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    /*
     *  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 <asm/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 */
    void get_filesystem(struct file_system_type *fs)
    {
    	__module_get(fs->owner);
    }
    
    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\n",
    			(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\n",
    			(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);
    
    	if (dot && fs && !(fs->fs_flags & FS_HAS_SUBTYPE)) {
    		put_filesystem(fs);
    		fs = NULL;
    	}
    	return fs;
    }
    
    EXPORT_SYMBOL(get_fs_type);