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//! Gas metering instrumentation.
use std::collections::BTreeMap;
use walrus::{ir::*, FunctionBuilder, GlobalId, LocalFunction, Module};
use crate::Error;
/// Name of the exported global that holds the gas limit.
pub const EXPORT_GAS_LIMIT: &str = "gas_limit";
/// Name of the exported global that holds the gas limit exhausted flag.
pub const EXPORT_GAS_LIMIT_EXHAUSTED: &str = "gas_limit_exhausted";
/// Configures the gas limit on the given instance.
pub fn set_gas_limit<C>(
instance: &wasm3::Instance<'_, '_, C>,
gas_limit: u64,
) -> Result<(), Error> {
instance
.set_global(EXPORT_GAS_LIMIT, gas_limit)
.map_err(|err| Error::ExecutionFailed(err.into()))
}
/// Returns the remaining gas.
pub fn get_remaining_gas<C>(instance: &wasm3::Instance<'_, '_, C>) -> u64 {
instance.get_global(EXPORT_GAS_LIMIT).unwrap_or_default()
}
/// Returns the amount of gas requested that was over the limit.
pub fn get_exhausted_amount<C>(instance: &wasm3::Instance<'_, '_, C>) -> u64 {
instance
.get_global(EXPORT_GAS_LIMIT_EXHAUSTED)
.unwrap_or_default()
}
/// Attempts to use the given amount of gas.
pub fn use_gas<C>(instance: &wasm3::Instance<'_, '_, C>, amount: u64) -> Result<(), wasm3::Trap> {
let gas_limit: u64 = instance
.get_global(EXPORT_GAS_LIMIT)
.map_err(|_| wasm3::Trap::Abort)?;
if gas_limit < amount {
let _ = instance.set_global(EXPORT_GAS_LIMIT_EXHAUSTED, amount);
return Err(wasm3::Trap::Abort);
}
instance
.set_global(EXPORT_GAS_LIMIT, gas_limit - amount)
.map_err(|_| wasm3::Trap::Abort)?;
Ok(())
}
/// Inject gas metering instrumentation into the module.
pub fn transform(module: &mut Module) {
let gas_limit_global = module.globals.add_local(
walrus::ValType::I64,
true,
walrus::InitExpr::Value(Value::I64(0)),
);
let gas_limit_exhausted_global = module.globals.add_local(
walrus::ValType::I64,
true,
walrus::InitExpr::Value(Value::I64(0)),
);
module.exports.add(EXPORT_GAS_LIMIT, gas_limit_global);
module
.exports
.add(EXPORT_GAS_LIMIT_EXHAUSTED, gas_limit_exhausted_global);
for (_, func) in module.funcs.iter_local_mut() {
transform_function(func, gas_limit_global, gas_limit_exhausted_global);
}
}
/// Instruction cost function.
fn instruction_cost(instr: &Instr) -> u64 {
match instr {
Instr::Loop(_) | Instr::Block(_) => 2,
Instr::LocalGet(_)
| Instr::LocalSet(_)
| Instr::LocalTee(_)
| Instr::GlobalGet(_)
| Instr::GlobalSet(_)
| Instr::Const(_) => 1,
Instr::Call(_) => 15,
Instr::CallIndirect(_) => 20,
Instr::Br(_) | Instr::BrIf(_) => 3,
Instr::BrTable(_) => 4,
Instr::Binop(op) => match op.op {
BinaryOp::I32Eq
| BinaryOp::I32Ne
| BinaryOp::I32LtS
| BinaryOp::I32LtU
| BinaryOp::I32GtS
| BinaryOp::I32GtU
| BinaryOp::I32LeS
| BinaryOp::I32LeU
| BinaryOp::I32GeS
| BinaryOp::I32GeU
| BinaryOp::I64Eq
| BinaryOp::I64Ne
| BinaryOp::I64LtS
| BinaryOp::I64LtU
| BinaryOp::I64GtS
| BinaryOp::I64GtU
| BinaryOp::I64LeS
| BinaryOp::I64LeU
| BinaryOp::I64GeS
| BinaryOp::I64GeU
| BinaryOp::I32Add
| BinaryOp::I32Sub
| BinaryOp::I32Mul
| BinaryOp::I32And
| BinaryOp::I32Or
| BinaryOp::I32Xor
| BinaryOp::I32Shl
| BinaryOp::I32ShrS
| BinaryOp::I32ShrU
| BinaryOp::I32Rotl
| BinaryOp::I32Rotr
| BinaryOp::I64Add
| BinaryOp::I64Sub
| BinaryOp::I64Mul
| BinaryOp::I64And
| BinaryOp::I64Or
| BinaryOp::I64Xor
| BinaryOp::I64Shl
| BinaryOp::I64ShrS
| BinaryOp::I64ShrU
| BinaryOp::I64Rotl
| BinaryOp::I64Rotr => 1,
BinaryOp::I32DivS
| BinaryOp::I32DivU
| BinaryOp::I32RemS
| BinaryOp::I32RemU
| BinaryOp::I64DivS
| BinaryOp::I64DivU
| BinaryOp::I64RemS
| BinaryOp::I64RemU => 4,
_ => 3,
},
Instr::Unop(op) => match op.op {
UnaryOp::I32Eqz
| UnaryOp::I32Clz
| UnaryOp::I32Ctz
| UnaryOp::I32Popcnt
| UnaryOp::I64Eqz
| UnaryOp::I64Clz
| UnaryOp::I64Ctz
| UnaryOp::I64Popcnt => 1,
_ => 3,
},
_ => 10,
}
}
/// A block of instructions which is metered.
