byte.saturating_mul(salt_len.into())),
			InstantiateSurchargeTransfer => s.instantiate_transfer_surcharge,
			HashSha256(len) => s
				.hash_sha2_256
				.saturating_add(s.hash_sha2_256_per_byte.saturating_mul(len.into())),
			HashKeccak256(len) => s
				.hash_keccak_256
				.saturating_add(s.hash_keccak_256_per_byte.saturating_mul(len.into())),
			HashBlake256(len) => s
				.hash_blake2_256
				.saturating_add(s.hash_blake2_256_per_byte.saturating_mul(len.into())),
			HashBlake128(len) => s
				.hash_blake2_128
				.saturating_add(s.hash_blake2_128_per_byte.saturating_mul(len.into())),
			EcdsaRecovery => s.ecdsa_recover,
			Sr25519Verify(len) => s
				.sr25519_verify
				.saturating_add(s.sr25519_verify_per_byte.saturating_mul(len.into())),
			ChainExtension(weight) | CallRuntime(weight) | CallXcmExecute(weight) => weight,
			SetCodeHash => s.set_code_hash,
			EcdsaToEthAddress => s.ecdsa_to_eth_address,
			ReentrantCount => s.reentrance_count,
			AccountEntranceCount => s.account_reentrance_count,
			InstantiationNonce => s.instantiation_nonce,
			LockDelegateDependency => s.lock_delegate_dependency,
			UnlockDelegateDependency => s.unlock_delegate_dependency,
		}
	}
}

/// Same as [`Runtime::charge_gas`].
///
/// We need this access as a macro because sometimes hiding the lifetimes behind
/// a function won't work out.
macro_rules! charge_gas {
	($runtime:expr, $costs:expr) => {{
		$runtime.ext.gas_meter_mut().charge($costs)
	}};
}

/// The kind of call that should be performed.
enum CallType {
	/// Execute another instantiated contract
	Call { callee_ptr: u32, value_ptr: u32, deposit_ptr: u32, weight: Weight },
	/// Execute deployed code in the context (storage, account ID, value) of the caller contract
	DelegateCall { code_hash_ptr: u32 },
}

impl CallType {
	fn cost(&self) -> RuntimeCosts {
		match self {
			CallType::Call { .. } => RuntimeCosts::CallBase,
			CallType::DelegateCall { .. } => RuntimeCosts::DelegateCallBase,
		}
	}
}

/// This is only appropriate when writing out data of constant size that does not depend on user
/// input. In this case the costs for this copy was already charged as part of the token at
/// the beginning of the API entry point.
fn already_charged(_: u32) -> Option<RuntimeCosts> {
	None
}

/// Ensure that the XCM program is executable, by checking that it does not contain any [`Transact`]
/// instruction with a call that is not allowed by the CallFilter.
fn ensure_executable<T: Config>(message: &VersionedXcm<CallOf<T>>) -> DispatchResult {
	use frame_support::traits::Contains;
	use xcm::prelude::{Transact, Xcm};

	let mut message: Xcm<CallOf<T>> =
		message.clone().try_into().map_err(|_| Error::<T>::XCMDecodeFailed)?;

	message.iter_mut().try_for_each(|inst| -> DispatchResult {
		let Transact { ref mut call, .. } = inst else { return Ok(()) };
		let call = call.ensure_decoded().map_err(|_| Error::<T>::XCMDecodeFailed)?;

		if !<T as Config>::CallFilter::contains(call) {
			return Err(frame_system::Error::<T>::CallFiltered.into())
		}

		Ok(())
	})?;

	Ok(())
}

/// Can only be used for one call.
pub struct Runtime<'a, E: Ext + 'a> {
	ext: &'a mut E,
	input_data: Option<Vec<u8>>,
	memory: Option<Memory>,
	chain_extension: Option<Box<<E::T as Config>::ChainExtension>>,
}

impl<'a, E: Ext + 'a> Runtime<'a, E> {
	pub fn new(ext: &'a mut E, input_data: Vec<u8>) -> Self {
		Runtime {
			ext,
			input_data: Some(input_data),
			memory: None,
			chain_extension: Some(Box::new(Default::default())),
		}
	}

	pub fn memory(&self) -> Option<Memory> {
		self.memory
	}

	pub fn set_memory(&mut self, memory: Memory) {
		self.memory = Some(memory);
	}

	/// Converts the sandbox result and the runtime state into the execution outcome.
	pub fn to_execution_result(self, sandbox_result: Result<(), wasmi::Error>) -> ExecResult {
		use wasmi::core::TrapCode::OutOfFuel;
		use TrapReason::*;

