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From:	"Gundolf" <news1 AT vonbachhaus DOT net>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Generating a CRC over a DJGPP programs binary code in memory?
Date:	21 Jul 2005 10:44:03 -0700
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To:	djgpp AT delorie DOT com
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Hello all,

Maybe someone can help me, I need to get an application to self-check
that certain portions of its binary code haven't been altered once it
has been started.
To do that, I'd like to generate a CRC over the code between certain
markers inserted into the binary code via something like "asm DB ..."
(assuming there is an equivalent for that in DJGPP)

So in a nutshell, the source code could look like something like this:

... bunch of code not to be crc'd

void start_marker ()
 {
 asm db 'S','T','A','R','T';
 }

void some_code_to_crc ()
 {
 ...
 }

void end_marker ()
 {
 asm db 'E','N',D';
 }

... more code not to be crc'd

The idea is to then run through the binary code in memory, look for the
string "START" and generate a CRC until "END" is found.

The next sub-problem: Does anybody how I can access the loaded code,
and scan it from position "0" to the "end".
As a fallback, I could always scan the .EXE file on disk, but I would
prefer the memory option as it's more reliable and the .EXE could then
be packed.

--- So to summarize it, I'm looking for solutions the following 3
sub-problems:

1) How can I insert arbitrary bytes into the program code, like with
"asm db ..." or "__emit__ (...)" in other compilers?
2a) How can I figure out the beginning and end of the program code in
memory once the program has been loaded, and having done that ...
2b) How can I read the program code byte for byte.


BTW: Using static variables for the markers doesn't work - variables
seem to be stored at the beginnig/end of the code, not where they are
placed in-between functions. I guess it's a code/data segment thing,
and I need to use the code segment.

Any tips would be very much appreciated! Thanks ..!

Gundolf

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