Wednesday, June 12, 2013

Windbg Tricks - Javascript Windbg Instrumentation

This post is going to cover three levels of usefulness of windbg instrumentation via javascript : subpar, normal, and abnormal.


The most basic way of instrumenting windbg via javascript is to set a breakpoint on a simple function, such as Math.atan, call Math.atan at the appropriate time in javascript to force windbg to break, and then do whatever you need to do in windbg. Useful, yes, but it's lame and gets extremely tiring after the first time of doing it.


A better way to instrument windbg via javascript is to create a way for javascript to print a message in windbg (and trigger a break):
bu jscript!Js::Math::Atan ".printf \"DEBUG: %mu\\n\", poi(poi(esp+10)+c) ; g"
(If you want to break, remove the ; g)

That's cool, but what if you want to do something a little more complicated, like track all allocations of a specific size after certain javascript statements have been executed. With the previous method, the javascript would have to look something like this:
function log(msg) {

function track_all_allocations_and_frees_size_x20() {

log("Executing main javascript");

log("Track all allocations and frees now");
... and the windbg breakpoints would be something like this:
bu jscript!Js::Math::Atan ".printf \"DEBUG: %mu\\n\", poi(poi(esp+10)+c) ; g"
bu jscript!Js::Math::Asin "bp ntdll!RtlAllocateHeap .if(poi(esp+c) == 0x20) { .echo ALLOCATED ONE ; knL } ; g"
This is more useful, but is still very inflexible. For every new javascript<-->windbg binding you might want, you'd need to also modify your breakpoints in windbg.


Below is an abnormally useful way to instrument windbg with javascript:
bu jscript!Js::Math::Atan ".block { .shell -ci \".printf \\\"%mu\\\\n\\\", poi(poi(esp+10)+c)\" find /v \"13333333337\" > cmd_to_exec.txt & exit } ; $$><cmd_to_exec.txt"
This lets you execute windbg commands directly from javascript. The breakpoint basically does an eval("WINDBG_CMD") with a string from memory. Broken down, the breakpoint goes like this:
.block {
    .shell -ci ".printf \"%mu\\n\", poi(poi(esp+10)+c)" find /v \"13333333337\" > cmd_to_exec.txt
Using .block helps to end the .shell command, since semicolons don't work as statement endings for the .shell command (see this article on msdn for more details).

find /v "13333333337" > cmd_to_exec.txt simply saves what was printf'd to the file cmd_to_exec.txt. Specifically, the find command filters out all lines from stdin that contain 13333333337. Any string here will work as long as you never expect to see it in a windbg command that you'd execute via javascript.

$$<>cmd_to_exec.txt runs the string we saved to cmd_to_exec.txt as a windbg script.

This method makes things much simpler. Going back to the first example, we can now do things like this:
function exec(cmd) {
function log(msg) {
    exec(".echo " + msg);
function track_allocations(size) {
    exec('bp ntdll!RtlAllocateHeap ".if(poi(esp+c) == 0n' + size + '){ .echo ALLOCATED ONE ; knL } ; g" ; g');

log("Executing main javascript");

var alloc_size = 0x20;
log("Tracking allocations of size " + alloc_size.toString(0x10));
Almost makes you wish you could write a javascript interface to windbg, doesn't it?

Abnormally useful. Laters.

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