root/fs/dcookies.c

/*
 * dcookies.c
 *
 * Copyright 2002 John Levon <levon@movementarian.org>
 *
 * Persistent cookie-path mappings. These are used by
 * profilers to convert a per-task EIP value into something
 * non-transitory that can be processed at a later date.
 * This is done by locking the dentry/vfsmnt pair in the
 * kernel until released by the tasks needing the persistent
 * objects. The tag is simply an unsigned long that refers
 * to the pair and can be looked up from userspace.
 */

#include <linux/config.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/mount.h>
#include <linux/dcache.h>
#include <linux/mm.h>
#include <linux/errno.h>
#include <linux/dcookies.h>
#include <asm/uaccess.h>

/* The dcookies are allocated from a kmem_cache and
 * hashed onto a small number of lists. None of the
 * code here is particularly performance critical
 */
struct dcookie_struct {
        struct dentry * dentry;
        struct vfsmount * vfsmnt;
        struct list_head hash_list;
};

static LIST_HEAD(dcookie_users);
static DECLARE_MUTEX(dcookie_sem);
static kmem_cache_t * dcookie_cache;
static struct list_head * dcookie_hashtable;
static size_t hash_size;

static inline int is_live(void)
{
        return !(list_empty(&dcookie_users));
}


/* The dentry is locked, its address will do for the cookie */
static inline unsigned long dcookie_value(struct dcookie_struct * dcs)
{
        return (unsigned long)dcs->dentry;
}


static size_t dcookie_hash(unsigned long dcookie)
{
        return (dcookie >> L1_CACHE_SHIFT) & (hash_size - 1);
}


static struct dcookie_struct * find_dcookie(unsigned long dcookie)
{
        struct dcookie_struct * found = 0;
        struct dcookie_struct * dcs;
        struct list_head * pos;
        struct list_head * list;

        list = dcookie_hashtable + dcookie_hash(dcookie);

        list_for_each(pos, list) {
                dcs = list_entry(pos, struct dcookie_struct, hash_list);
                if (dcookie_value(dcs) == dcookie) {
                        found = dcs;
                        break;
                }
        }

        return found;
}


static void hash_dcookie(struct dcookie_struct * dcs)
{
        struct list_head * list = dcookie_hashtable + dcookie_hash(dcookie_value(dcs));
        list_add(&dcs->hash_list, list);
}


static struct dcookie_struct * alloc_dcookie(struct dentry * dentry,
        struct vfsmount * vfsmnt)
{
        struct dcookie_struct * dcs = kmem_cache_alloc(dcookie_cache, GFP_KERNEL);
        if (!dcs)
                return NULL;

        atomic_inc(&dentry->d_count);
        atomic_inc(&vfsmnt->mnt_count);
        dentry->d_cookie = dcs;

        dcs->dentry = dentry;
        dcs->vfsmnt = vfsmnt;
        hash_dcookie(dcs);

        return dcs;
}


/* This is the main kernel-side routine that retrieves the cookie
 * value for a dentry/vfsmnt pair.
 */
int get_dcookie(struct dentry * dentry, struct vfsmount * vfsmnt,
        unsigned long * cookie)
{
        int err = 0;
        struct dcookie_struct * dcs;

        down(&dcookie_sem);

        if (!is_live()) {
                err = -EINVAL;
                goto out;
        }

        dcs = dentry->d_cookie;

        if (!dcs)
                dcs = alloc_dcookie(dentry, vfsmnt);

        if (!dcs) {
                err = -ENOMEM;
                goto out;
        }

        *cookie = dcookie_value(dcs);

out:
        up(&dcookie_sem);
        return err;
}


/* And here is where the userspace process can look up the cookie value
 * to retrieve the path.
 */
asmlinkage long sys_lookup_dcookie(u64 cookie64, char __user * buf, size_t len)
{
        unsigned long cookie = (unsigned long)cookie64;
        int err = -EINVAL;
        char * kbuf;
        char * path;
        size_t pathlen;
        struct dcookie_struct * dcs;