		match sandbox_result {
			// Contract returned from main function -> no data was returned.
			Ok(_) => Ok(ExecReturnValue { flags: ReturnFlags::empty(), data: Vec::new() }),
			// `OutOfGas` when host asks engine to consume more than left in the _store_.
			// We should never get this case, as gas meter is being charged (and hence raises error)
			// first.
			Err(wasmi::Error::Store(_)) => Err(Error::<E::T>::OutOfGas.into()),
			// Contract either trapped or some host function aborted the execution.
			Err(wasmi::Error::Trap(trap)) => {
				if let Some(OutOfFuel) = trap.trap_code() {
					// `OutOfGas` during engine execution.
					return Err(Error::<E::T>::OutOfGas.into())
				}
				// If we encoded a reason then it is some abort generated by a host function.
				if let Some(reason) = &trap.downcast_ref::<TrapReason>() {
					match &reason {
						Return(ReturnData { flags, data }) => {
							let flags = ReturnFlags::from_bits(*flags)
								.ok_or(Error::<E::T>::InvalidCallFlags)?;
							return Ok(ExecReturnValue { flags, data: data.to_vec() })
						},
						Termination =>
							return Ok(ExecReturnValue {
								flags: ReturnFlags::empty(),
								data: Vec::new(),
							}),
						SupervisorError(error) => return Err((*error).into()),
					}
				}
				// Otherwise the trap came from the contract itself.
				Err(Error::<E::T>::ContractTrapped.into())
			},
			// Any other error is returned only if instantiation or linking failed (i.e.
			// wasm binary tried to import a function that is not provided by the host).
			// This shouldn't happen because validation process ought to reject such binaries.
			//
			// Because panics are really undesirable in the runtime code, we treat this as
			// a trap for now. Eventually, we might want to revisit this.
			Err(_) => Err(Error::<E::T>::CodeRejected.into()),
		}
	}

	/// Get a mutable reference to the inner `Ext`.
	///
	/// This is mainly for the chain extension to have access to the environment the
	/// contract is executing in.
	pub fn ext(&mut self) -> &mut E {
		self.ext
	}

	/// Charge the gas meter with the specified token.
	///
	/// Returns `Err(HostError)` if there is not enough gas.
	pub fn charge_gas(&mut self, costs: RuntimeCosts) -> Result<ChargedAmount, DispatchError> {
		charge_gas!(self, costs)
	}

	/// Adjust a previously charged amount down to its actual amount.
	///
	/// This is when a maximum a priori amount was charged and then should be partially
	/// refunded to match the actual amount.
	pub fn adjust_gas(&mut self, charged: ChargedAmount, actual_costs: RuntimeCosts) {
		self.ext.gas_meter_mut().adjust_gas(charged, actual_costs);
	}

	/// Charge, Run and adjust gas, for executing the given dispatchable.
	fn call_dispatchable<ErrorReturnCode: Get<ReturnErrorCode>>(
		&mut self,
		dispatch_info: DispatchInfo,
		runtime_cost: impl Fn(Weight) -> RuntimeCosts,
		run: impl FnOnce(&mut Self) -> DispatchResultWithPostInfo,
	) -> Result<ReturnErrorCode, TrapReason> {
		use frame_support::dispatch::extract_actual_weight;
		let charged = self.charge_gas(runtime_cost(dispatch_info.weight))?;
		let result = run(self);
		let actual_weight = extract_actual_weight(&result, &dispatch_info);
		self.adjust_gas(charged, runtime_cost(actual_weight));
		match result {
			Ok(_) => Ok(ReturnErrorCode::Success),
			Err(e) => {
				if self.ext.debug_buffer_enabled() {
					self.ext.append_debug_buffer("call failed with: ");
					self.ext.append_debug_buffer(e.into());
				};
				Ok(ErrorReturnCode::get())
			},
		}
	}

	/// Read designated chunk from the sandbox memory.
	///
	/// Returns `Err` if one of the following conditions occurs:
	///
	/// - requested buffer is not within the bounds of the sandbox memory.
	pub fn read_sandbox_memory(
		&self,
		memory: &[u8],
		ptr: u32,
		len: u32,
	) -> Result<Vec<u8>, DispatchError> {
		ensure!(len <= self.ext.schedule().limits.max_memory_size(), Error::<E::T>::OutOfBounds);
		let mut buf = vec![0u8; len as usize];
		self.read_sandbox_memory_into_buf(memory, ptr, buf.as_mut_slice())?;
		Ok(buf)
	}

