QuantumG's Blog

Sometimes copyright law is just stoopid. Sometimes the rules just don't apply. Have you heard the story of Steve McDonald and White Stripes? Here's nice flash animation of what happened, or you can keep reading.. Steve McDonald is a veteran member of the band Redd Kross. He likes the White Stripes, but he thought they would sound better if they had a bass guitarist. So he appointed himself. He had the equipment, and the skill, so he made up some bass tracks and added them to his favourite White Stripes songs. He then posted those songs on the Redd Kross website - without permission. Of course, this could land him in hot water, but luckily he bumped into Jack White who gave his assurances that he wasn't going to sue. Before that happened I'm guessing Steve McDonald just didn't give a damn.. after all, he's a rocker, man. I'm not a rocker, but I am a rebel, ask anyone. I once posted the full c99 standard to my web site so people wouldn't have ...

Hehe, after struggling to place the head, torso and legs of pedestrians together to make the animations work in FreeSynd for the last week or so I happened to find this information about HELE-0.ANI, HFRA-0.ANI and HSTA-0.ANI. Turns out these three files contain just about everything you need to know to draw objects in the game. There are 1970 animations which are made up of frames taken from a pool of 8949, each of which are made up of elements taken from a pool of 10486, each of which index into the available 1180 game sprites. The amount of work required to manually reproduce these animations (and that I had intended on doing) is phenomonal. A conservative estimate is that this information has knocked a year off the development time. In just one day (today) I have figured out the exact placement of units on the map, fixed the drawing of animations and cleaned up a lot of very ugly code. The result is that agents can now walk around, with and without all the different weapons ...

Breaking my own rule that improvements should be belayed until such time as the original game has been reimplemented, I've added waypoints and path finding to agent movement in FreeSynd . I found this documentation of the A* path finding algorithm to be most enlightening.. although, frankly, the sketch is more than adequate to implement a useful algorithm for agent movement. It always annoyed me when I told my agents to go to point X and they got stuck inside buildings or took some inordinately long path. Now you can hold down the ctrl key and enter waypoints manually, or you can let the path finder do its job (this is the default). Pressing the ctrl key without clicking the mouse will display the selected agents' current path(s) as a bright yellow line. At the moment the planning algorithm does not take elevated terrain into account, so trying to send agents onto bridges or up stairs simply won't work just yet. There's still a hell of a lot to do before the game ...

Back when Mike and I started the Boomerang decompiler project, we had the choice to start from scratch or to use our work on the UQBT project as a base. There was pros and cons to both approaches, but in the end we decided that UQBT had quite a lot of value in it, so we went with that. Unfortunately, at the time Mike and I started working on a decompiler based on UQBT, the copyright was owned by a number of parties. We had to talk to lawyers from the University of Queensland, Sun Microsystems and the individual people who had worked on the codebase over the years. Initially the lawyers were of a single opinion: no way. The individual contributors, however, were of mixed opinions: what for? and which license? As it turned out, answering the second question also answered the first question and started to put the lawyers minds at ease. The goal of the Boomerang decompiler project was decided, way back in 2001, to be a guiding force in bringing about a market for decompilation techn...

I've recently been working on some game related projects. First, there's the RPG engine I've started which I call Stallion . I've received just about no interested about this from anyone so I havn't been paying it much attention. Now and then I get the desire to hack on an RPG and none of the available codebases for graphical RPGs interest me as much as the ones for textual RPGs. So why not hack on a textual RPG? Because it's just so dry without players to consider, and with players to consider you've just gotta be so dedicated. Then there's my other project. I've always been a fan of Bullfrog's Syndicate. My fan page gets a lot of hits (probably because I give out copies of the game on it). If you've never played Syndicate you probably won't understand the attraction. You'll also think we're all crazy for fooling around with DosBox or old hardware to get the game to actually run and never consider doing the like yourse...

Havn't been working on Boomerang much lately. I made a few changes to make the exports of DLL files appear as entrypoints in the GUI. Seems to work, will check it in soon. I also made some recent changes that handle gotos inside switches better. This means that magic like Duff's Device now decompile sensibly.

A small number of library procedures are "called" but never actually return. Eventually I'd like to have a way to specify these procedures with anotations in the signature files, but for the moment they are hard coded in the frontend. That's acceptable for the moment as there is only five: _exit, exit, ExitProcess, abort and _assert . Thing is, what happens when you have a branch over one of these calls, as you often do. Typically you end up with phi statements that refer to the call or the statements before it because there's no way to show that these definitions are killed by the call but not defined by it. We could add this to the SSA form but a simpler solution is available. Whenever the decompiler determines that the destination of a call is one of these no return procedures then we simply drop the out edge of the containing basic block. Without an out edge the definitions cannot flow beyond the call. Using dataflow based type analysis and some of m...

