WAIL is a Javascript library for instrumenting WebAssemby binaries from within the browser. WAIL is designed to handle large WASM binaries quickly and memory-efficiently.
Check out the slides for Hacking WebAssembly Games with Binary Instrumentation at Defcon 27.
See examples/
For a more complex example, see Cetus
WAIL can add elements to almost any specification-defined section. (Exceptions are TABLE, MEMORY, and START sections since each only contains one element in the MVP)
const parser = new WailParser(bufferSource);
const newTypeEntry = parser.addTypeEntry({
form: "func",
params: [ "i32", "f32" ],
return: "i32"
});Entries in some sections require references to other sections. In this example, we create a TYPE entry and use it as part of our newly-created IMPORT entry.
const parser = new WailParser(bufferSource);
const newTypeEntry = parser.addTypeEntry({
form: "func",
params: [],
});
const newImportEntry = parser.addImportEntry({
moduleStr: "moduleName",
fieldStr: "fieldName",
kind: "func",
type: newTypeEntry
});const parser = new WailParser(bufferSource);
const newTypeEntry = parser.addTypeEntry({
form: "func",
params: [],
});
const newFuncEntry = parser.addFunctionEntry({
type: newTypeEntry,
});
parser.addCodeEntry({
locals: [ "i32" ],
code: [ OP_RETURN, OP_END ]
});const parser = new WailParser(bufferSource);
const newGlobalEntry = parser.addGlobalEntry({
globalType: {
contentType: "i32",
mutability: true,
},
initExpr: [ OP_I32_CONST, VarUint32(0x00), OP_END ]
});const parser = new WailParser(bufferSource);
const adjustedFunctionIndex = parser.getFunctionIndex(1000);
parser.addExportEntry({
fieldStr: "fieldStr",
kind: "func",
index: adjustedFunctionIndex,
});const parser = new WailParser(bufferSource);
const adjustedFunctionIndex = parser.getFunctionIndex(1000);
parser.addElementEntry({
index: 0,
offset: [ OP_I32_CONST, VarUint32(0), OP_END ],
elems: [
adjustedFunctionIndex
]
});const parser = new WailParser(bufferSource);
parser.addDataEntry({
index: 0,
offset: [ OP_I32_CONST, VarUint32(0), OP_END ],
data: [ "hello world" ]
});You may have noticed in the examples above we use getFunctionIndex() before any function indexes we plan on using. This is because some modifications will change the function or global tables, causing these indexes to change.
As such, it is highly recommended that you use these two functions rather than referencing any function indexes by directly their index.
const adjustedGlobalIndex = parser.getGlobalIndex(10);
const adjustedFunctionIndex = parser.getFunctionIndex(1000);