#[derive(Debug)]
struct MeteredBlock {
/// Instruction sequence where metering code should be injected.
seq_id: InstrSeqId,
/// Start index of instruction within the instruction sequence before which the metering code
/// should be injected.
start_index: usize,
/// Instruction cost.
cost: u64,
/// Indication of whether the metered block can be merged in case instruction sequence and start
/// index match. In case the block cannot be merged this contains the index
merge_index: Option<usize>,
}
impl MeteredBlock {
fn new(seq_id: InstrSeqId, start_index: usize) -> Self {
Self {
seq_id,
start_index,
cost: 0,
merge_index: None,
}
}
/// Create a mergable version of this metered block with the given start index.
fn mergable(&self, start_index: usize) -> Self {
Self {
seq_id: self.seq_id,
start_index,
cost: 0,
merge_index: Some(self.start_index),
}
}
}
/// A map of finalized metered blocks.
#[derive(Default)]
struct MeteredBlocks {
blocks: BTreeMap<InstrSeqId, Vec<MeteredBlock>>,
}
impl MeteredBlocks {
/// Finalize the given metered block. This means that the cost associated with the block cannot
/// change anymore.
fn finalize(&mut self, block: MeteredBlock) {
if block.cost > 0 {
self.blocks.entry(block.seq_id).or_default().push(block);
}
}
}
fn determine_metered_blocks(func: &LocalFunction) -> BTreeMap<InstrSeqId, Vec<MeteredBlock>> {
// NOTE: This is based on walrus::ir::dfs_in_order but we need more information.
let mut blocks = MeteredBlocks::default();
let mut stack: Vec<(InstrSeqId, usize, MeteredBlock)> = vec![(
func.entry_block(), // Initial instruction sequence to visit.
0, // Instruction offset within the sequence.
MeteredBlock::new(func.entry_block(), 0), // Initial metered block.
)];
'traversing_blocks: while let Some((seq_id, index, mut metered_block)) = stack.pop() {
let seq = func.block(seq_id);
'traversing_instrs: for (index, (instr, _)) in seq.instrs.iter().enumerate().skip(index) {
// NOTE: Current instruction is always included in the current metered block.
metered_block.cost += instruction_cost(instr);
// Determine whether we need to end/start a metered block.
match instr {
Instr::Block(Block { seq }) => {
// Do not start a new metered block as blocks are unconditional and metered
// blocks can encompass many of them to avoid injecting unnecessary
// instructions.
stack.push((seq_id, index + 1, metered_block.mergable(index + 1)));
stack.push((*seq, 0, metered_block));
continue 'traversing_blocks;
}
Instr::Loop(Loop { seq }) => {
// Finalize current metered block.
blocks.finalize(metered_block);
// Start a new metered block for remainder of block.
stack.push((seq_id, index + 1, MeteredBlock::new(seq_id, index + 1)));
// Start a new metered block for loop body.
stack.push((*seq, 0, MeteredBlock::new(*seq, 0)));
continue 'traversing_blocks;
}
Instr::IfElse(IfElse {
consequent,
alternative,
}) => {
// Finalize current metered block.
blocks.finalize(metered_block);
// Start a new metered block for remainder of block.
