#include <linux/fs.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/namei.h>
#include <linux/mount.h>
#include <linux/dcache.h>
#include <linux/list.h>
#include <linux/err.h>
#include <linux/mm.h>
#include <linux/string.h>
#include <linux/printk.h>

#define LIBFS_MAGIC        0xA0B0C0D0
#define LIBFS_BOOKS_MODE   01777
#define LIBFS_AUTHORS_MODE 0777
#define LIBFS_SUBDIR_MODE  01777

static struct super_block *libfs_sb;
static struct inode *books_inode;
static struct inode *authors_inode;
static struct dentry *authors_dentry;

struct link_entry {
    struct hlist_node node;
    struct dentry *link;
};

static void libfs_evict_inode(struct inode *inode);
static int libfs_create(struct mnt_idmap *idmap, struct inode *dir,
                        struct dentry *dentry, umode_t mode, bool excl);
static int libfs_mkdir(struct mnt_idmap *idmap, struct inode *dir,
                       struct dentry *dentry, umode_t mode);
static int libfs_unlink(struct inode *dir, struct dentry *dentry);
static int libfs_rmdir(struct inode *dir, struct dentry *dentry);
static int libfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
                        struct dentry *old_dentry, struct inode *new_dir,
                        struct dentry *new_dentry, unsigned int flags);
static int libfs_link(struct dentry *old_dentry, struct inode *dir,
                      struct dentry *new_dentry);

static const struct file_operations libfs_file_ops = {
        .read_iter  = generic_file_read_iter,
        .write_iter = generic_file_write_iter,
        .llseek     = generic_file_llseek,
};

static const struct inode_operations libfs_file_iops = {
        .setattr = simple_setattr,
        .getattr = simple_getattr,
};

static const struct inode_operations libfs_dir_iops = {
        .lookup  = simple_lookup,
        .create  = libfs_create,
        .unlink  = libfs_unlink,
        .mkdir   = libfs_mkdir,
        .rmdir   = libfs_rmdir,
        .rename  = libfs_rename,
        .link    = libfs_link,
        .mknod   = NULL,    /* запрещаем */
        .symlink = NULL,    /* запрещаем */
};

static const struct super_operations statsfs_sops = {
        .statfs      = simple_statfs,
        .drop_inode  = generic_delete_inode,
        .evict_inode = libfs_evict_inode,
};

static struct inode *libfs_get_inode(struct super_block *sb, umode_t mode)
{
    struct inode *inode;

    pr_info("statsfs: libfs_get_inode(sb=%p, mode=%o)\n", sb, mode);
    inode = new_inode(sb);
    if (!inode) {
        pr_err("statsfs: new_inode failed for mode %o\n", mode);
        return NULL;
    }

    inode->i_ino   = get_next_ino();
    inode->i_sb    = sb;
    inode->i_mode  = mode;

    if (S_ISDIR(mode)) {
        inode->i_op  = &libfs_dir_iops;
        inode->i_fop = &simple_dir_operations;
        pr_info("statsfs: created dir inode #%lu\n", inode->i_ino);
    } else {
        inode->i_op  = &libfs_file_iops;
        inode->i_fop = &libfs_file_ops;

        inode->i_private = kzalloc(sizeof(struct hlist_head), GFP_KERNEL);
        if (!inode->i_private) {
            pr_err("statsfs: kzalloc for inode #%lu failed\n", inode->i_ino);
            iput(inode);
            return NULL;
        }
        INIT_HLIST_HEAD((struct hlist_head *)inode->i_private);
        pr_info("statsfs: created file inode #%lu with private list\n", inode->i_ino);
    }

    return inode;
}

static void cleanup_links(struct inode *victim)
{
    struct hlist_head *list;
    struct link_entry *le;
    struct hlist_node *tmp;

    if (!S_ISREG(victim->i_mode) || !victim->i_private)
        return;

    pr_info("statsfs: cleanup_links for inode #%lu\n", victim->i_ino);
    list = (struct hlist_head *)victim->i_private;

    hlist_for_each_entry_safe(le, tmp, list, node) {
        struct dentry *link = le->link;
        struct dentry *pd;
        struct inode  *parent;

        if (!link || d_unlinked(link)) {
            pr_info("statsfs: link already gone, freeing entry\n");
            hlist_del(&le->node);
            dput(link);
            kfree(le);
            continue;
        }

        pd = dget_parent(link);
        if (pd) {
            parent = d_inode(pd);
            if (WARN_ON(!parent))
                goto skip_unlink;
            inode_lock(parent);
            vfs_unlink(NULL, parent, link, NULL);
            inode_unlock(parent);
            pr_info("statsfs: unlinked %pd from parent inode #%lu\n",
                    link, parent->i_ino);
            dput(pd);
        }

        skip_unlink:
        hlist_del(&le->node);
        dput(link);
        kfree(le);
    }

    kfree(victim->i_private);
    victim->i_private = NULL;
}

static bool is_protected(const struct dentry *d)
{
    return !strcmp(d->d_name.name, "books") ||
           !strcmp(d->d_name.name, "authors");
}

static int libfs_create(struct mnt_idmap *idmap, struct inode *dir,
                        struct dentry *dentry, umode_t mode, bool excl)
{
    struct inode *inode;

    pr_info("statsfs: CREATE %pd in dir inode #%lu mode=%o\n",
            dentry, dir->i_ino, mode);
    if (dir != books_inode)
        return -EPERM;

    inode = libfs_get_inode(dir->i_sb, S_IFREG | mode);
    if (!inode)
        return -ENOMEM;

    inode->i_uid = current_fsuid();
    inode->i_gid = current_fsgid();
    d_add(dentry, inode);
    pr_info("statsfs: added file %pd as inode #%lu\n", dentry, inode->i_ino);
    return 0;
}

