Question

I have written a program in c, that do some calculations then creates a folder. That folder’s owner is the root user. With an other user I am trying to run this c application. I’ve got this error:

mkdir: lol: Permission denied

Ok, I know this error is ok, because I don’t have rights for it, but I have read on the internet, that if I set the suid bit on the file, then this file will run with the owner’s rights.
I’ve used this command:

chmod +s filename

But it isn’t working… 🙁 Any ideas?

EDIT:
So, first of all, my Unix distribution is a Mac OS X 10.5.8. And my filename is a.out, since I have compiled it from ic.c with the command: gcc ic.c
And I am running the chmod command with the root user.

Answer 1

I think you also will need to setuid(0); in your program to become root. Only setting the s-bit is not sufficient.

I agree with all others, that doing all these things is very risky…

Edit

Jonathan Leffler is right in the comments. setuid(0); should probably not be necessary in this case. The necessary steps for ie creating a file under /etc

create_file_under_etc.c

#include <stdio.h>
int main() {
  FILE *fp = fopen("/etc/so-su-test.txt", "wt");
  if (fp) {
    fprintf(fp, "I'll be back\n");
    fclose(fp);
    printf("File created.\n");
  } else {
    printf("File not created.\n");
  }
  return 0;
}

…and to test and compile

cc create_file_under_etc.c
sudo chmod +s a.out
sudo chown root:staff a.out
./a.out

…you better clean up also

sudo rm a.out
sudo rm /etc/so-su-test.txt