Set owner of the new created file in C

Question

I have a program write.c, which creates a new file. I compiled that through root user and set the sticky bit for setuid using chmod u+s write.

Now, if a user2 executes this program. A new file is created with the root as owner, why ? The owner of the file should be user2. For that, I changed the uid using setuid() and seteuid() to user2. And then created the file. But this also creates the file with root as owner. I want to create the file as user2 as owner.

"I compiled that through root user" - never do that! And that's not about programming, but system administration. Such questions are off-tiopoc here. — too honest for this site, Feb 12 '17 at 17:01
"setuid() and setuid()" is one of them `seteuid` by any chance? Have you checked the return value of those? — n. m. could be an AI, Feb 12 '17 at 17:16
Too broad and likely an XY problem. Don't run programs as root without absolute need. Even then, release the root ownership **asap**. If that is Linux, you better use capabilities instead to get finer grain of permissions. — too honest for this site, Feb 12 '17 at 17:19

score 1 · Accepted Answer · edited May 23 '17 at 12:17

Post an mcve. What you describe works just fine on my system. This:

#!/bin/sh -e

cat > main.c <<EOF
#define _GNU_SOURCE
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char** argv)
{

    int fd;
    uid_t ruid,euid,suid;
    struct stat sb;
    getresuid(&ruid,&euid,&suid);
    printf("ruid=%ld euid=%ld suid=%ld\n", (long)ruid,(long)euid,(long)suid);

    if(0>(fd = open(argv[1], O_CREAT|O_RDWR, 0660))){
        perror(0); 
        exit(1);
    }
    fstat(fd,&sb);
    printf("owner=%ld\n", (long)sb.st_uid);

    close(fd);
    seteuid(ruid);
    getresuid(&ruid,&euid,&suid);
    printf("ruid=%ld euid=%ld suid=%ld\n", (long)ruid,(long)euid,(long)suid);

    if(0>(fd = open(argv[2], O_CREAT|O_RDWR, 0660))){
        perror(0); 
        exit(1);
    }
    fstat(fd,&sb);
    printf("owner=%ld\n", (long)sb.st_uid);

    close(fd);
}
EOF

gcc main.c  
sudo chown root a.out  
sudo chmod u+s a.out
rm -f roots mine  
./a.out roots mine

gets me:

ruid=1008 euid=0 suid=0
owner=0
ruid=1008 euid=1008 suid=0
owner=1008

i.e., the seteuid call succesfully resets my uid and the second file is no longer owner by root.

Set owner of the new created file in C

1 Answers1