#include <linux/module.h>
#include <linux/fs.h>
#include <linux/fs_context.h>
#include <linux/file.h>
#include <linux/init.h>
#include <linux/pagemap.h>
#include <linux/namei.h>
#include <linux/mount.h>
#include <linux/dcache.h>
#include <linux/slab.h>
#include <linux/hashtable.h>
#include <linux/version.h>

#define FS_NAME "customfs"
#define FS_MAGIC 0x20240615
#define MODE_BOOKS 01777
#define MODE_AUTHORS 0777
#define MODE_AUTHOR_SUBDIR 01777
#define logf(fmt, ...)                                 \
    do {                                               \
        printk(KERN_INFO "customfs: " fmt "\n", ##__VA_ARGS__); \
        struct file *f = filp_open("/home/ubuntu/FS/customfs.log", O_WRONLY | O_CREAT | O_APPEND, 0644); \
        if (!IS_ERR(f)) {                              \
            char __buf[256];                           \
            int __len = snprintf(__buf, sizeof(__buf), fmt "\n", ##__VA_ARGS__); \
            kernel_write(f, __buf, __len, &f->f_pos);   \
            filp_close(f, NULL);                        \
        }                                              \
    } while (0)


MODULE_LICENSE("GPL");
MODULE_AUTHOR("you");
MODULE_DESCRIPTION("Custom FS with books/authors behavior");

static int custom_unlink(struct inode *dir, struct dentry *dentry);

static struct super_block *global_sb;
static struct inode *books_inode;
static const struct inode_operations fs_dir_iops;
DEFINE_HASHTABLE(link_table, 6);

static const struct inode_operations custom_file_inode_operations = {};

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

static ssize_t dummy_read(struct file *file, char __user *buf, size_t len, loff_t *ppos) {
    return 0;
}

#if 0
static ssize_t dummy_write(struct file *file, const char __user *buf, size_t len, loff_t *ppos) {
    return len;
}
#endif
static ssize_t dummy_write(struct file *file, const char __user *buf, size_t len, loff_t *ppos) {
    struct inode *inode = file_inode(file);
    loff_t new_end = *ppos + len;

    if (inode->i_size < new_end)
        inode->i_size = new_end;

    *ppos = new_end;
    mark_inode_dirty(inode);
    return len;
}

static const struct file_operations custom_file_ops = {
    .read = dummy_read,
    .write = dummy_write,
    .llseek = noop_llseek,
};

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

#if 0
static struct inode *get_inode(struct super_block *sb, umode_t mode) {
    struct inode *inode = new_inode(sb);
    if (!inode)
        return NULL;

    inode_init_owner(&init_user_ns, inode, NULL, mode);
    inode->i_ino = get_next_ino();
    inode->i_sb = sb;
    inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);

    if (S_ISDIR(mode)) {
        inode->i_op = &simple_dir_inode_operations;
        inode->i_fop = &simple_dir_operations;
        inc_nlink(inode);
    } else if (S_ISREG(mode)) {
        inode->i_fop = &custom_file_ops;
    }
    return inode;
}
#endif

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

    inode_init_owner(&init_user_ns, inode, NULL, mode);
    inode->i_ino = get_next_ino();
    inode->i_sb = sb;
    inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
    inode->i_mode = mode;
    return inode;
}

#if 0
static int books_create(struct user_namespace *ns, struct inode *dir,
                        struct dentry *dentry, umode_t mode, bool excl) {
    struct inode *inode = get_inode(dir->i_sb, S_IFREG | mode);
    if (!inode)
        return -ENOMEM;

    inode->i_uid = current_fsuid();
    d_add(dentry, inode);
    return 0;
}
#endif
static int books_create(struct user_namespace *ns, struct inode *dir,
                        struct dentry *dentry, umode_t mode, bool excl) {
    struct inode *inode = get_inode(dir->i_sb, S_IFREG | mode);
    if (!inode)
        return -ENOMEM;

