linux/fs/dcookies.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * 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/syscalls.h>
#include <linux/export.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/mount.h>
#include <linux/capability.h>
#include <linux/dcache.h>
#include <linux/mm.h>
#include <linux/err.h>
#include <linux/errno.h>
#include <linux/dcookies.h>
#include <linux/mutex.h>
#include <linux/path.h>
#include <linux/compat.h>
#include <linux/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 path path;
	struct list_head hash_list;
};

static LIST_HEAD(dcookie_users);
static DEFINE_MUTEX(dcookie_mutex);
static struct kmem_cache *dcookie_cache __read_mostly;
static struct list_head *dcookie_hashtable __read_mostly;
static size_t hash_size __read_mostly;

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->path.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 = NULL;
	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(const struct path *path)
{
	struct dcookie_struct *dcs = kmem_cache_alloc(dcookie_cache,
							GFP_KERNEL);
	struct dentry *d;
	if (!dcs)
		return NULL;

	d = path->dentry;
	spin_lock(&d->d_lock);
	d->d_flags |= DCACHE_COOKIE;
	spin_unlock(&d->d_lock);

	dcs->path = *path;
	path_get(path);
	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(const struct path *path, unsigned long *cookie)
{
	int err = 0;
	struct dcookie_struct * dcs;

	mutex_lock(&dcookie_mutex);

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

	if (path->dentry->d_flags & DCACHE_COOKIE) {
		dcs = find_dcookie((unsigned long)path->dentry);
	} else {
		dcs = alloc_dcookie(path);
		if (!dcs) {
			err = -ENOMEM;
			goto out;
		}
	}

	*cookie = dcookie_value(dcs);

out:
	mutex_unlock(&dcookie_mutex);
	return err;
}


/* And here is where the userspace process can look up the cookie value
 * to retrieve the path.
 */
static int do_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;

	mutex_lock(&dcookie_mutex);

	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->path, kbuf, PAGE_SIZE);

	mutex_unlock(&dcookie_mutex);

	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);
	return err;
out:
	mutex_unlock(&dcookie_mutex);
	return err;
}

SYSCALL_DEFINE3(lookup_dcookie, u64, cookie64, char __user *, buf, size_t, len)
{
	return do_lookup_dcookie(cookie64, buf, len);
}

#ifdef CONFIG_COMPAT
COMPAT_SYSCALL_DEFINE4(lookup_dcookie, u32, w0, u32, w1, char __user *, buf, compat_size_t, len)
{
#ifdef __BIG_ENDIAN
	return do_lookup_dcookie(((u64)w0 << 32) | w1, buf, len);
#else
	return do_lookup_dcookie(((u64)w1 << 32) | w0, buf, len);
#endif
}
#endif

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);

	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)
{
	struct dentry *d = dcs->path.dentry;

	spin_lock(&d->d_lock);
	d->d_flags &= ~DCACHE_COOKIE;
	spin_unlock(&d->d_lock);

	path_put(&dcs->path);
	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;

	mutex_lock(&dcookie_mutex);

	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:
	mutex_unlock(&dcookie_mutex);
	return user;
out_free:
	kfree(user);
	user = NULL;
	goto out;
}


void dcookie_unregister(struct dcookie_user * user)
{
	mutex_lock(&dcookie_mutex);

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

	if (!is_live())
		dcookie_exit();

	mutex_unlock(&dcookie_mutex);
}

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