	/// Read designated chunk from the sandbox memory into the supplied buffer.
	///
	/// Returns `Err` if one of the following conditions occurs:
	///
	/// - requested buffer is not within the bounds of the sandbox memory.
	pub fn read_sandbox_memory_into_buf(
		&self,
		memory: &[u8],
		ptr: u32,
		buf: &mut [u8],
	) -> Result<(), DispatchError> {
		let ptr = ptr as usize;
		let bound_checked =
			memory.get(ptr..ptr + buf.len()).ok_or_else(|| Error::<E::T>::OutOfBounds)?;
		buf.copy_from_slice(bound_checked);
		Ok(())
	}

	/// Reads and decodes a type with a size fixed at compile time from contract memory.
	///
	/// # Note
	///
	/// The weight of reading a fixed value is included in the overall weight of any
	/// contract callable function.
	pub fn read_sandbox_memory_as<D: Decode + MaxEncodedLen>(
		&self,
		memory: &[u8],
		ptr: u32,
	) -> Result<D, DispatchError> {
		let ptr = ptr as usize;
		let mut bound_checked = memory.get(ptr..).ok_or_else(|| Error::<E::T>::OutOfBounds)?;

		let decoded = D::decode_with_depth_limit(MAX_DECODE_NESTING, &mut bound_checked)
			.map_err(|_| DispatchError::from(Error::<E::T>::DecodingFailed))?;
		Ok(decoded)
	}

	/// Read designated chunk from the sandbox memory and attempt to decode into the specified type.
	///
	/// Returns `Err` if one of the following conditions occurs:
	///
	/// - requested buffer is not within the bounds of the sandbox memory.
	/// - the buffer contents cannot be decoded as the required type.
	///
	/// # Note
	///
	/// There must be an extra benchmark for determining the influence of `len` with
	/// regard to the overall weight.
	pub fn read_sandbox_memory_as_unbounded<D: Decode>(
		&self,
		memory: &[u8],
		ptr: u32,
		len: u32,
	) -> Result<D, DispatchError> {
		let ptr = ptr as usize;
		let mut bound_checked =
			memory.get(ptr..ptr + len as usize).ok_or_else(|| Error::<E::T>::OutOfBounds)?;

		let decoded = D::decode_all_with_depth_limit(MAX_DECODE_NESTING, &mut bound_checked)
			.map_err(|_| DispatchError::from(Error::<E::T>::DecodingFailed))?;

		Ok(decoded)
	}

	/// Write the given buffer and its length to the designated locations in sandbox memory and
	/// charge gas according to the token returned by `create_token`.
	//
	/// `out_ptr` is the location in sandbox memory where `buf` should be written to.
	/// `out_len_ptr` is an in-out location in sandbox memory. It is read to determine the
	/// length of the buffer located at `out_ptr`. If that buffer is large enough the actual
	/// `buf.len()` is written to this location.
	///
	/// If `out_ptr` is set to the sentinel value of `SENTINEL` and `allow_skip` is true the
	/// operation is skipped and `Ok` is returned. This is supposed to help callers to make copying
	/// output optional. For example to skip copying back the output buffer of an `seal_call`
	/// when the caller is not interested in the result.
	///
	/// `create_token` can optionally instruct this function to charge the gas meter with the token
	/// it returns. `create_token` receives the variable amount of bytes that are about to be copied
	/// by this function.
	///
	/// In addition to the error conditions of `write_sandbox_memory` this functions returns
	/// `Err` if the size of the buffer located at `out_ptr` is too small to fit `buf`.
	pub fn write_sandbox_output(
		&mut self,
		memory: &mut [u8],
		out_ptr: u32,
		out_len_ptr: u32,
		buf: &[u8],
		allow_skip: bool,
		create_token: impl FnOnce(u32) -> Option<RuntimeCosts>,
	) -> Result<(), DispatchError> {
		if allow_skip && out_ptr == SENTINEL {
			return Ok(())
		}

		let buf_len = buf.len() as u32;
		let len: u32 = self.read_sandbox_memory_as(memory, out_len_ptr)?;

		if len < buf_len {
			return Err(Error::<E::T>::OutputBufferTooSmall.into())
		}

		if let Some(costs) = create_token(buf_len) {
			self.charge_gas(costs)?;
		}

		self.write_sandbox_memory(memory, out_ptr, buf)?;
		self.write_sandbox_memory(memory, out_len_ptr, &buf_len.encode())
	}