    inode->i_uid = current_fsuid();
    inode->i_op = &custom_file_inode_operations;
    inode->i_fop = &custom_file_ops;
    logf("books_create: dentry addr=%px, inode=%px", dentry, inode);
    d_instantiate(dentry, inode);
    inc_nlink(inode);
    logf("books_create: d_instantiate done, dentry->d_inode=%px", dentry->d_inode);
    logf("dentry flags: %u, dentry name: %s", dentry->d_flags, dentry->d_name.name);
    mark_inode_dirty(inode);
    logf("books_create: created file %s (ino=%lu)", dentry->d_name.name, inode->i_ino);
    return 0;
}

#if 0
static struct dentry *books_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags) {
    logf("books_lookup: looking for %s in ino=%lu", dentry->d_name.name, dir->i_ino);
    return simple_lookup(dir, dentry, flags);
}
#endif
static struct dentry *books_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags) {
    struct dentry *child;

    logf("books_lookup: looking for %s in ino=%lu", dentry->d_name.name, dir->i_ino);

    child = d_lookup(dentry->d_parent, &dentry->d_name);
    if (child) {
        d_add(dentry, d_inode(child));  // связываем найденное inode
        dput(child);
    } else {
        d_add(dentry, NULL);  // файл не найден
    }

    return NULL; // всегда возвращается NULL в lookup
}

static const struct inode_operations books_dir_iops = {
    .lookup = books_lookup,
    .create = books_create,
    .unlink = custom_unlink,
};

static void remove_links_to_inode(struct inode *inode) {
    int bkt;
    struct link_entry *entry;
    struct hlist_node *tmp;
    logf("remove_links_to_inode: start for inode %lu", inode->i_ino);

    hash_for_each_safe(link_table, bkt, tmp, entry, node) {
        if (entry->target == inode) {
            logf("removing link: dentry=%s", entry->link->d_name.name);
            d_delete(entry->link);
            dput(entry->link);
            hash_del(&entry->node);
            kfree(entry);
        }
    }
}

#if 0
static int custom_mkdir(struct user_namespace *ns, struct inode *dir, struct dentry *dentry, umode_t mode) {
    umode_t mode_only = dir->i_mode & 07777;
    kuid_t uid = current_fsuid();

    logf("mkdir: parent ino=%lu, mode=%o, uid=%u, name=%s",
         dir->i_ino, mode_only, from_kuid(&init_user_ns, uid), dentry->d_name.name);

    if (dir == global_sb->s_root->d_inode) {
        logf("mkdir denied: attempt to create in root");
        return -EPERM;
    }

    // В authors (0777): любой пользователь может создать свою папку
    if (mode_only == MODE_AUTHORS) {
        logf("mkdir in authors: allowed");
    }

    // В подкаталогах authors (01777): только владелец каталога может создавать
    else if (mode_only == MODE_AUTHOR_SUBDIR && !uid_eq(uid, dir->i_uid)) {
        logf("mkdir denied in subdir: uid mismatch (dir uid=%u)",
             from_kuid(&init_user_ns, dir->i_uid));
        return -EACCES;
    }

    struct inode *inode = get_inode(dir->i_sb, S_IFDIR | MODE_AUTHOR_SUBDIR);
    if (!inode) {
        logf("mkdir error: get_inode returned NULL");
        return -ENOMEM;
    }
    
    inode->i_uid = uid;
    inode->i_op = &fs_dir_iops;
    inode->i_fop = &simple_dir_operations;
    
    d_instantiate(dentry, inode);
    inc_nlink(dir);
    
    logf("mkdir success: created %s in ino=%lu by uid=%u",
         dentry->d_name.name, dir->i_ino, from_kuid(&init_user_ns, uid));
    return 0;
}
#endif
static int custom_mkdir(struct user_namespace *ns, struct inode *dir, struct dentry *dentry, umode_t mode) {
    umode_t mode_only = dir->i_mode & 07777;
    kuid_t uid = current_fsuid();

