[ubuntu] file permission s flag topic

hi, i'm currently reading 'Hacking, the art of exploitation' and try to reproduce some of the examples there. One example would be to change the file permissions seeting the s flag on file, so it can be executed by any user as if it would be the file owner. what i'm doing is the following...

Quote:

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[kbear@HECHTNY]:~/Dev\> ls -la uidcheck
-rwxr-xr-x 1 kbear kbear 8621 Feb 6 15:42 uidcheck
[kbear@HECHTNY]:~/Dev\> ./uidcheck
real uid: 1000
effective uid: 1000
[kbear@HECHTNY]:~/Dev\> sudo chown root:root uidcheck
[kbear@HECHTNY]:~/Dev\> ls -la uidcheck
-rwxr-xr-x 1 root root 8621 Feb 6 15:42 uidcheck
[kbear@HECHTNY]:~/Dev\> sudo chmod u+s uidcheck
[kbear@HECHTNY]:~/Dev\> ls -la uidcheck
-rwsr-xr-x 1 root root 8621 Feb 6 15:42 uidcheck
[kbear@HECHTNY]:~/Dev\> ./uidcheck
real uid: 1000
effective uid: 1000
[kbear@HECHTNY]:~/Dev\>

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the uidcheck file is a simple program that prints the userid as well as the effective userid (using getuid(), geteuid() respectively, see code below). after changing the owner to root and changing the file permission w/ the s flag i would expect the ids to be different and not the same. i did the same thing on the live-cd provided w/ the book and there it seems to work all fine.

Am i missing something or is this a security feature preventing this kind of change of file permissions.

thanks for the help

regards

Code:

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#include <stdio.h>

int main(){
    printf("real uid: %d\n",getuid());
    printf("effective uid: %d\n",geteuid());
}

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