	/// Write the given buffer to the designated location in the sandbox memory.
	///
	/// Returns `Err` if one of the following conditions occurs:
	///
	/// - designated area is not within the bounds of the sandbox memory.
	fn write_sandbox_memory(
		&self,
		memory: &mut [u8],
		ptr: u32,
		buf: &[u8],
	) -> Result<(), DispatchError> {
		let ptr = ptr as usize;
		let bound_checked =
			memory.get_mut(ptr..ptr + buf.len()).ok_or_else(|| Error::<E::T>::OutOfBounds)?;
		bound_checked.copy_from_slice(buf);
		Ok(())
	}

	/// Computes the given hash function on the supplied input.
	///
	/// Reads from the sandboxed input buffer into an intermediate buffer.
	/// Returns the result directly to the output buffer of the sandboxed memory.
	///
	/// It is the callers responsibility to provide an output buffer that
	/// is large enough to hold the expected amount of bytes returned by the
	/// chosen hash function.
	///
	/// # Note
	///
	/// The `input` and `output` buffers may overlap.
	fn compute_hash_on_intermediate_buffer<F, R>(
		&self,
		memory: &mut [u8],
		hash_fn: F,
		input_ptr: u32,
		input_len: u32,
		output_ptr: u32,
	) -> Result<(), DispatchError>
	where
		F: FnOnce(&[u8]) -> R,
		R: AsRef<[u8]>,
	{
		// Copy input into supervisor memory.
		let input = self.read_sandbox_memory(memory, input_ptr, input_len)?;
		// Compute the hash on the input buffer using the given hash function.
		let hash = hash_fn(&input);
		// Write the resulting hash back into the sandboxed output buffer.
		self.write_sandbox_memory(memory, output_ptr, hash.as_ref())?;
		Ok(())
	}

	/// Fallible conversion of `DispatchError` to `ReturnErrorCode`.
	fn err_into_return_code(from: DispatchError) -> Result<ReturnErrorCode, DispatchError> {
		use ReturnErrorCode::*;

		let transfer_failed = Error::<E::T>::TransferFailed.into();
		let no_code = Error::<E::T>::CodeNotFound.into();
		let not_found = Error::<E::T>::ContractNotFound.into();

		match from {
			x if x == transfer_failed => Ok(TransferFailed),
			x if x == no_code => Ok(CodeNotFound),
			x if x == not_found => Ok(NotCallable),
			err => Err(err),
		}
	}

	/// Fallible conversion of a `ExecResult` to `ReturnErrorCode`.
	fn exec_into_return_code(from: ExecResult) -> Result<ReturnErrorCode, DispatchError> {
		use crate::exec::ErrorOrigin::Callee;

		let ExecError { error, origin } = match from {
			Ok(retval) => return Ok(retval.into()),
			Err(err) => err,
		};

		match (error, origin) {
			(_, Callee) => Ok(ReturnErrorCode::CalleeTrapped),
			(err, _) => Self::err_into_return_code(err),
		}
	}
	fn decode_key(
		&self,
		memory: &[u8],
		key_type: KeyType,
		key_ptr: u32,
	) -> Result<crate::exec::Key<E::T>, TrapReason> {
		let res = match key_type {
			KeyType::Fix => {
				let key = self.read_sandbox_memory(memory, key_ptr, 32u32)?;
				Key::try_from_fix(key)
			},
			KeyType::Var(len) => {
				ensure!(
					len <= <<E as Ext>::T as Config>::MaxStorageKeyLen::get(),
					Error::<E::T>::DecodingFailed
				);
				let key = self.read_sandbox_memory(memory, key_ptr, len)?;
				Key::try_from_var(key)
			},
		};

		res.map_err(|_| Error::<E::T>::DecodingFailed.into())
	}

	fn set_storage(
		&mut self,
		memory: &[u8],
		key_type: KeyType,
		key_ptr: u32,
		value_ptr: u32,
		value_len: u32,
	) -> Result<u32, TrapReason> {
		let max_size = self.ext.max_value_size();
		let charged = self
			.charge_gas(RuntimeCosts::SetStorage { new_bytes: value_len, old_bytes: max_size })?;
		if value_len > max_size {
			return Err(Error::<E::T>::ValueTooLarge.into())
		}
		let key = self.decode_key(memory, key_type, key_ptr)?;
		let value = Some(self.read_sandbox_memory(memory, value_ptr, value_len)?);
		let write_outcome = self.ext.set_storage(&key, value, false)?;

