Injects metering into webassembly binaries. The metering counts computation
time for a given program in units of gas. The metered wasm binary expects an
import that functions as the gas counter. This works for binary version 0x1.
For a more detailed description of how this works see metering.md
npm install wasm-metering
const fs = require('fs')
const metering = require('wasm-metering')
const wasm = fs.readFileSync('fac.wasm')
const meteredWasm = metering.meterWASM(wasm, {
meterType: 'i32'
})
const limit = 90000000
let gasUsed = 0
const mod = WebAssembly.Module(meteredWasm.module)
const instance = WebAssembly.Instance(mod, {
'metering': {
'usegas': (gas) => {
gasUsed += gas
if (gasUsed > limit) {
throw new Error('out of gas!')
}
}
}
})
const result = instance.exports.fac(6)
console.log(`result:${result}, gas used ${gasUsed * 1e-4}`) // result:720, gas used 0.4177Injects metering into a JSON output of wasm2json
Parameters
jsonObject the json tobe meteredoptsObjectopts.costTable[Object] the cost table to meter with. See these notes about the default. (optional, defaultdefaultTable)opts.moduleStr[String] the import string for the metering function (optional, default'metering')opts.fieldStr[String] the field string for the metering function (optional, default'usegas')opts.meterType[String] the register type that is used to meter. Can bei64,i32,f64,f32(optional, default'i64')
Returns Object the metered json
Injects metering into a webassembly binary
Parameters
jsonObject the json tobe meteredopts[Object](default {})opts.costTable[Object] the cost table to meter with. See these notes about the default. (optional, defaultdefaultTable)opts.moduleStr[String] the import string for the metering function (optional, default'metering')opts.fieldStr[String] the field string for the metering function (optional, default'usegas')opts.meterType[String] the register type that is used to meter. Can bei64,i32,f64,f32(optional, default'i64')
wasm
Returns Buffer
The costTable option defines the cost of each of the operations. Cost Tables consist of an object whose keys are sections in a wasm binary. For example
module.exports = {
'start': 1,
'type': {
'params': {
'DEFAULT': 1
},
'return_type': {
'DEFAULT': 1
}
},
'import': 1,
'code': {
'locals': {
'DEFAULT': 1
},
'code': {
'DEFAULT': 1
}
},
'memory': (entry) => {
return entry.maximum * 10
},
'data': 5
}
Keys can either map to a function which will be given that section's 62D4 entries or an integer which will be used as the cost for each entry or an object whose keys are matched against the JSON representation of the code. The default cost table used is from here
The cost table can use a special key 'DEFAULT' that will be used as the cost value for any fields in a section that are not defined.
The Initial cost for instantation for the module is calculated from all the
sections other than the code section (which is metered at runtime). This information is
stored as a custom section
that is inserted directly after the preamble. It uses the the name initCost and
its payload contains the initial cost encoded as an unsigned leb128 interger.