// SPDX-License-Identifier: GPL-2.0

  /*
   *  linux/fs/filesystems.c
   *
   *  Copyright (C) 1991, 1992  Linus Torvalds
   *
   *  table of configured filesystems
   */
  
  #include <linux/syscalls.h>
  #include <linux/fs.h>

  #include <linux/proc_fs.h>
  #include <linux/seq_file.h>

  #include <linux/kmod.h>
  #include <linux/init.h>
  #include <linux/module.h>

  #include <linux/slab.h>

  #include <linux/uaccess.h>

  
  /*
   * Handling of filesystem drivers list.
   * Rules:
   *	Inclusion to/removals from/scanning of list are protected by spinlock.
   *	During the unload module must call unregister_filesystem().
   *	We can access the fields of list element if:
   *		1) spinlock is held or
   *		2) we hold the reference to the module.
   *	The latter can be guaranteed by call of try_module_get(); if it
   *	returned 0 we must skip the element, otherwise we got the reference.
   *	Once the reference is obtained we can drop the spinlock.
   */
  
  static struct file_system_type *file_systems;
  static DEFINE_RWLOCK(file_systems_lock);
  
  /* WARNING: This can be used only if we _already_ own a reference */

  struct file_system_type *get_filesystem(struct file_system_type *fs)

  {
  	__module_get(fs->owner);

  	return fs;

  }
  
  void put_filesystem(struct file_system_type *fs)
  {
  	module_put(fs->owner);
  }

  static struct file_system_type **find_filesystem(const char *name, unsigned len)

  {
  	struct file_system_type **p;

  	for (p = &file_systems; *p; p = &(*p)->next)
  		if (strncmp((*p)->name, name, len) == 0 &&
  		    !(*p)->name[len])

  			break;
  	return p;
  }
  
  /**
   *	register_filesystem - register a new filesystem
   *	@fs: the file system structure
   *
   *	Adds the file system passed to the list of file systems the kernel
   *	is aware of for mount and other syscalls. Returns 0 on success,
   *	or a negative errno code on an error.
   *
   *	The &struct file_system_type that is passed is linked into the kernel 
   *	structures and must not be freed until the file system has been
   *	unregistered.
   */
   
  int register_filesystem(struct file_system_type * fs)
  {
  	int res = 0;
  	struct file_system_type ** p;

  	BUG_ON(strchr(fs->name, '.'));

  	if (fs->next)
  		return -EBUSY;

  	write_lock(&file_systems_lock);

  	p = find_filesystem(fs->name, strlen(fs->name));

  	if (*p)
  		res = -EBUSY;
  	else
  		*p = fs;
  	write_unlock(&file_systems_lock);
  	return res;
  }
  
  EXPORT_SYMBOL(register_filesystem);
  
  /**
   *	unregister_filesystem - unregister a file system
   *	@fs: filesystem to unregister
   *
   *	Remove a file system that was previously successfully registered
   *	with the kernel. An error is returned if the file system is not found.
   *	Zero is returned on a success.
   *	
   *	Once this function has returned the &struct file_system_type structure
   *	may be freed or reused.
   */
   
  int unregister_filesystem(struct file_system_type * fs)
  {
  	struct file_system_type ** tmp;
  
  	write_lock(&file_systems_lock);
  	tmp = &file_systems;
  	while (*tmp) {
  		if (fs == *tmp) {
  			*tmp = fs->next;
  			fs->next = NULL;
  			write_unlock(&file_systems_lock);

  			synchronize_rcu();

  			return 0;
  		}
  		tmp = &(*tmp)->next;
  	}
  	write_unlock(&file_systems_lock);

  	return -EINVAL;
  }
  
  EXPORT_SYMBOL(unregister_filesystem);

  #ifdef CONFIG_SYSFS_SYSCALL

  static int fs_index(const char __user * __name)
  {
  	struct file_system_type * tmp;

  	struct filename *name;

  	int err, index;
  
  	name = getname(__name);
  	err = PTR_ERR(name);
  	if (IS_ERR(name))
  		return err;
  
  	err = -EINVAL;
  	read_lock(&file_systems_lock);
  	for (tmp=file_systems, index=0 ; tmp ; tmp=tmp->next, index++) {

  		if (strcmp(tmp->name, name->name) == 0) {

  			err = index;
  			break;
  		}
  	}
  	read_unlock(&file_systems_lock);
  	putname(name);
  	return err;
  }
  
  static int fs_name(unsigned int index, char __user * buf)
  {
  	struct file_system_type * tmp;
  	int len, res;
  
  	read_lock(&file_systems_lock);
  	for (tmp = file_systems; tmp; tmp = tmp->next, index--)
  		if (index <= 0 && try_module_get(tmp->owner))
  			break;
  	read_unlock(&file_systems_lock);
  	if (!tmp)
  		return -EINVAL;
  