		self.adjust_gas(
			charged,
			RuntimeCosts::SetStorage { new_bytes: value_len, old_bytes: write_outcome.old_len() },
		);
		Ok(write_outcome.old_len_with_sentinel())
	}

	fn clear_storage(
		&mut self,
		memory: &[u8],
		key_type: KeyType,
		key_ptr: u32,
	) -> Result<u32, TrapReason> {
		let charged = self.charge_gas(RuntimeCosts::ClearStorage(self.ext.max_value_size()))?;
		let key = self.decode_key(memory, key_type, key_ptr)?;
		let outcome = self.ext.set_storage(&key, None, false)?;

		self.adjust_gas(charged, RuntimeCosts::ClearStorage(outcome.old_len()));
		Ok(outcome.old_len_with_sentinel())
	}

	fn get_storage(
		&mut self,
		memory: &mut [u8],
		key_type: KeyType,
		key_ptr: u32,
		out_ptr: u32,
		out_len_ptr: u32,
	) -> Result<ReturnErrorCode, TrapReason> {
		let charged = self.charge_gas(RuntimeCosts::GetStorage(self.ext.max_value_size()))?;
		let key = self.decode_key(memory, key_type, key_ptr)?;
		let outcome = self.ext.get_storage(&key);

		if let Some(value) = outcome {
			self.adjust_gas(charged, RuntimeCosts::GetStorage(value.len() as u32));
			self.write_sandbox_output(
				memory,
				out_ptr,
				out_len_ptr,
				&value,
				false,
				already_charged,
			)?;
			Ok(ReturnErrorCode::Success)
		} else {
			self.adjust_gas(charged, RuntimeCosts::GetStorage(0));
			Ok(ReturnErrorCode::KeyNotFound)
		}
	}

	fn contains_storage(
		&mut self,
		memory: &[u8],
		key_type: KeyType,
		key_ptr: u32,
	) -> Result<u32, TrapReason> {
		let charged = self.charge_gas(RuntimeCosts::ContainsStorage(self.ext.max_value_size()))?;
		let key = self.decode_key(memory, key_type, key_ptr)?;
		let outcome = self.ext.get_storage_size(&key);

		self.adjust_gas(charged, RuntimeCosts::ClearStorage(outcome.unwrap_or(0)));
		Ok(outcome.unwrap_or(SENTINEL))
	}

	fn call(
		&mut self,
		memory: &mut [u8],
		flags: CallFlags,
		call_type: CallType,
		input_data_ptr: u32,
		input_data_len: u32,
		output_ptr: u32,
		output_len_ptr: u32,
	) -> Result<ReturnErrorCode, TrapReason> {
		self.charge_gas(call_type.cost())?;
		let input_data = if flags.contains(CallFlags::CLONE_INPUT) {
			let input = self.input_data.as_ref().ok_or(Error::<E::T>::InputForwarded)?;
			charge_gas!(self, RuntimeCosts::CallInputCloned(input.len() as u32))?;
			input.clone()
		} else if flags.contains(CallFlags::FORWARD_INPUT) {
			self.input_data.take().ok_or(Error::<E::T>::InputForwarded)?
		} else {
			self.charge_gas(RuntimeCosts::CopyFromContract(input_data_len))?;
			self.read_sandbox_memory(memory, input_data_ptr, input_data_len)?
		};

		let call_outcome = match call_type {
			CallType::Call { callee_ptr, value_ptr, deposit_ptr, weight } => {
				let callee: <<E as Ext>::T as frame_system::Config>::AccountId =
					self.read_sandbox_memory_as(memory, callee_ptr)?;
				let deposit_limit: BalanceOf<<E as Ext>::T> = if deposit_ptr == SENTINEL {
					BalanceOf::<<E as Ext>::T>::zero()
				} else {
					self.read_sandbox_memory_as(memory, deposit_ptr)?
				};
				let value: BalanceOf<<E as Ext>::T> =
					self.read_sandbox_memory_as(memory, value_ptr)?;
				if value > 0u32.into() {
					self.charge_gas(RuntimeCosts::CallSurchargeTransfer)?;
				}
				self.ext.call(
					weight,
					deposit_limit,
					callee,
					value,
					input_data,
					flags.contains(CallFlags::ALLOW_REENTRY),
				)
			},
			CallType::DelegateCall { code_hash_ptr } => {
				if flags.contains(CallFlags::ALLOW_REENTRY) {
					return Err(Error::<E::T>::InvalidCallFlags.into())
				}
				let code_hash = self.read_sandbox_memory_as(memory, code_hash_ptr)?;
				self.ext.delegate_call(code_hash, input_data)
			},
		};