stack.push((seq_id, index + 1, MeteredBlock::new(seq_id, index + 1)));
// Start new metered blocks for alternative and consequent blocks.
stack.push((*alternative, 0, MeteredBlock::new(*alternative, 0)));
stack.push((*consequent, 0, MeteredBlock::new(*consequent, 0)));
continue 'traversing_blocks;
}
Instr::Call(_)
| Instr::CallIndirect(_)
| Instr::Br(_)
| Instr::BrIf(_)
| Instr::BrTable(_)
| Instr::Return(_) => {
// Finalize current metered block and start a new one for the remainder.
blocks.finalize(std::mem::replace(
&mut metered_block,
MeteredBlock::new(seq_id, index + 1),
));
continue 'traversing_instrs;
}
_ => continue 'traversing_instrs,
}
}
// Check if we can merge the blocks.
if let Some((_, _, upper)) = stack.last_mut() {
match upper.merge_index {
Some(index)
if upper.seq_id == metered_block.seq_id
&& index == metered_block.start_index =>
{
// Blocks can be merged, so overwrite upper.
*upper = metered_block;
continue 'traversing_blocks;
}
_ => {
// Blocks cannot be merged so treat as new block.
}
}
}
blocks.finalize(metered_block);
}
blocks.blocks
}
fn transform_function(
func: &mut LocalFunction,
gas_limit_global: GlobalId,
gas_limit_exhausted_global: GlobalId,
) {
// First pass: determine where metering instructions should be injected.
let blocks = determine_metered_blocks(func);
// Second pass: actually emit metering instructions in correct positions.
let builder = func.builder_mut();
for (seq_id, blocks) in blocks {
let mut seq = builder.instr_seq(seq_id);
let instrs = seq.instrs_mut();
let original_instrs = std::mem::take(instrs);
let new_instrs_len = instrs.len() + METERING_INSTRUCTION_COUNT * blocks.len();
let mut new_instrs = Vec::with_capacity(new_instrs_len);
let mut block_iter = blocks.into_iter().peekable();
for (index, (instr, loc)) in original_instrs.into_iter().enumerate() {
match block_iter.peek() {
Some(block) if block.start_index == index => {
inject_metering(
builder,
&mut new_instrs,
block_iter.next().unwrap(),
gas_limit_global,
gas_limit_exhausted_global,
);
}
_ => {}
}
// Push original instruction.
new_instrs.push((instr, loc));
}
let mut seq = builder.instr_seq(seq_id);
let instrs = seq.instrs_mut();
*instrs = new_instrs;
}
}
/// Number of injected metering instructions (needed to calculate final instruction size).
const METERING_INSTRUCTION_COUNT: usize = 8;
fn inject_metering(
builder: &mut FunctionBuilder,
instrs: &mut Vec<(Instr, InstrLocId)>,
block: MeteredBlock,
gas_limit_global: GlobalId,
gas_limit_exhausted_global: GlobalId,
) {
let mut builder = builder.dangling_instr_seq(None);
let seq = builder
// if unsigned(globals[gas_limit]) < unsigned(block.cost) { throw(); }
.global_get(gas_limit_global)
.i64_const(block.cost as i64)
.binop(BinaryOp::I64LtU)
.if_else(
None,
|then| {
then.i64_const(block.cost as i64)
.global_set(gas_limit_exhausted_global)
.unreachable();
},
|_else| {},
)
// globals[gas_limit] -= block.cost;
.global_get(gas_limit_global)
.i64_const(block.cost as i64)
.binop(BinaryOp::I64Sub)
.global_set(gas_limit_global);
instrs.append(seq.instrs_mut());
}
#[cfg(test)]
mod test {
use pretty_assertions::assert_eq;
macro_rules! test_transform {
(name = $name:ident, source = $src:expr, expected = $expected:expr) => {
#[test]
fn $name() {
let src = wat::parse_str($src).unwrap();
let expected = wat::parse_str($expected).unwrap();
let mut result_module = walrus::ModuleConfig::new()
.generate_producers_section(false)
.parse(&src)
.unwrap();
super::transform(&mut result_module);
