filesystems.c

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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 (strlen((*p)->name) == len &&
            strncmp((*p)->name, name, len) == 0)
            break;
    return p;
}

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);

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);

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;
}

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("%.*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);