static int libfs_unlink(struct inode *dir, struct dentry *dentry)
{
    struct inode *victim = d_inode(dentry);
    int err;

    pr_info("statsfs: UNLINK %pd from dir inode #%lu\n",
            dentry, dir->i_ino);
    if (dir == libfs_sb->s_root->d_inode && is_protected(dentry))
        return -EPERM;

    err = simple_unlink(dir, dentry);
    if (!err && dir == books_inode) {
        cleanup_links(victim);
    }
    return err;
}

static int libfs_mkdir(struct mnt_idmap *idmap, struct inode *dir,
                       struct dentry *dentry, umode_t mode)
{
    struct inode *inode;

    pr_info("statsfs: MKDIR %pd in dir inode #%lu\n",
            dentry, dir->i_ino);
    if (dir == libfs_sb->s_root->d_inode || dir == books_inode)
        return -EPERM;
    if (dir != authors_inode)
        return -EPERM;

    inode = libfs_get_inode(dir->i_sb, S_IFDIR | LIBFS_SUBDIR_MODE);
    if (!inode)
        return -ENOMEM;

    inode->i_uid = current_fsuid();
    d_add(dentry, inode);
    inc_nlink(dir);
    pr_info("statsfs: created subdir %pd as inode #%lu\n", dentry, inode->i_ino);
    return 0;
}

static int libfs_rmdir(struct inode *dir, struct dentry *dentry)
{
    pr_info("statsfs: RMDIR %pd in dir inode #%lu\n",
            dentry, dir->i_ino);
    if (dir == libfs_sb->s_root->d_inode && is_protected(dentry))
        return -EPERM;
    if (dir == authors_inode)
        return -EPERM;
    return simple_rmdir(dir, dentry);
}

static int libfs_rename(struct mnt_idmap *idmap, struct inode *odir,
                        struct dentry *odent, struct inode *ndir,
                        struct dentry *ndent, unsigned int flags)
{
    pr_info("statsfs: RENAME %pd (ino#%lu) -> %pd (ino#%lu)\n",
            odent, odir->i_ino, ndent, ndir->i_ino);
    if ((odir == libfs_sb->s_root->d_inode || ndir == libfs_sb->s_root->d_inode) &&
        (is_protected(odent) || is_protected(ndent)))
        return -EPERM;
    if (odir == authors_inode || ndir == authors_inode)
        return -EPERM;
    return simple_rename(idmap, odir, odent, ndir, ndent, flags);
}

static int libfs_link(struct dentry *old_dentry, struct inode *dir,
                      struct dentry *new_dentry)
{
    struct inode *t = d_inode(old_dentry);
    struct link_entry *le;
    int err;

    pr_info("statsfs: LINK %pd -> %pd in dir inode #%lu\n",
            old_dentry, new_dentry, dir->i_ino);
    if (!S_ISREG(t->i_mode))
        return -EPERM;
    if (!authors_dentry ||
        new_dentry->d_parent->d_parent != authors_dentry)
        return -EPERM;

    err = simple_link(old_dentry, dir, new_dentry);
    if (err)
        return err;

    le = kmalloc(sizeof(*le), GFP_KERNEL);
    if (!le)
        return -ENOMEM;

    le->link = dget(new_dentry);
    hlist_add_head(&le->node, (struct hlist_head *)t->i_private);
    pr_info("statsfs: recorded link entry for inode #%lu\n", t->i_ino);
    return 0;
}

static int libfs_fill_super(struct super_block *sb, void *data, int silent)
{
    struct inode *root;

    pr_info("statsfs: fill_super\n");
    libfs_sb     = sb;
    sb->s_magic  = LIBFS_MAGIC;
    sb->s_op     = &statsfs_sops;

    root = libfs_get_inode(sb, S_IFDIR | 0755);
    if (!root)
        return -ENOMEM;

    root->i_op  = &libfs_dir_iops;
    root->i_fop = &simple_dir_operations;
    sb->s_root  = d_make_root(root);
    if (!sb->s_root)
        return -ENOMEM;

    books_inode   = libfs_get_inode(sb, S_IFDIR | LIBFS_BOOKS_MODE);
    d_add(d_alloc_name(sb->s_root, "books"), books_inode);

    authors_inode = libfs_get_inode(sb, S_IFDIR | LIBFS_AUTHORS_MODE);
    authors_dentry = d_alloc_name(sb->s_root, "authors");
    d_add(authors_dentry, authors_inode);

    pr_info("statsfs: superblock initialized\n");
    return 0;
}

static struct dentry *libfs_mount(struct file_system_type *fs,
                                  int flags, const char *dev, void *data)
{
    pr_info("statsfs: mounting\n");
    return mount_nodev(fs, flags, data, libfs_fill_super);
}

static void libfs_evict_inode(struct inode *inode)
{
    pr_info("statsfs: evict_inode inode #%lu\n", inode->i_ino);
    truncate_inode_pages_final(&inode->i_data);
    cleanup_links(inode);
    clear_inode(inode);
}

static struct file_system_type libfs_type = {
        .owner   = THIS_MODULE,
        .name    = "libfs",
        .mount   = libfs_mount,
        .kill_sb = kill_litter_super,
};

static int __init libfs_init(void)
{
    pr_info("statsfs: module init\n");
    return register_filesystem(&libfs_type);
}

static void __exit libfs_exit(void)
{
    pr_info("statsfs: module exit\n");
    dput(authors_dentry);
    cleanup_links(books_inode);
    iput(books_inode);
    iput(authors_inode);
    unregister_filesystem(&libfs_type);
}

module_init(libfs_init);
module_exit(libfs_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("you");
MODULE_DESCRIPTION("Custom library FS with books and authors (with debug logs)");