debugger - Dave's Blog

Search

Data breakpoints in JavaScript

2016 Jun 17, 5:44

The other day I had to debug a JavaScript UWA that was failing when trying to use an undefined property. In a previous OS build this code would run and the property was defined. I wanted something similar to windbg/cdb's ba command that lets me set a breakpoint on read or writes to a memory location so I could see what was creating the object in the previous OS build and what that code was doing now in the current OS build. I couldn't find such a breakpoint mechanism in Visual Studio or F12 so I wrote a little script to approximate JavaScript data breakpoints.

The script creates a stub object with a getter and setter. It actually performs the get or set but also calls debugger; to break in the debugger. In order to handle my case of needing to break when window.object1.object2 was created or accessed, I further had it recursively set up such stub objects for the matching property names.

Its not perfect because it is an enumerable property and shows up in hasOwnProperty and likely other places. But for your average code that checks for the existence of a property via if (object.property) it works well.

PermalinkCommentsdebug debugging javascript

Cdb/Windbg Commands for Runtime Patching

2016 Feb 8, 1:47

You can use conditional breakpoints and debugging commands in windbg and cdb that together can amount to effectively patching a binary at runtime. This can be useful if you have symbols but you can't easily rebuild the binary. Or if the patch is small and the binary requires a great deal of time to rebuild.

Skipping code

If you want to skip a chunk of code you can set a breakpoint at the start address of the code to skip and set the breakpoint's command to change the instruction pointer register to point to the address at the end of the code to skip and go. Voila you're skipping over that code now. For example:

bp 0x6dd6879b "r @eip=0x6dd687c3 ; g"

Changing parameters

You may want to modify parameters or variables and this is simple of course. In the following example a conditional breakpoint ANDs out a bit from dwFlags. Now when we run its as if no one is passing in that flag.

bp wiwi!RelativeCrack "?? dwFlags &= 0xFDFFFFFF;g"

Slightly more difficult is to modify string values. If the new string length is the same size or smaller than the previous, you may be able to modify the string value in place. But if the string is longer or the string memory isn't writable, you'll need a new chunk of memory into which to write your new string. You can use .dvalloc to allocate some memory and ezu to write a string into the newly allocated memory. In the following example I then overwrite the register containing the parameter I want to modify:

.dvalloc 100
ezu 000002a9`d4eb0000 "mfcore.dll"
r rcx = 000002a9`d4eb0000

Calling functions

You can also use .call to actually make new calls to methods or functions. Read more about that on the Old New Thing: Stupid debugger tricks: Calling functions and methods. Again, all of this can be used in a breakpoint command to effectively patch a binary.

PermalinkCommentscdb debug technical windbg

High Performance Web Sites :: HAR to Page Speed

2010 May 2, 2:52'HAR to Page Speed' is a tool that takes a HAR file (Http ARchive) supported by various HTTP debuggers and produces a page speed score. This is a great example of the value of a cross HTTP debugger file format.PermalinkCommentshttp tool debug performance web technical

WPAD Server Fiddler Extension

2010 Jan 5, 7:42

I've made a WPAD server Fiddler extension and in a fit of creativity I've named it: WPAD Server Fiddler Extension.

Of course you know about Fiddler, Eric's awesome HTTP debugger tool, the HTTP proxy that lets you inspect, visualize and modify the HTTP traffic that flows through it. And on the subject you've probably definitely heard of WPAD, the Web Proxy Auto Discovery protocol that allows web browsers like IE to use DHCP or DNS to automatically discover HTTP proxies on their network. While working on a particularly nasty WPAD bug towards the end of IE8 I really wished I had a way to see the WPAD requests and responses and modify PAC responses in Fiddler. Well the wishes of me of the past are now fulfilled by present day me as this Fiddler extension will respond to WPAD DHCP requests telling those clients (by default) that Fiddler is their proxy.

When I started working on this project I didn't really understand how DHCP worked especially with respect to WPAD. I won't bore you with my misconceptions: it works by having your one DHCP server on your network respond to regular DHCP requests as well as WPAD DHCP requests. And Windows I've found runs a DHCP client service (you can start/stop it via Start|Run|'services.msc', scroll to DHCP Client or via the command line with "net start/stop 'DHCP Client'") that caches DHCP server responses making it just slightly more difficult to test and debug my extension. If a Windows app uses the DHCP client APIs to ask for the WPAD option, this service will send out a DHCP request and take the first DHCP server response it gets. That means that if you're on a network with a DHCP server, my extension will be racing to respond to the client. If the DHCP server wins then the client ignores the WPAD response from my extension.

