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| const toolkit = require('wasm-json-toolkit')
const leb128 = require('leb128/unsigned')
const text2json = toolkit.text2json
const SECTION_IDS = require('wasm-json-toolkit/json2wasm').SECTION_IDS
const defaultCostTable = require('./defaultCostTable.json')
// gets the cost of an operation for entry in a section from the cost table
function getCost (json, costTable = {}, defaultCost = 0) {
let cost = 0
// finds the default cost
defaultCost = costTable['DEFAULT'] !== undefined ? costTable['DEFAULT'] : 0
if (typeof costTable === 'function') {
cost = costTable(json)
} else if (Array.isArray(json)) {
json.forEach(el => {
cost += getCost(el, costTable)
})
} else if (typeof costTable === 'number') {
cost = costTable
} else if (typeof json === 'object') {
for (const propName in json) {
const propCost = costTable[propName]
if (propCost) {
cost += getCost(json[propName], propCost, defaultCost)
}
}
} else if (costTable[json] === undefined) {
cost = defaultCost
} else {
cost = costTable[json]
}
return cost
}
// meters a single code entrie
function meterCodeEntry (entry, costTable, meterFuncIndex, meterType, cost = 0) {
function meteringStatement (cost, meteringImportIndex) {
return text2json(`${meterType}.const ${cost} call ${meteringImportIndex}`)
}
function remapOp (op, funcIndex) {
if (op.name === 'call' && op.immediates >= funcIndex) {
op.immediates = (++op.immediates).toString()
}
}
function meterTheMeteringStatement () {
const code = meteringStatement(0, 0)
// sum the operations cost
return code.reduce(
(sum, op) => sum + getCost(op.name, costTable.code)
, 0)
}
// operations that can possible cause a branch
const branchingOps = new Set(['end', 'br', 'br_table', 'br_if', 'if', 'else', 'return', 'loop'])
const meteringOverHead = meterTheMeteringStatement()
let code = entry.code.slice()
let meteredCode = []
cost += getCost(entry.locals, costTable.local)
while (code.length) {
let i = 0
// meters a segment of wasm code
while (true) {
const op = code[i++]
remapOp(op, meterFuncIndex)
cost += getCost(op.name, costTable.code)
if (branchingOps.has(op.name)) {
break
}
}
// add the metering statement
if (cost !== 0) {
// add the cost of metering
cost += meteringOverHead
meteredCode = meteredCode
.concat(meteringStatement(cost, meterFuncIndex))
}
// start a new segment
meteredCode = meteredCode
.concat(code.slice(0, i))
code = code.slice(i)
cost = 0
}
entry.code = meteredCode
return entry
}
/**
* Injects metering into a JSON output of [wasm2json](https://github.com/ewasm/wasm-json-toolkit#wasm2json)
* @param {Object} json the json tobe metered
* @param {Object} opts
* @param {Object} [opts.costTable=defaultTable] the cost table to meter with. See these notes about the default.
* @param {String} [opts.moduleStr='metering'] the import string for the metering function
* @param {String} [opts.fieldStr='usegas'] the field string for the metering function
* @param {String} [opts.meterType='i64'] the regerster type that is used to meter. Can be `i64`, `i32`, `f64`, `f32`
* @return {Object} the metered json
*/
exports.meterJSON = (json, opts) => {
function findSection (module, sectionName) {
return module.find(section => section.name === sectionName)
}
function createSection (module, sectionName) {
const newSectionId = SECTION_IDS[sectionName]
for (let index in module) {
const section = module[index]
const sectionId = SECTION_IDS[section.name]
if (sectionId) {
if (newSectionId < sectionId) {
// inject a new section
module.splice(index, 0, {
name: sectionName,
entries: []
})
return
}
}
}
}
let funcIndex = 0
let initialCost = 0
let functionModule, typeModule
let {costTable, moduleStr, fieldStr, meterType} = opts
// set defaults
if (!costTable) costTable = defaultCostTable
if (!moduleStr) moduleStr = 'metering'
if (!fieldStr) fieldStr = 'usegas'
if (!meterType) meterType = 'i64'
// add nessicarry sections iff they don't exist
if (!findSection(json, 'type')) createSection(json, 'type')
if (!findSection(json, 'import')) createSection(json, 'import')
const importJson = {
'moduleStr': moduleStr,
'fieldStr': fieldStr,
'kind': 'function'
}
const importType = {
'form': 'func',
'params': [meterType]
}
json = json.slice(0)
for (let section of json) {
section = Object.assign(section)
switch (section.name) {
case 'type':
// mark the import index
importJson.type = section.entries.push(importType) - 1
// save for use for the code section
typeModule = section
break
case 'function':
// save for use for the code section
functionModule = section
break
case 'import':
for (const entry of section.entries) {
initialCost += getCost(entry, costTable.import)
if (entry.kind === 'function') {
funcIndex++
}
}
// append the metering import
section.entries.push(importJson)
break
case 'export':
for (const entry of section.entries) {
initialCost += getCost(entry, costTable.global)
if (entry.kind === 'function' && entry.index >= funcIndex) {
entry.index++
}
}
break
case 'element':
for (const entry of section.entries) {
initialCost += getCost(entry, costTable.element)
// remap elements indices
entry.elements = entry.elements.map(el => el >= funcIndex ? ++el : el)
}
break
case 'start':
initialCost += getCost(section, costTable.start)
// remap start index
if (section.index >= funcIndex) section.index++
break
case 'code':
for (const i in section.entries) {
const entry = section.entries[i]
const typeIndex = functionModule.entries[i]
const type = typeModule.entries[typeIndex]
const cost = getCost(type, costTable.type)
meterCodeEntry(entry, costTable.code, funcIndex, meterType, cost)
}
break
default:
if (section.entries) {
for (const entry of section.entries) {
initialCost += getCost(entry, costTable[section.name])
}
}
}
}
// add custom section with the initial cost
json.splice(1, 0, {
'name': 'custom',
'sectionName': 'initCost',
'payload': leb128.encode(initialCost)
})
return json
}
/**
* Injects metering into a webassembly binary
* @param {Object} json the json tobe metered
* @param {Object} opts
* @param {Object} [opts.costTable=defaultTable] the cost table to meter with. See these notes about the default.
* @param {String} [opts.moduleStr='metering'] the import string for the metering function
* @param {String} [opts.fieldStr='usegas'] the field string for the metering function
* @param {String} [opts.meterType='i64'] the regerster type that is used to meter. Can be `i64`, `i32`, `f64`, `f32`
* @return {Buffer}
*/
exports.meterWASM = (wasm, opts = {}) => {
let json = toolkit.wasm2json(wasm)
json = exports.meterJSON(json, opts)
return toolkit.json2wasm(json)
}
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