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 Forums >>  Brainstorms - General  >>  Memory Structure

Topic created on: January 21, 2007 15:24 CST by Cubic .

Hi all,

I doing some RE work and I've stumbled upon something that I haven't seen before. Basically there is a local variable defined as var_60 (size 60). Before a call is made to a function, the code loads the address of var_60 into esi and the whole function uses esi+SOME_CONST to access and store data.

Two questions:

1. What is the best representation of this asm code in C?
2. Sine there are no variable, but only esi+SOME_CONST, it's quite confusing to follow the program. Is there any way to help me navigate through this?

Thanks!

  Raindog     January 21, 2007 17:36.45 CST
It's a structure. SOME_CONST = offset of member.

  PSUJobu     January 22, 2007 09:56.53 CST
If you're using IDA Pro, define a structure and add fields at each offset used in the function. You can use the t key (with the default key mapping) to turn [esi+20h] into [esi+MyStruct.field_20]. As you find out what different fields do, you may eventually see [esi+Connection.hSocket] or something (contrived example).

I don't know if OllyDbg offers any equivalent, so YMMV. What you're describing is extremely common in any compiled C or C++ code, so get used to seeing it a lot.

Tip: A function that initializes most offsets of a structure after a new operator (or equivalent) is very useful for generating a good structure definition. The first challenge is to know what fields are present (e.g., 4 individual BYTEs or a single DWORD), and such initializer functions do nicely to solve this first challenge. The second (and generally more difficult) challenge is to figure out what they do...

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