setuid login wrapper that let's you only be you

relogin.c

#include <pwd.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>

static void check(const char **name, const char *user) {
    if (*name != NULL || user == NULL)
        return;

    struct passwd *pw = getpwnam(user);
    if (pw != NULL && pw->pw_uid == getuid())
        *name = user;
}

int main() {
    const char *name = NULL;
    check(&name, getenv("LOGNAME"));
    check(&name, getlogin());

    struct passwd *pw = getpwuid(getuid());
    if (pw != NULL)
        check(&name, pw->pw_name);

    if (name == NULL)
        return 1;

    const char *login = "/usr/bin/login";
    if (access("/bin/login", X_OK) == 0)
        login = "/bin/login";

    char host[64] = "";
    snprintf(host, sizeof(host), "local:pid=%d", getppid());

    if (setuid(0) != 0 || setgid(0) != 0)
        return 1;

    execl(login, "login", "-f", "-h", host, "--", name, NULL);
    return 1;
}