#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>

bool check_user(char *filename, char *username, unsigned long *new_start) {
    /*
     * Check whether the user exists in the uid file (subuid, subgid).
     * Return exists or not, and set the new_start if need it.
     *
     * Args:
     *      filename (char *): means the uid file (subuid or subgid).
     *      username (char *): the user who run this excutable file.
     *      new_start (unsigned long *): it's a pointer, save the new start point.
     * Return:
     *      exist (bool): exist or not
     */
    FILE *fp = fopen(filename, "r");
    
    if (fp == NULL) {
        printf("[ERROR] Please contact Unix Manager.\n");
        exit(1);
    }

    // read data from subuid, subgid
    char line[100];
    char user[30];
    unsigned long start, size;     // 0~2^32-1, equal to subuid
    bool exist = false;
    while (fgets(line, sizeof(line), fp)) {
        sscanf(line, "%[^:]:%lu:%lu", user, &start, &size);
        *new_start = start + size;
        if (strcmp(username, user) == 0) exist = true;
    }
    fclose(fp);
    return exist;
}

void insert_user(char *filename, char *username, unsigned long start) {
    /*
     * Insert the user into uid file
     * It will setup the next start uid automatically
     * == it will change /etc/subuid or /etc/subgid ==
     *
     * Args:
     *      filename (char *): means the uid file (subuid or subgid).
     *      username (char *): the user who run this excutable file.
     *      new_start (unsigned long ): it's a value, means the next start uid
     * Return:
     *      No
     */
    FILE *fp = fopen(filename, "a");
    
    if (fp == NULL) {
        printf("[ERROR] Please contact Unix Manager.\n");
        exit(1);
    }
    char new_line[100];
    sprintf(new_line, "%s:%lu:%d\n", username, start, 65536);
    fputs(new_line, fp);
    fclose(fp);
    printf("[SUCCESS] %s has been inserted in %s\n", username, filename);
}


int main() {
    int ret;
    char *username = getlogin();

    uid_t ruid, euid, suid;
    getresuid(&ruid, &euid, &suid);
    printf("ruid=%d, euid=%d, suid=%d\n", ruid, euid, suid);

    ret = setuid(0);
    printf("ret=%d\n", ret);
    if (ret != 0) {
        perror("Error:");
        exit(1);
    }

    getresuid(&ruid, &euid, &suid);
    printf("ruid=%d, euid=%d, suid=%d\n", ruid, euid, suid);

    char filenames[2][20] = {
        "/etc/subuid", "/etc/subgid"
    };
    
    bool exist;
    unsigned long new_start;        // 0~2^32-1, equal to subuid
    for (int i=0; i<2; i++) {
        new_start = 100000;         // subuid, subgid default from 100000~165535
        exist = check_user(filenames[i], username, &new_start);
        if (exist) {
            printf("[WARNING] %s has been in %s, no change\n", username, filenames[i]);
        } else {
            insert_user(filenames[i], username, new_start);
        }
    }
    return 0;
}