        /* we could leak path information to users
         * without dir read permission without this
         */
        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;

        down(&dcookie_sem);

        if (!is_live()) {
                err = -EINVAL;
                goto out;
        }

        if (!(dcs = find_dcookie(cookie)))
                goto out;

        err = -ENOMEM;
        kbuf = kmalloc(PAGE_SIZE, GFP_KERNEL);
        if (!kbuf)
                goto out;

        /* FIXME: (deleted) ? */
        path = d_path(dcs->dentry, dcs->vfsmnt, kbuf, PAGE_SIZE);

        if (IS_ERR(path)) {
                err = PTR_ERR(path);
                goto out_free;
        }

        err = -ERANGE;
 
        pathlen = kbuf + PAGE_SIZE - path;
        if (pathlen <= len) {
                err = pathlen;
                if (copy_to_user(buf, path, pathlen))
                        err = -EFAULT;
        }

out_free:
        kfree(kbuf);
out:
        up(&dcookie_sem);
        return err;
}


static int dcookie_init(void)
{
        struct list_head * d;
        unsigned int i, hash_bits;
        int err = -ENOMEM;

        dcookie_cache = kmem_cache_create("dcookie_cache",
                sizeof(struct dcookie_struct),
                0, 0, NULL, NULL);

        if (!dcookie_cache)
                goto out;

        dcookie_hashtable = kmalloc(PAGE_SIZE, GFP_KERNEL);
        if (!dcookie_hashtable)
                goto out_kmem;

        err = 0;

        /*
         * Find the power-of-two list-heads that can fit into the allocation..
         * We don't guarantee that "sizeof(struct list_head)" is necessarily
         * a power-of-two.
         */
        hash_size = PAGE_SIZE / sizeof(struct list_head);
        hash_bits = 0;
        do {
                hash_bits++;
        } while ((hash_size >> hash_bits) != 0);
        hash_bits--;

        /*
         * Re-calculate the actual number of entries and the mask
         * from the number of bits we can fit.
         */
        hash_size = 1UL << hash_bits;

        /* And initialize the newly allocated array */
        d = dcookie_hashtable;
        i = hash_size;
        do {
                INIT_LIST_HEAD(d);
                d++;
                i--;
        } while (i);

out:
        return err;
out_kmem:
        kmem_cache_destroy(dcookie_cache);
        goto out;
}


static void free_dcookie(struct dcookie_struct * dcs)
{
        dcs->dentry->d_cookie = NULL;
        dput(dcs->dentry);
        mntput(dcs->vfsmnt);
        kmem_cache_free(dcookie_cache, dcs);
}


static void dcookie_exit(void)
{
        struct list_head * list;
        struct list_head * pos;
        struct list_head * pos2;
        struct dcookie_struct * dcs;
        size_t i;

        for (i = 0; i < hash_size; ++i) {
                list = dcookie_hashtable + i;
                list_for_each_safe(pos, pos2, list) {
                        dcs = list_entry(pos, struct dcookie_struct, hash_list);
                        list_del(&dcs->hash_list);
                        free_dcookie(dcs);
                }
        }

        kfree(dcookie_hashtable);
        kmem_cache_destroy(dcookie_cache);
}


struct dcookie_user {
        struct list_head next;
};
 
struct dcookie_user * dcookie_register(void)
{
        struct dcookie_user * user;

        down(&dcookie_sem);

        user = kmalloc(sizeof(struct dcookie_user), GFP_KERNEL);
        if (!user)
                goto out;

        if (!is_live() && dcookie_init())
                goto out_free;

        list_add(&user->next, &dcookie_users);

out:
        up(&dcookie_sem);
        return user;
out_free:
        kfree(user);
        user = NULL;
        goto out;
}


void dcookie_unregister(struct dcookie_user * user)
{
        down(&dcookie_sem);

        list_del(&user->next);
        kfree(user);

        if (!is_live())
                dcookie_exit();

        up(&dcookie_sem);
}

EXPORT_SYMBOL_GPL(dcookie_register);
EXPORT_SYMBOL_GPL(dcookie_unregister);
EXPORT_SYMBOL_GPL(get_dcookie);