crypto: null - Add default null skcipher

This patch adds a default null skcipher for users such as gcm
to perform copies on SG lists.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/crypto_null.c

@@ -25,6 +25,10 @@
 #include <linux/mm.h>
 #include <linux/string.h>
 
+static DEFINE_MUTEX(crypto_default_null_skcipher_lock);
+static struct crypto_blkcipher *crypto_default_null_skcipher;
+static int crypto_default_null_skcipher_refcnt;
+
 static int null_compress(struct crypto_tfm *tfm, const u8 *src,
 			 unsigned int slen, u8 *dst, unsigned int *dlen)
 {
@@ -149,6 +153,41 @@ MODULE_ALIAS_CRYPTO("compress_null");
 MODULE_ALIAS_CRYPTO("digest_null");
 MODULE_ALIAS_CRYPTO("cipher_null");
 
+struct crypto_blkcipher *crypto_get_default_null_skcipher(void)
+{
+	struct crypto_blkcipher *tfm;
+
+	mutex_lock(&crypto_default_null_skcipher_lock);
+	tfm = crypto_default_null_skcipher;
+
+	if (!tfm) {
+		tfm = crypto_alloc_blkcipher("ecb(cipher_null)", 0, 0);
+		if (IS_ERR(tfm))
+			goto unlock;
+
+		crypto_default_null_skcipher = tfm;
+	}
+
+	crypto_default_null_skcipher_refcnt++;
+
+unlock:
+	mutex_unlock(&crypto_default_null_skcipher_lock);
+
+	return tfm;
+}
+EXPORT_SYMBOL_GPL(crypto_get_default_null_skcipher);
+
+void crypto_put_default_null_skcipher(void)
+{
+	mutex_lock(&crypto_default_null_skcipher_lock);
+	if (!--crypto_default_null_skcipher_refcnt) {
+		crypto_free_blkcipher(crypto_default_null_skcipher);
+		crypto_default_null_skcipher = NULL;
+	}
+	mutex_unlock(&crypto_default_null_skcipher_lock);
+}
+EXPORT_SYMBOL_GPL(crypto_put_default_null_skcipher);
+
 static int __init crypto_null_mod_init(void)
 {
 	int ret = 0;

include/crypto/null.h

@@ -8,4 +8,7 @@
 #define NULL_DIGEST_SIZE	0
 #define NULL_IV_SIZE		0
 
+struct crypto_blkcipher *crypto_get_default_null_skcipher(void);
+void crypto_put_default_null_skcipher(void);
+
 #endif