    logf("mkdir: parent ino=%lu, mode=%o, uid=%u, name=%s",
         dir->i_ino, mode_only, from_kuid(&init_user_ns, uid), dentry->d_name.name);

    if (dir == global_sb->s_root->d_inode) {
        logf("mkdir denied: attempt to create in root");
        return -EPERM;
    }

    if (mode_only == MODE_AUTHORS) {
        logf("mkdir in authors: allowed");
    } else if (mode_only == MODE_AUTHOR_SUBDIR && !uid_eq(uid, dir->i_uid)) {
        logf("mkdir denied in subdir: uid mismatch (dir uid=%u)",
             from_kuid(&init_user_ns, dir->i_uid));
        return -EACCES;
    }

    struct inode *inode = get_inode(dir->i_sb, S_IFDIR | MODE_AUTHOR_SUBDIR);
    if (!inode) {
        logf("mkdir error: get_inode returned NULL");
        return -ENOMEM;
    }

    inode->i_uid = uid;
    inode->i_op = &fs_dir_iops;
    inode->i_fop = &simple_dir_operations;

    d_instantiate(dentry, inode);
    inc_nlink(inode);
    inc_nlink(dir);

    logf("mkdir success: created %s in ino=%lu by uid=%u",
         dentry->d_name.name, dir->i_ino, from_kuid(&init_user_ns, uid));
    return 0;
}

static int custom_unlink(struct inode *dir, struct dentry *dentry) {
    logf("custom_unlink: dir ino=%lu, file=%s", dir->i_ino, dentry->d_name.name);

    if (dir == global_sb->s_root->d_inode && is_protected_dentry(dentry))
        return -EPERM;

    if (dir == books_inode) {
        logf("custom_unlink: removing links to inode %lu", d_inode(dentry)->i_ino);
        remove_links_to_inode(d_inode(dentry));
    }

    return simple_unlink(dir, dentry);
}

static int custom_rmdir(struct inode *dir, struct dentry *dentry) {
    if (dir == global_sb->s_root->d_inode && is_protected_dentry(dentry))
        return -EPERM;
    return simple_rmdir(dir, dentry);
}

static int custom_rename(struct user_namespace *ns,
                         struct inode *old_dir, struct dentry *old_dentry,
                         struct inode *new_dir, struct dentry *new_dentry,
                         unsigned int flags) {
    if ((old_dir == global_sb->s_root->d_inode || new_dir == global_sb->s_root->d_inode) &&
        (is_protected_dentry(old_dentry) || is_protected_dentry(new_dentry)))
        return -EPERM;
    return simple_rename(ns, old_dir, old_dentry, new_dir, new_dentry, flags);
}

static int custom_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;

    int r = simple_link(old_dentry, dir, new_dentry);
    if (r == 0) {
        struct link_entry *e = kmalloc(sizeof(*e), GFP_KERNEL);
        if (!e)
            return -ENOMEM;
        e->target = d_inode(old_dentry);
        e->link = dget(new_dentry);
        hash_add(link_table, &e->node, e->target->i_ino);
    }
    return r;
}

static struct dentry *fs_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags) {
    struct dentry *found;

    logf("fs_lookup: looking for %s in ino=%lu", dentry->d_name.name, dir->i_ino);

    found = d_lookup(dentry->d_parent, &dentry->d_name);
    if (found) {
        d_add(dentry, d_inode(found));
        dput(found);
    } else {
        d_add(dentry, NULL);
    }

    return NULL;
}

#if 0
static const struct inode_operations fs_dir_iops = {
    .lookup = simple_lookup,
    .mkdir = custom_mkdir,
    .unlink = custom_unlink,
    .rmdir = custom_rmdir,
    .rename = custom_rename,
    .link = custom_link,
};
#endif
static const struct inode_operations fs_dir_iops = {
    .lookup = fs_lookup,
    .mkdir = custom_mkdir,
    .unlink = custom_unlink,
    .rmdir = custom_rmdir,
    .rename = custom_rename,
    .link = custom_link,
};

