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

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

EXPORT_SYMBOL(get_fs_type);