Continuing with my ongoing test program extract_kyra.exe from the scummvm tools I've been looking at the very first call in main . It would appear that this exe was compiled with stack checking runtime code enabled. That very first call is to a short little procedure that takes a single parameter in the eax register; the number of bytes to subtract from esp . Here's a disassembly of the procedure: push ecx mov ecx, esp add ecx, 8 loc_405336: cmp eax, 1000h jb short loc_40534D sub ecx, 1000h or dword ptr [ecx], 0 sub eax, 1000h jmp short loc_405336 loc_40534D: sub ecx, eax or dword ptr [ecx], 0 mov eax, esp mov esp, ecx mov ecx, [eax] mov eax, [eax+4] jmp eax It not only s...

I've been promising to do this for a while, so here goes. I have downloaded a small open source utility called extract_kyra which is part of the scummvm tools . It doesn't really matter what the utility does. In fact, I prefer not to know as it gives me an unfair advantage compared to, say, decompiling malware. What's important is that this tool is under the GPL, so it is permisible for me to decompile it. I will be writing a number of these posts that describe all the issues I've run into and the steps I've completed to produce the decompilation. I promise that I will not go and download the source code for this program until such time as I believe my decompilation is "complete". The first problem with decompiling this program is that Boomerang has failed to find main() . I can tell this because the load phase of the workflow does not list main as a found entrypoint, it only lists start . This is because this exe was compiled with MinGW and it...

After reading this paper I've starting implementing a brand new type analysis in Boomerang. The first step is calculating reverse dominance frontiers. A regular dominance frontier is the set of nodes that have a predecessor that is dominated by the given node, but is not itself dominated. These are the nodes where phi statements are to be placed when transforming a program into SSA form. The reverse dominance frontier is much the same, except it is the successors that must be post -dominated. Boomerang already calculates immediate post-dominators for use by the code generation phase, but we've never before had a use for reverse dominator frontiers. The paper describes an extension to SSA form called static single information form (SSI) which introduces a new kind of assignment: the sigma function statement which are placed on the reverse dominator frontiers. The purpose of this new statement is to split definitions before uses on different code paths. I will be using...

I've checked in some code that detects branches to branches and merges them if they meet some basic requirements. As such, Boomerang can now generate code like: if (a // x } else { // y } instead of generating a goto statement. I've checked in a couple of test programs that exercise this analysis. I havn't looked at how this analysis effects loops or more obscure control structures.. so there could well be bugs here.

I received an email of inspiration from Mike a week ago outlining how similar conjunctions and disjunctions in short circuited if statements are at the binary level. After looking at the problem myself I found it was a pretty simple problem. If one of the two out edges of a branch node is another branch node which contains only a branch statement, and the destination of that statement is shared with the first branch statement then you can merge the condition of the second branch into the condition of the first branch. Exactly what combination of fall/taken edges are shared by the two branches is what determines whether an || or a && should be used. This is a pretty easy transformation to do just before code generation (or just after decompilation) and I'm about half way through implementing it. Unfortunately I got sidetracked. My work, you know, the people who pay me, they have me doing - wait for it - Java development. I moaned about not just being a one trick pony ...

A Mail Transfer Agent is that bit of software that listens on port 25 and accepts mail when you send it. There's a lot of them available, but which ones are truely free? I find that a good moral compass on questions of licensing is to look at the OpenBSD project . What they use is typically the most free you can get. So what do they use? Sendmail , which has these license terms . They're pretty ass. Basically you can distribute it if you're "open source" in the GPL sense of the term; you have to promise to hand over source code, or if you are "freeware". So yeah, if you want to make a binary only CD of OpenBSD and include Sendmail you're going to have to promise whoever you give it to that you'll give them the source if they ask, or you can't charge them anything more than distribution costs. Seems kind of anti-OpenBSD-philosophy to me. But maybe there's nothing better out there. What about qmail ? Ask anyone and they'll te...

