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Comment by viraptor | Hacker News Reader

A silly decompiler that doesn't even recognise for loop with a break (https://github.com/Laurelai/decompile-dump/blob/master/outpu...)

That said... I wonder how useful would github be as a collaborative decompilation space - grab a file, annotate / simplify as much as possible, send pull request, repeat...

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'collaborative decompilation space' (github-based or not) sounds like a sick idea, actually.

I wonder if Google's translation tools could be adapted to the task: "Suggest a better decompilation," perhaps?

Does anybody have a good reference for how decompilers work? When converting assembly to C by hand I always went through multiple passes (not necessarily in this order): first a literal pass that looked a lot like this dump with gotos and variables named after registers, then I'd identify if/else and switch blocks, then I'd convert any reverse gotos into a while loop, convert while loops into for loops, identify data structures, duplicate lines that had been optimized to a single reference with jmps, etc.

Pretty much that, yes.

Also, you have specific idioms for popular compilers (vc, gcc, icc, borland) and value propagation. You could even include an SMT solver to identify constraints and propagate the range to child basic blocks.

Bincrowd

http://www.zynamics.com/bincrowd.html

The plugin is open source and the community server is free.

I wanted to post about this, but I could not remember the name and googling things like '(collaborative OR crowdsource) static binary analysis' was futile. Thanks, it was driving me insane.

Although in this case, the object code may have been deliberately obscured. On reflection, I can't help wondering how much of the seemingly pointless diddling of local variables is the result of turning a naive decompiler on a bytestream where the actual instructions are somehow encrypted or masked.

I really don't think so. The decompilation result is genuinely silly. For example look at:

    if (*(int*)(local2 + global0) == 0x4550) {
        local5 = *(unsigned short*)(local2 + global0 + 20);

Either local2 or global0 are pointers to a structure. Neither is detected. Most sensible decompilers would change the names and just make it `local5 = global0[local2].blah` instead. Advanced decompilers also can avoid gotos in almost any case. It also uses function calls without parameters or output, assuming some stack layout probably.

It's a really really bad output, not a result of obscured compilation.

I've seen HexRays do the same thing - most of the time you need to actually tell it that a variable is a pointer before it tries to do that.

Stating the function type explicitly also helps it display the right arguments given to it. This output could be significantly improved if the assembly version were available as well.

This came from one of the better known commercial decompilers. Which ones do you consider 'advanced'?

Pointed out before: http://news.ycombinator.com/item?id=2214052 - this is from boomerang - not a known commercial decompiler really.

Hex-rays is much much better than this. You'll have to see the actual output to understand, but at least here are some examples. It does not do such a bad job with type identification, although it certainly isn't perfect.

http://www.hex-rays.com/hexcomp11.shtml

Stuxnet didn't really do anything to actively thwart reverse engineering. See here: http://rdist.root.org/2011/01/17/stuxnet-is-embarrassing-not...

Truly obfuscated code will just plain cause the Hex-Rays decompiler to crash, producing absolutely no output at all.

I don't think it would necessarily crash, unless you intentionally wrote code to target a specific bug in Hex-rays itself.

You can certainly get Hex-rays to output confusing information on purpose without actually targeting it. If this was obfuscated, you'd expect that would be much more likely.

You could compile the decompiled code with an optimizing compiler, then decompile that, or use GCC's C backend, or something to automatically simplify the program?

No. There is just too much information lost going from source to binary and when you use optimization the results are more information lost, not less.

The idea works well enough if you just want get optimized asm though. http://events.ccc.de/congress/2010/Fahrplan/events/4096.en.h...

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