develop a 'customized ld'

[Nick Clifton]

Hi Sean,

>>> I would like to collect the information of call to some libc 
>>> functions. 

Why not use the "--wrap <func-name>" linker command line switch ?  That 
way you can intercept calls to specific libc functions, add your own 
wrapper code to collect the information you require and then call the 
normal libc version.

> I want to have the code obfuscated at link time also, say, insert some 
> junk(nop) to prevent others understanding it easily.

This is a separate issue.  As a general principle I would argue against 
obfuscation, it should never really be needed.  If you must do it, then 
just inserting junk instructions is not really going to help.  They can 
easily be detected and ignored by a determined examiner.  Your best bet 
would be to encrypt the binary and hope that you can keep your keys safe 
so that an unauthorized viewer cannot decode the executable.

Getting back to your original question:

> Could someone give me some hint where should I start? Can any expert 
 > assess the difficulty of achieving such functionality?

Conceptually intercepting certain instructions and modifying them is 
very similar to relaxation, so I would suggest that you look at the 
linker's support for this feature.  Have a look at the various 
*_relax_section() functions in the bfd/ directory for examples of this.

As for difficulty - well this is not really something I would ask a 
binutils newbie to do.  You have the potential to corrupt the binaries 
you are producing in quite nasty and/or subtle ways.  Good luck though!

Cheers
   Nick