Which order you decompile the procedures of a program in can determine how much information you have available to make good analysis. In Boomerang we've developed a system of depth first search of the call graph, which takes into account recursion, to have the most information about parameters available when needed. For example, if a program consists of two procedures, one called by the other, it makes the most sense to decompile the procedure which is the leaf in the call graph so that the procedure that calls it has the most information about what parameters it needs to pass to it. What happens if the way the leaf procedure takes a parameter that is a pointer to a compound type? By the parameter's usage the decompiler may be able to determine that it is a pointer, it might even be able to determine that it is a pointer to a compound, but unless every member in the compound is accessed in a way that restricts that member's type sufficiently, the type that the decompiler...

I've added a new binary release of Boomerang for win32 to the sourceforge project. The Linux binaries are also up. If you'd like to try making a release for some other platform, please let me know. What can the new release do? Well, it crashes less. It supports ELF .o files much better than previous releases. It includes some changes that make for better output in some instances. Overall, just general improvements. What can't the new release do? This is still an alpha release. That means we don't expect you to be able to do very much work with it. Running it on /bin/ls will still give you horrendous output, but try /bin/arch.

The type analysis in Boomerang remains a nice big mess. There have been three attempts at a type analysis system: dataflow based, constraint based and adhoc. At the moment the adhoc gives the best results, and the other two crash, a lot. Sometimes there is an absolute orgy of types in an input program, and the type analysis will assign the type void to a local. I've just added some more adhoc type analysis that will recognise when the programmer is assigning the elements of an object of unknown type to an object of known type and copy the known types for the elements to the unknown type. This is very specific but hopefully it occurs in more than just the one input program I was looking at. In C the programmer would have written something like this: struct foo *p = someFuncThatReturnsAFoo(); p->name = global.name; p->count = global.count; p->pos = global.pos; p->other = 0; If that call is to a library proc we will have the struct foo , and know that p is a pointe...

A common source of confusion for new C programmers is the conflation of pointers and arrays that C does. I often think of the dynamic semantics of the language when I'm thinking deeply about passing arrays to functions. Typically, you can tell an experienced programmer that C always passes arrays by reference, never by value, and they won't go wrong. Not all languages are like this, so in Boomerang we try to represent pointers and arrays as seperate non-conflated types. In our type system an array is a type used to describe those bytes in memory that contain a finite number of objects of a particular base type. Similarly, a pointer is a type used to describe those bytes in memory that contain an address, which if followed will reveal a single object of a particular base type. As such, it is necessary to refer to somethings explicitly using the Boomerang type system that are typically implied by the C type system. For example, a C string is often written in the C type syste...

Strength reduction is a compiler optimisation that tries to replace expensive operations involving a loop variant with less expensive operations. The most common example of strength reduction is the replacement of array indexing with pointer arithmetic. Consider this program fragment: for (int i = 0; i < 25; i++) arr[i] = 9; where arr is an array of integers. An unoptimised compilation, in an RTL form, might look like this: 1 *32* r25 := 0 2 BRANCH 6 Highlevel: r25 >= 25 3 *32* m[a[arr] + r25 * 4] := 9 4 *32* r25 := r25 + 1 5 GOTO 2 which would actually be some very nice RTL for the decompiler to discover because it is easy to recognise that arr is an array with stride 4 and replace statement 3 with: 3 *32* a[r24] := 9 unfortunately, the compiler will have gotten to the RTL before we have, and most any compiler will do some strength reduction to get rid of that multiply in the middle of the left of statement 3. So what the decompiler will see is more like this: 1 *32* r...

I had an major issue where the Boehm garbage collector was crashing and spitting errors because of my use of Qt4's QThread class. The problem was simple enough, Qt4 calls pthread_create when it should be calling GC_pthread_create. I could have solved this problem by modifying qthread_private.cpp to do this, but that would mean distributing a modified Qt4, which is just silly for such a small change. So after much annoyance, I managed to come up with a solution that, although not pretty, seems to work. As such, there will be a Linux version of the GUI available to download when I make binary packages sometime in the next week.

A while ago I added a bool to the Signature class that allowed the user to specify that the signature was already fully formed and did not need any processing. This was part of the "symbol file" hack that we used to do procedure-at-a-time decompilation using the command line tool. I noticed today that we were not honouring the forced bit anymore, for example, we were removing unused parameters and returns from the signature, so I fixed that. It occured to me that any procedure we discover via a function pointer was an excellent candidate for setting the forced bit on. The results were pretty spectacular as locals and globals were soon inheriting type information from the parameters of the signature. Unfortunately, the same could not be said of returns. In particular, consider some code like this: mystruct *proc5() 12 { *v** r24 } := malloc(343) 13 m[r24{12} + 4] := "foo" 14 RET r24{12} It's pretty clear that any local we create for r24 should be of type m...