#include <linux/fs.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/pagemap.h>
#include <linux/slab.h>
#include <linux/namei.h>
#include <linux/mount.h>
#include <linux/dcache.h>
#include <linux/list.h>
#include <linux/fs_context.h>
#include <linux/err.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 = NULL;
static struct vfsmount *libfs_mnt;

static const struct super_operations libfs_sops = {
        .statfs = simple_statfs,
        .drop_inode = generic_delete_inode,
};

static const struct file_operations libfs_file_ops = {
        .llseek = no_llseek,
        .read = NULL,
        .write = NULL,
};

static struct inode *libfs_get_inode(struct super_block *sb, umode_t mode) {
    struct inode *inode = new_inode(sb);
    if (!inode) return NULL;

    inode->i_ino = get_next_ino();
    inode->i_sb = sb;
    inode->i_op = &simple_dir_inode_operations;
    if (S_ISDIR(mode)) {
        inode->i_fop = &simple_dir_operations;
    } else {
        inode->i_fop = &libfs_file_ops;
    }
    inode->i_mode = mode;
    return inode;
}

static bool is_protected_dentry(struct dentry *dentry) {
    return (strcmp(dentry->d_name.name, "books") == 0 || strcmp(dentry->d_name.name, "authors") == 0);
}

struct cleanup_ctx {
    struct inode *victim;
};

static void libfs_cleanup_links(struct inode *victim)
{
    struct dentry *root = libfs_sb->s_root;

    struct qstr authors_q = QSTR_INIT("authors", 7);
    struct dentry *authors = d_lookup(root, &authors_q);
    if (!authors || d_really_is_negative(authors)) {
        dput(authors);
        return;
    }

    // Здесь конкретный пример: если ты сам знаешь имя линка на victim
    // То можешь через d_lookup() найти его и удалить
    struct qstr filename = QSTR_INIT("some_symlink", 12);
    struct dentry *maybe_link = d_lookup(authors, &filename);

    if (maybe_link && maybe_link->d_inode == victim) {
        inode_lock(authors->d_inode);
        vfs_unlink(NULL, authors->d_inode, maybe_link, NULL);
        inode_unlock(authors->d_inode);
    }

    dput(maybe_link);
    dput(authors);
}

static int libfs_unlink(struct inode *dir, struct dentry *dentry) {
    if (dir == libfs_sb->s_root->d_inode && is_protected_dentry(dentry))
        return -EPERM;

    struct inode *victim = dentry->d_inode;
    int ret = simple_unlink(dir, dentry);
    if (ret == 0 && dir == books_inode) {
        libfs_cleanup_links(victim);
    }
    return ret;
}

static int libfs_mkdir(struct mnt_idmap *idmap, struct inode *dir, struct dentry *dentry, umode_t mode) {
    if (dir == libfs_sb->s_root->d_inode) {
        return -EPERM;
    }
    struct inode *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);
    return 0;
}

static int libfs_rmdir(struct inode *dir, struct dentry *dentry) {
    if (dir == libfs_sb->s_root->d_inode && is_protected_dentry(dentry))
        return -EPERM;
    return simple_rmdir(dir, 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) {
    if ((old_dir == libfs_sb->s_root->d_inode || new_dir == libfs_sb->s_root->d_inode) &&
        (is_protected_dentry(old_dentry) || is_protected_dentry(new_dentry)))
        return -EPERM;
    return simple_rename(idmap, old_dir, old_dentry, new_dir, new_dentry, flags);
}

static int libfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_dentry) {
    if (dir->i_sb != old_dentry->d_inode->i_sb)
        return -EXDEV;
    if (!S_ISREG(old_dentry->d_inode->i_mode))
        return -EPERM;
    return simple_link(old_dentry, dir, new_dentry);
}

static const struct inode_operations libfs_dir_iops = {
        .lookup = simple_lookup,
        .mkdir = libfs_mkdir,
        .unlink = libfs_unlink,
        .rmdir = libfs_rmdir,
        .rename = libfs_rename,
        .link = libfs_link,
};

static int libfs_fill_super(struct super_block *sb, void *data, int silent) {
    struct inode *inode;
    struct dentry *root;
    libfs_sb = sb;
    sb->s_magic = LIBFS_MAGIC;
    sb->s_op = &libfs_sops;

    inode = libfs_get_inode(sb, S_IFDIR | 0755);
    if (!inode)
        return -ENOMEM;
    inode->i_op = &libfs_dir_iops;
    inode->i_fop = &simple_dir_operations;

    root = d_make_root(inode);
    if (!root)
        return -ENOMEM;
    sb->s_root = root;

    // /books
    {
        struct inode *books = libfs_get_inode(sb, S_IFDIR | LIBFS_BOOKS_MODE);
        books_inode = books;
        struct dentry *d = d_alloc_name(root, "books");
        d_add(d, books);
    }
    // /authors
    {
        struct inode *authors = libfs_get_inode(sb, S_IFDIR | LIBFS_AUTHORS_MODE);
        struct dentry *d = d_alloc_name(root, "authors");
        d_add(d, authors);
    }
    return 0;
}

static struct dentry *libfs_mount(struct file_system_type *fs_type, int flags, const char *dev_name, void *data) {
    return mount_nodev(fs_type, flags, data, libfs_fill_super);
}

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) {
    int ret = register_filesystem(&libfs_type);
    if (ret)
        return ret;
    libfs_mnt = kern_mount(&libfs_type);
    return PTR_ERR_OR_ZERO(libfs_mnt);
}

static void __exit libfs_exit(void) {
    kern_unmount(libfs_mnt);
    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");