daemon to monitor files being modified
 

Is there some kernel module which can log all files modified not their access date or time but only those whose contents have changed .

Perhaps inotifywait would meet your requirements. It can be run in daemon mode to monitor many events. The modify event would indicate anytime a file changed. There is the problem that multiple events could be logged.

If this is just for a few of your own files, that would work.
If this is more for a system-wide security situation, the check out Samhain/Aide/Tripwire and ask the Mods (via the Report button) to move this to the Security forum.

I was looking for monitoring all mounted file systems even the external pluggable USB .

I will read https://github.com/rvoicilas/inotify...iki/#wiki-info , http://xaroumenosloukoumas.wordpress...h-inotifywait/
Is there any other link which provides my need ?