every c program is converted to machine code, if this binary is distributed. Since the instruction set of a computer is well known, is it possible to get back the C original program?
+10
A:
You can never get back to the exact same source since there is no meta-data about that saved with the compiled code.
But you can re-create code out from the assembly-code.
Check out this book if you are interested in these things: Reversing: Secrets of Reverse Engineering.
Edit
Some compilers-101 here, if you were to define a compiler with another word and not as technical as "compiler", what would it be?
Answer: Translator
A compiler translates the syntax / phrases you have written into another language a C compiler translates to Assembly or even Machine-code. C# Code is translated to IL and so forth.
The executable you have is just a translation of your original text / syntax and if you want to "reverse it" hence "translate it back" you will most likely not get the same structure as you had at the start.
A more real life example would be if you Translate from English to German and the from German back to English, the sentance structure will most likely be different, other words might be used but the meaning, the context, will most likely not have changed.
The same goes for a compiler / translator if you go from C to ASM, the logic is the same, it's just a different way of reading it ( and of course its optimized ).
Filip Ekberg
2010-02-16 11:48:12
+4
A:
It depends on what you mean by original C program. Things like local variable names, comments, etc... are not included in the binary, so there's no way to get the exact same source code as the one used to produce the binary. Tools such as IDA Pro might help you disassemble a binary.
Darin Dimitrov
2010-02-16 11:48:25
no not to the level of variable names, but atleast the logic of the program
2010-02-16 11:50:40
If you use objdump -S you could get more information.
Tom
2010-02-16 17:55:09
(and the program was compiled with -g ?)
Tom
2010-02-16 17:55:27
+2
A:
Working on tools that do this is a research activity. That is, it is possible to get something in the easy cases (you won't recover local variables names unless debug symbols are present, for instance). It's nearly impossible in practice for large programs or if the programmer had decided to make it difficult.
Pascal Cuoq
2010-02-16 11:49:01
+1
A:
There is not a 1:1 mapping between a C program and the ASM/machine code it will produce - one C program can compile to a different result on different compilers or with different settings) and sometimes two different bits of C could produce the same machine code.
You definitely can generate C code from a compiled EXE. You just can't know how similar in structure it will be to the original code - apart from variable/function names being lost, I assume it won't know the original way the code was split amongst many files.
John
2010-02-16 12:01:40
+2
A:
I would guestimate the conversion rate of a really skilled hacker at about 1 kilobyte of machine code per day. At common Western salaries, that puts the price of, say, a 100 KB executable at about $25,000. After spending that much money, all that's gained is a chunk of C code that does exactly what yours does, minus the benefit of comments and whatnot. It is no way competitive with your version, you'll be able to deliver updates and improvements much quicker. Reverse engineering those updates is a non trivial effort as well.
If that price tag doesn't impress you, you can arbitrarily raise the conversion cost by adding more code. Just keep in mind that skilled hackers that can tackle large programs like this have something much better to do. They write their own code.
Hans Passant
2010-02-16 13:10:58
Doing a quick analysis of a few files reveals that 1KB is only a few hundred lines of machine code, and it tends to be less the larger the program. Analyzing that much would be relatively easily for a not so skilled hacker. However I doubt somebody would be willing to pay for raw C code that would need further reverse engineering, so the programmer would need to rewrite or otherwise document the code in a usable way. If you just want to steal an algorithm, or interface with a specific secret API it could be cost effective, but else, it sounds like just too expensive to bother.
jbcreix
2010-02-16 14:22:48
+1
A:
The Hex-Rays decompiler (extension to IDA Pro) can do exactly that. It's still fairly recent and upcoming but showing great promise. It takes a little getting used to but can potentially speed up the reversing process. It's not a "silver bullet" - no c decompiler is, but it's a great asset.
joveha
2010-02-16 14:55:19
How does the decompiler differentiate between `for`, `while` and `do-while` loops when then can be coded differently in the source? Also, how does it get the names of the original functions? How does it "un-optimize" the code to match the original? How does it determine the variable names? Perform a search of *news:comp.lang.c" for *hamburger* and *cow*.
Thomas Matthews
2010-02-16 18:21:01
Does it matter? Either will do if I care about the logic and not the form
joveha
2010-02-16 18:32:39
+1
A:
The common name for this procedure is "turning hamburger back into cows." It's possible to reverse engineer binary code into a functionally equivalent C program, but whether that C code bears a close resemblance to the original is an open question.
John Bode
2010-02-16 17:44:14
+2
A:
One of the best works on this topic that I know about is:
Pigs from sausages? Reengineering from assembler to C via FermaT.
The claim is you get back a reasonable C program, even if the original asm code was not written in C! Lots of caveats apply.
Ira Baxter
2010-02-18 16:21:40
Nice. I have one of the caveats. The result of running one of my programs through it produces a C program but the resulting C program doesn't work due to the assembly code assuming my nonstandard stack frame.
Joshua
2010-05-01 18:45:21
You actually used the FermaT converter? ... I'd expect that stack frame layout wouldn't make a bit of difference; a stack frame is just a kind of "struct" and if you can propose reasonable structs for heap values, you should be able to do it for stack frames. Can you give a specific example?
Ira Baxter
2010-05-01 21:03:44
+1
A:
You can try hex-rays.com, it has a really nice decompiler which can decompile assembly code into C with 99% accuracy.
Yuriy Y. Yermilov
2010-03-15 22:05:51