		// `TAIL_CALL` only matters on an `OK` result. Otherwise the call stack comes to
		// a halt anyways without anymore code being executed.
		if flags.contains(CallFlags::TAIL_CALL) {
			if let Ok(return_value) = call_outcome {
				return Err(TrapReason::Return(ReturnData {
					flags: return_value.flags.bits(),
					data: return_value.data,
				}))
			}
		}

		if let Ok(output) = &call_outcome {
			self.write_sandbox_output(
				memory,
				output_ptr,
				output_len_ptr,
				&output.data,
				true,
				|len| Some(RuntimeCosts::CopyToContract(len)),
			)?;
		}
		Ok(Runtime::<E>::exec_into_return_code(call_outcome)?)
	}

	fn instantiate(
		&mut self,
		memory: &mut [u8],
		code_hash_ptr: u32,
		weight: Weight,
		deposit_ptr: u32,
		value_ptr: u32,
		input_data_ptr: u32,
		input_data_len: u32,
		address_ptr: u32,
		address_len_ptr: u32,
		output_ptr: u32,
		output_len_ptr: u32,
		salt_ptr: u32,
		salt_len: u32,
	) -> Result<ReturnErrorCode, TrapReason> {
		self.charge_gas(RuntimeCosts::InstantiateBase { input_data_len, salt_len })?;
		let deposit_limit: BalanceOf<<E as Ext>::T> = if deposit_ptr == SENTINEL {
			BalanceOf::<<E as Ext>::T>::zero()
		} else {
			self.read_sandbox_memory_as(memory, deposit_ptr)?
		};
		let value: BalanceOf<<E as Ext>::T> = self.read_sandbox_memory_as(memory, value_ptr)?;
		if value > 0u32.into() {
			self.charge_gas(RuntimeCosts::InstantiateSurchargeTransfer)?;
		}
		let code_hash: CodeHash<<E as Ext>::T> =
			self.read_sandbox_memory_as(memory, code_hash_ptr)?;
		let input_data = self.read_sandbox_memory(memory, input_data_ptr, input_data_len)?;
		let salt = self.read_sandbox_memory(memory, salt_ptr, salt_len)?;
		let instantiate_outcome =
			self.ext.instantiate(weight, deposit_limit, code_hash, value, input_data, &salt);
		if let Ok((address, output)) = &instantiate_outcome {
			if !output.flags.contains(ReturnFlags::REVERT) {
				self.write_sandbox_output(
					memory,
					address_ptr,
					address_len_ptr,
					&address.encode(),
					true,
					already_charged,
				)?;
			}
			self.write_sandbox_output(
				memory,
				output_ptr,
				output_len_ptr,
				&output.data,
				true,
				|len| Some(RuntimeCosts::CopyToContract(len)),
			)?;
		}
		Ok(Runtime::<E>::exec_into_return_code(instantiate_outcome.map(|(_, retval)| retval))?)
	}

	fn terminate(&mut self, memory: &[u8], beneficiary_ptr: u32) -> Result<(), TrapReason> {
		self.charge_gas(RuntimeCosts::Terminate)?;
		let beneficiary: <<E as Ext>::T as frame_system::Config>::AccountId =
			self.read_sandbox_memory_as(memory, beneficiary_ptr)?;
		self.ext.terminate(&beneficiary)?;
		Err(TrapReason::Termination)
	}
}

// This is the API exposed to contracts.
//
// # Note
//
// Any input that leads to a out of bound error (reading or writing) or failing to decode
// data passed to the supervisor will lead to a trap. This is not documented explicitly
// for every function.
#[define_env(doc)]
pub mod env {
	/// Set the value at the given key in the contract storage.
	/// See [`pallet_contracts_uapi::HostFn::set_storage`]
	#[prefixed_alias]
	fn set_storage(
		ctx: _,
		memory: _,
		key_ptr: u32,
		value_ptr: u32,
		value_len: u32,
	) -> Result<(), TrapReason> {
		ctx.set_storage(memory, KeyType::Fix, key_ptr, value_ptr, value_len).map(|_| ())