If you use fork+exec
you can change euid
of the child process while staying as root in the parent.
The code could look something like this:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
int runAs(int gid, int uid, char **env, char *command) {
int status;
int child = fork();
if (child == 0) {
setgid(100);
setuid(1000);
do {
putenv(*env);
env++;
} while (env != null);
exec(command);
} else if (child > 0) {
waitpid(child,&status,WUNTRACED | WCONTINUED);
} else {
// Error: fork() failed!
}
}
int main(int argc, char *argv[] )
{
char *env[3];
if ( argc != 2) {
printf("usage: %s command\n",argv[0]);
exit(1);
}
env[0] = "HOME=/home/caroline";
env[1] = "DISPLAY=:0";
env[2] = NULL;
runAs(100, 1000, env, argv[1]);
env[0] = "HOME=/home/john";
env[1] = "DISPLAY=:1";
runAs(100, 1001, env, argv[1]);
return 0;
}