  	/* OK, we got the reference, so we can safely block */
  	len = strlen(tmp->name) + 1;
  	res = copy_to_user(buf, tmp->name, len) ? -EFAULT : 0;
  	put_filesystem(tmp);
  	return res;
  }
  
  static int fs_maxindex(void)
  {
  	struct file_system_type * tmp;
  	int index;
  
  	read_lock(&file_systems_lock);
  	for (tmp = file_systems, index = 0 ; tmp ; tmp = tmp->next, index++)
  		;
  	read_unlock(&file_systems_lock);
  	return index;
  }
  
  /*
   * Whee.. Weird sysv syscall. 
   */

  SYSCALL_DEFINE3(sysfs, int, option, unsigned long, arg1, unsigned long, arg2)

  {
  	int retval = -EINVAL;
  
  	switch (option) {
  		case 1:
  			retval = fs_index((const char __user *) arg1);
  			break;
  
  		case 2:
  			retval = fs_name(arg1, (char __user *) arg2);
  			break;
  
  		case 3:
  			retval = fs_maxindex();
  			break;
  	}
  	return retval;
  }

  #endif

  int __init get_filesystem_list(char *buf)

  {
  	int len = 0;
  	struct file_system_type * tmp;
  
  	read_lock(&file_systems_lock);
  	tmp = file_systems;
  	while (tmp && len < PAGE_SIZE - 80) {
  		len += sprintf(buf+len, "%s\t%s
  ",
  			(tmp->fs_flags & FS_REQUIRES_DEV) ? "" : "nodev",
  			tmp->name);
  		tmp = tmp->next;
  	}
  	read_unlock(&file_systems_lock);
  	return len;
  }

  #ifdef CONFIG_PROC_FS
  static int filesystems_proc_show(struct seq_file *m, void *v)
  {
  	struct file_system_type * tmp;
  
  	read_lock(&file_systems_lock);
  	tmp = file_systems;
  	while (tmp) {
  		seq_printf(m, "%s\t%s
  ",
  			(tmp->fs_flags & FS_REQUIRES_DEV) ? "" : "nodev",
  			tmp->name);
  		tmp = tmp->next;
  	}
  	read_unlock(&file_systems_lock);
  	return 0;
  }
  
  static int filesystems_proc_open(struct inode *inode, struct file *file)
  {
  	return single_open(file, filesystems_proc_show, NULL);
  }
  
  static const struct file_operations filesystems_proc_fops = {
  	.open		= filesystems_proc_open,
  	.read		= seq_read,
  	.llseek		= seq_lseek,
  	.release	= single_release,
  };
  
  static int __init proc_filesystems_init(void)
  {
  	proc_create("filesystems", 0, NULL, &filesystems_proc_fops);
  	return 0;
  }
  module_init(proc_filesystems_init);
  #endif

  static struct file_system_type *__get_fs_type(const char *name, int len)

  {
  	struct file_system_type *fs;
  
  	read_lock(&file_systems_lock);

  	fs = *(find_filesystem(name, len));

  	if (fs && !try_module_get(fs->owner))
  		fs = NULL;
  	read_unlock(&file_systems_lock);

  	return fs;
  }
  
  struct file_system_type *get_fs_type(const char *name)
  {
  	struct file_system_type *fs;
  	const char *dot = strchr(name, '.');
  	int len = dot ? dot - name : strlen(name);
  
  	fs = __get_fs_type(name, len);

  	if (!fs && (request_module("fs-%.*s", len, name) == 0)) {

  		fs = __get_fs_type(name, len);

  		WARN_ONCE(!fs, "request_module fs-%.*s succeeded, but still no fs?
  ", len, name);
  	}

  
  	if (dot && fs && !(fs->fs_flags & FS_HAS_SUBTYPE)) {
  		put_filesystem(fs);
  		fs = NULL;
  	}

  	return fs;
  }
  
  EXPORT_SYMBOL(get_fs_type);