setuid not working, linux rhel6, simple example

Question

This is a repost after being referred to "Calling a script from a setuid root C program - script does not run as root" for a solution

My problem is different in that I do not want to run the C program (and c-shell script called inside) as root. Rather, I want to run as the owner of the c program file.

Also, I tried with "setuid(0)" as this was the solution in the referenced post. This is reflected in the edited code below. Same results.

Also, I opened permissions up all the way this time with "chmod 7775" (just in case it was a permissions problem)

Here's the original note with edits to reflect the change to "setuid(0)"

I'm having a problem implementing an simple example that would demonstrate how setuid can be made to run a binary with the uid of the file owner of the binary. What I would eventually like to do is run a c-shell script using this technique. I read that this will not work for shell scripts but also heard of a work-around using a C program to run a system() call that'll run the c-shell script ( e.g. system("source my.csh") ). I wrote a C program that attempts this, plus simply reports the current uid. This is what I tried...

From my current shell, I did a "su katman" to become the user of the binary I want to run as user katman.

I created a C program try.c. ...

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int main()
{
  setuid(0);
  printf("In try.c ... sourcing katwhoami.csh...\n");
  system( "source /home/me/su_experiments/katwhoami.csh");
  printf("In try.c ... using straight system call...\n");
  system("whoami");

  return 0;
}

I "setuid(0)" as recommended by a reference to a different note. But earlier, I tried setting it to the uid of the C program's owner as obtained with "id -u".

The katwhoami.csh shell script is simply...

date
echo "In katwhoami.csh, I am "`whoami`
echo "In katwhoami.csh, I am "$USER
exit

Then I compiled the C program and set the bit...

% gcc -o try try.c
% chmod 7775 try
% ls -l try
-rwsrwsr-x 1 katman design 6784 Mar  1 11:59 try

And then I test it...

% try
In try.c ... sourcing katwhoami.csh...
Thu Mar  1 12:28:28 EST 2018
In katwhoami.csh, I am ktadmin
In katwhoami.csh, I am ktadmin
In try.c ... using straight system call...
ktadmin

...which is what I expected.

I exit to get back to the shell I started from, hoping that if I run try there, it'll tell me I'm "katman"....

% exit
% whoami
daveg
% try
In try.c ... sourcing katwhoami.csh...
Thu Mar  1 12:30:04 EST 2018
In katwhoami.csh, I am daveg
In katwhoami.csh, I am daveg
In try.c ... using straight system call...
daveg

... which is not what I was hoping for :-(

As you can probably tell, I'm new at using this.
Any help would be appreciated !

Update....

I tried a new sample program, a.c...

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int main()
{
  FILE *fh;

  setuid(1234);
  system("whoami"); 

  fh=fopen("test.file","w");
  fprintf(fh,"Here I am\n");
  fclose(fh);

  return 0;
}

I compiled and set the bit

gcc -o a a.c
chmod 4775 a

Then I exited the "su" and ran as a user that is NOT the owner of the binary. Same result as before with regard to reported uid (the current uid), BUT, the owner of the file that the C program created ion this version was the owner of the C program !!! So that worked.

Is there something fishy about the "whoami" command? system() call ?

If I do a "system("source some.csh") does it create a new shell which assumes the original uid (not the owner of the C binary) ? Something like that ?

I really need the new uid to "stick" as far as child processes.