CRC-16

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#CRC-16

CRC-16是一种广泛使用的错误检测码，常用于数据通信和存储中的错误检测。本文深入探讨了CRC-16的工作原理，包括生成多项式选择、计算过程和校验码的生成。通过实例解析，了解如何应用CRC-16进行数据完整性校验，确保数据在传输或存储过程中不被篡改。

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/*
 * T10 Data Integrity Field CRC16 calculation
 *
 * Copyright (c) 2007 Oracle Corporation.  All rights reserved.
 * Written by Martin K. Petersen <martin.petersen@oracle.com>
 *
 * This source code is licensed under the GNU General Public License,
 * Version 2. See the file COPYING for more details.
 */

#include <linux/types.h>
#include <linux/module.h>
#include <linux/crc-t10dif.h>
#include <linux/err.h>
#include <linux/init.h>
#include <crypto/hash.h>
#include <crypto/algapi.h>
#include <linux/static_key.h>
#include <linux/notifier.h>

static struct crypto_shash __rcu *crct10dif_tfm;
static struct static_key crct10dif_fallback __read_mostly;
static DEFINE_MUTEX(crc_t10dif_mutex);

static int crc_t10dif_rehash(struct notifier_block *self, unsigned long val, void *data)
{
	struct crypto_alg *alg = data;
	struct crypto_shash *new, *old;

	if (val != CRYPTO_MSG_ALG_LOADED ||
	    static_key_false(&crct10dif_fallback) ||
	    strncmp(alg->cra_name, CRC_T10DIF_STRING, strlen(CRC_T10DIF_STRING)))
		return 0;

	mutex_lock(&crc_t10dif_mutex);
	old = rcu_dereference_protected(crct10dif_tfm,
					lockdep_is_held(&crc_t10dif_mutex));
	if (!old) {
		mutex_unlock(&crc_t10dif_mutex);
		return 0;
	}
	new = crypto_alloc_shash("crct10dif", 0, 0);
	if (IS_ERR(new)) {
		mutex_unlock(&crc_t10dif_mutex);
		return 0;
	}
	rcu_assign_pointer(crct10dif_tfm, new);
	mutex_unlock(&crc_t10dif_mutex);

	synchronize_rcu();
	crypto_free_shash(old);
	return 0;
}

static struct notifier_block crc_t10dif_nb = {
	.notifier_call = crc_t10dif_rehash,
};

__u16 crc_t10dif_update(__u16 crc, const unsigned char *buffer, size_t len)
{
	struct {
		struct shash_desc shash;
		char ctx[2];
	} desc;
	int err;

	if (static_key_false(&crct10dif_fallback))
		return crc_t10dif_generic(crc, buffer, len);

	rcu_read_lock();
	desc.shash.tfm = rcu_dereference(crct10dif_tfm);
	desc.shash.flags = 0;
	*(__u16 *)desc.ctx = crc;

	err = crypto_shash_update(&desc.shash, buffer, len);
	rcu_read_unlock();

	BUG_ON(err);

	return *(__u16 *)desc.ctx;
}
EXPORT_SYMBOL(crc_t10dif_update);

__u16 crc_t10dif(const unsigned char *buffer, size_t len)
{
	return crc_t10dif_update(0, buffer, len);
}
EXPORT_SYMBOL(crc_t10dif);

static int __init crc_t10dif_mod_init(void)
{
	crypto_register_notifier(&crc_t10dif_nb);
	crct10dif_tfm = crypto_alloc_shash("crct10dif", 0, 0);
	if (IS_ERR(crct10dif_tfm)) {
		static_key_slow_inc(&crct10dif_fallback);
		crct10dif_tfm = NULL;
	}
	return 0;
}

static void __exit crc_t10dif_mod_fini(void)
{
	crypto_unregister_notifier(&crc_t10dif_nb);
	crypto_free_shash(crct10dif_tfm);
}

module_init(crc_t10dif_mod_init);
module_exit(crc_t10dif_mod_fini);

static int crc_t10dif_transform_show(char *buffer, const struct kernel_param *kp)
{
	if (static_key_false(&crct10dif_fallback))
		return sprintf(buffer, "fallback\n");

	return sprintf(buffer, "%s\n",
		crypto_tfm_alg_driver_name(crypto_shash_tfm(crct10dif_tfm)));
}

module_param_call(transform, NULL, crc_t10dif_transform_show, NULL, 0644);

MODULE_DESCRIPTION("T10 DIF CRC calculation");
MODULE_LICENSE("GPL");
MODULE_SOFTDEP("pre: crct10dif");

/*
 *	crc16.h - CRC-16 routine
 *
 * Implements the standard CRC-16:
 *   Width 16
 *   Poly  0x8005 (x^16 + x^15 + x^2 + 1)
 *   Init  0
 *
 * Copyright (c) 2005 Ben Gardner <bgardner@wabtec.com>
 *
 * This source code is licensed under the GNU General Public License,
 * Version 2. See the file COPYING for more details.
 */

#ifndef __CRC16_H
#define __CRC16_H

#include <linux/types.h>

extern u16 const crc16_table[256];

extern u16 crc16(u16 crc, const u8 *buffer, size_t len);

static inline u16 crc16_byte(u16 crc, const u8 data)
{
	return (crc >> 8) ^ crc16_table[(crc ^ data) & 0xff];
}

#endif /* __CRC16_H */