let mut expected_module = walrus::ModuleConfig::new()
.generate_producers_section(false)
.parse(&expected)
.unwrap();
let result_wasm = result_module.emit_wasm();
let expected_wasm = expected_module.emit_wasm();
let result = wasmprinter::print_bytes(&result_wasm).unwrap();
let expected = wasmprinter::print_bytes(&expected_wasm).unwrap();
assert_eq!(result, expected);
}
};
}
test_transform! {
name = simple,
source = r#"
(module
(func (result i32)
(i32.const 1)))
"#,
expected = r#"
(module
(func (result i32)
(if
(i64.lt_u
(global.get 0)
(i64.const 1))
(then
(global.set 1
(i64.const 1))
(unreachable)))
(global.set 0
(i64.sub
(global.get 0)
(i64.const 1)))
(i32.const 1))
(global (;0;) (mut i64) (i64.const 0))
(global (;1;) (mut i64) (i64.const 0))
(export "gas_limit" (global 0))
(export "gas_limit_exhausted" (global 1)))
"#
}
test_transform! {
name = nested_blocks,
source = r#"
(module
(func (result i32)
(block
(block
(block
(i32.const 1)
(drop))))
(i32.const 1)))
"#,
expected = r#"
(module
(func (result i32)
(if
(i64.lt_u
(global.get 0)
(i64.const 18))
(then
(global.set 1
(i64.const 18))
(unreachable)))
(global.set 0
(i64.sub
(global.get 0)
(i64.const 18)))
(block
(block
(block
(i32.const 1)
(drop))))
(i32.const 1))
(global (;0;) (mut i64) (i64.const 0))
(global (;1;) (mut i64) (i64.const 0))
(export "gas_limit" (global 0))
(export "gas_limit_exhausted" (global 1)))
"#
}
test_transform! {
name = nested_blocks_with_loop,
source = r#"
(module
(func (result i32)
(block
(block
(block
(i32.const 1)
(drop))
(loop
(i32.const 1)
(drop)
(i32.const 1)
(drop)
(br 0))))
(i32.const 1)))
"#,
expected = r#"
(module
(func (result i32)
(if
(i64.lt_u
(global.get 0)
(i64.const 19))
(then
(global.set 1
(i64.const 19))
(unreachable)))
(global.set 0
(i64.sub
(global.get 0)
(i64.const 19)))
(block
(block
(block
(i32.const 1)
(drop))
(loop
(if
(i64.lt_u
(global.get 0)
(i64.const 25))
(then
(global.set 1
(i64.const 25))
(unreachable)))
(global.set 0
(i64.sub
(global.get 0)
(i64.const 25)))
(i32.const 1)
(drop)
(i32.const 1)
(drop)
(br 0))))
(if
(i64.lt_u
(global.get 0)
(i64.const 1))
(then
(global.set 1
(i64.const 1))
(unreachable)))
(global.set 0
(i64.sub
(global.get 0)
(i64.const 1)))
(i32.const 1))
(global (;0;) (mut i64) (i64.const 0))
(global (;1;) (mut i64) (i64.const 0))
(export "gas_limit" (global 0))
(export "gas_limit_exhausted" (global 1)))
"#
}
test_transform! {
name = if_else,
source = r#"
(module
(func (result i32)
(i32.const 1)
(if
(then
(i32.const 1)
(drop)
(i32.const 1)
(drop))
(else
(i32.const 1)
(drop)))
(i32.const 1)))
"#,
expected = r#"
(module
(func (result i32)
(if
(i64.lt_u
(global.get 0)
(i64.const 11))
(then
(global.set 1
(i64.const 11))
(unreachable)))
(global.set 0
(i64.sub
(global.get 0)
(i64.const 11)))
(i32.const 1)
(if
(then
(if
(i64.lt_u
(global.get 0)
(i64.const 22))
(then
(global.set 1
(i64.const 22))
(unreachable)))
(global.set 0
(i64.sub
(global.get 0)
(i64.const 22)))
(i32.const 1)
(drop)
(i32.const 1)
(drop)
)
(else
(if
(i64.lt_u
(global.get 0)
(i64.const 11))
(then
(global.set 1
(i64.const 11))
(unreachable)))
(global.set 0
(i64.sub
(global.get 0)
(i64.const 11)))
(i32.const 1)
(drop)
)
)
(if
(i64.lt_u
(global.get 0)
(i64.const 1))
(then
(global.set 1
(i64.const 1))
(unreachable)))
(global.set 0
(i64.sub
(global.get 0)
(i64.const 1)))
(i32.const 1))
(global (;0;) (mut i64) (i64.const 0))
(global (;1;) (mut i64) (i64.const 0))
(export "gas_limit" (global 0))
(export "gas_limit_exhausted" (global 1)))
"#
}
}