Various documents and tools I found useful while working on this:

PermalinkCommentsproxy fiddler http technical debug wpad pac tool dhcp

Become a Web Debugging Virtuoso with Fiddler :: Sessions :: Microsoft PDC09

2009 Nov 19, 3:32Eric's talk at PDC on Fiddler.PermalinkCommentshttp eric-lawrence fiddler pdc video proxy microsoft windows debug debugger technical

Creating Accelerators for Other People's Web Services

2009 Aug 18, 4:19

Before we shipped IE8 there were no Accelerators, so we had some fun making our own for our favorite web services. I've got a small set of tips for creating Accelerators for other people's web services. I was planning on writing this up as an IE blog post, but Jon wrote a post covering a similar area so rather than write a full and coherent blog post I'll just list a few points:

PermalinkCommentstechnical accelerator ie8 ie

ANTLRWorks: The ANTLR GUI Development Environment

2008 Oct 2, 9:37Cool graphical ANTLR IDE! They didn't have this the last time I used ANTLR. "ANTLRWorks is a novel grammar development environment for ANTLR v3 grammars written by Jean Bovet (with suggested use cases from Terence Parr). It combines an excellent grammar-aware editor with an interpreter for rapid prototyping and a language-agnostic debugger for isolating grammar errors. ANTLRWorks helps eliminate grammar nondeterminisms, one of the most difficult problems for beginners and experts alike, by highlighting nondeterministic paths in the syntax diagram associated with a grammar."PermalinkCommentsantlr ide graph grammar tool free download development opensource java

Debugging XSLT

2008 Aug 15, 4:02VS debugs XSLT. Didn't know that. Neat. "You can use the Visual Studio debugger to debug XSLT. The debugger supports setting breakpoints, viewing XSLT execution state, and so on. The debugger can be used to debug a style sheet, or to debug an XSLT transformation invoked from another application. XSLT debugging is available in the Visual Studio Team System and the Professional Edition." Unfortunately I couldn't figure out how to pass in parameter values... I just ended up setting the default value for my param elements. Otherwise, cool.PermalinkCommentsdebug visual-studio microsoft msdn reference xsl xslt xml

IEBlog : Scripting Debugging in Internet Explorer

2007 Sep 7, 11:29Info on debugging script in IE.PermalinkCommentsie ie7 blog javascript debug debugger debugging web development programming vbscript tools tool download browser reference microsoft msdn

Install Debugging Tools for Windows 32-bit Version

2007 May 20, 5:14Debugging tools for Windows executables.PermalinkCommentsmsdn microsoft c++ c debug debugger development download free programming software tool tools windbg windows cdb

Fiddler2 HTTP Debugger - Fiddler2

2006 Nov 27, 11:23Fiddler2 is a free tool that lets you view and fiddle with HTTP and now HTTPS traffic! Supports automated modification of traffic using javascript as well as manual modification using breakpoints. Very cool tool.PermalinkCommentseric-lawrence tool tools free internet http debugger debug fiddler fiddler2 microsoft proxy

Frequently Used Debugger Commands

2006 Apr 21, 4:52Quick list of Debugging Commands for the Microsoft debuggers windbg and cdbPermalinkCommentstony-schriner debug debugger windows microsoft windbg tools tool

Fiddler HTTP Debugger - Debugging Essentials

2006 Apr 7, 4:59Good tools for debugging IE problems.PermalinkCommentseric-lawrence fiddler tool tools debug development http http-header ie ie7 web internet

Download details: Script Debugger for Windows NT 4.0 and Later

2006 Mar 29, 11:27Debug Javascript in IEPermalinkCommentsdebug debugger script development ie javascript microsoft windows

Debugging Tools and Symbols: Getting Started

2006 Mar 9, 9:53PermalinkCommentsmicrosoft windows decompiler debug debugger x64 x86

Fiddler HTTP Debugger - Fiddler

2005 Apr 10, 12:52A great debugging tool from Mr. Eric Lawrence himselfPermalinkCommentshttp development ie security tools web eric-lawrence fiddler
Older Entries Creative Commons License Some rights reserved.