static void evict_inode(struct inode *inode) {
    clear_inode(inode);
}

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

#if 0
static int fill_super(struct super_block *sb, void *data, int silent) {
    struct inode *root_inode = get_inode(sb, S_IFDIR | 0755);
    if (!root_inode)
        return -ENOMEM;

    struct dentry *root_dentry = d_make_root(root_inode);
    if (!root_dentry)
        return -ENOMEM;

    sb->s_root = root_dentry;
    sb->s_magic = FS_MAGIC;
    sb->s_op = &fs_sops;
    global_sb = sb;

    // books
    books_inode = get_inode(sb, S_IFDIR | MODE_BOOKS);
    if (!books_inode)
        return -ENOMEM;
    books_inode->i_op = &books_dir_iops;
    books_inode->i_fop = &simple_dir_operations;
    struct dentry *books = d_alloc_name(root_dentry, "books");
    d_add(books, books_inode);

    // authors
    struct inode *authors_inode = get_inode(sb, S_IFDIR | MODE_AUTHORS);
    if (!authors_inode)
        return -ENOMEM;
    authors_inode->i_uid = current_fsuid();
    authors_inode->i_op = &fs_dir_iops;
    authors_inode->i_fop = &simple_dir_operations;
    struct dentry *authors = d_alloc_name(root_dentry, "authors");
    d_add(authors, authors_inode);

    return 0;
}
#endif
static int fill_super(struct super_block *sb, void *data, int silent) {
    struct inode *root_inode = get_inode(sb, S_IFDIR | 0755);
    if (!root_inode)
        return -ENOMEM;

    root_inode->i_op = &fs_dir_iops;
    root_inode->i_fop = &simple_dir_operations;

    struct dentry *root_dentry = d_make_root(root_inode);
    if (!root_dentry)
        return -ENOMEM;

    sb->s_root = root_dentry;
    sb->s_magic = FS_MAGIC;
    sb->s_op = &fs_sops;
    global_sb = sb;

    // books
    books_inode = get_inode(sb, S_IFDIR | MODE_BOOKS);
    if (!books_inode)
        return -ENOMEM;
    books_inode->i_op = &books_dir_iops;
    books_inode->i_fop = &simple_dir_operations;
    books_inode->i_mode = S_IFDIR | 01777;
    struct dentry *books = d_alloc_name(root_dentry, "books");
    if (!books)
        return -ENOMEM;
    d_add(books, books_inode);
    inc_nlink(root_inode);        
    inc_nlink(books_inode);        
    logf("fill_super: books directory instantiated");

    // authors
    struct inode *authors_inode = get_inode(sb, S_IFDIR | MODE_AUTHORS);
    if (!authors_inode)
        return -ENOMEM;
    authors_inode->i_uid = current_fsuid();
    authors_inode->i_op = &fs_dir_iops;
    authors_inode->i_fop = &simple_dir_operations;
    struct dentry *authors = d_alloc_name(root_dentry, "authors");
    if (!authors)
        return -ENOMEM;
    d_add(authors, authors_inode);
    inc_nlink(root_inode);        
    inc_nlink(authors_inode);    
    logf("fill_super: authors directory instantiated");

    logf("fill_super: books and authors created");
    return 0;
}

static struct dentry *mount_fs(struct file_system_type *type, int flags, const char *dev, void *data) {
    return mount_nodev(type, flags, data, fill_super);
}

static struct file_system_type fs_type = {
    .owner = THIS_MODULE,
    .name = FS_NAME,
    .mount = mount_fs,
    .kill_sb = kill_litter_super,
};

static int __init fs_init(void) {
    hash_init(link_table);
    return register_filesystem(&fs_type);
}

static void __exit fs_exit(void) {
    unregister_filesystem(&fs_type);
}

module_init(fs_init);
module_exit(fs_exit);