crypto: ecdsa - Use ecc_digits_from_bytes to convert signature

Since ecc_digits_from_bytes will provide zeros when an insufficient number
of bytes are passed in the input byte array, use it to convert the r and s
components of the signature to digits directly from the input byte
array. This avoids going through an intermediate byte array that has the
first few bytes filled with zeros.
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/ecdsa.c

@@ -38,7 +38,6 @@ static int ecdsa_get_signature_rs(u64 *dest, size_t hdrlen, unsigned char tag,
 	size_t bufsize = ndigits * sizeof(u64);
 	ssize_t diff = vlen - bufsize;
 	const char *d = value;
-	u8 rs[ECC_MAX_BYTES];
 
 	if (!value || !vlen)
 		return -EINVAL;
@@ -46,7 +45,7 @@ static int ecdsa_get_signature_rs(u64 *dest, size_t hdrlen, unsigned char tag,
 	/* diff = 0: 'value' has exacly the right size
 	 * diff > 0: 'value' has too many bytes; one leading zero is allowed that
 	 *           makes the value a positive integer; error on more
-	 * diff < 0: 'value' is missing leading zeros, which we add
+	 * diff < 0: 'value' is missing leading zeros
 	 */
 	if (diff > 0) {
 		/* skip over leading zeros that make 'value' a positive int */
@@ -61,14 +60,7 @@ static int ecdsa_get_signature_rs(u64 *dest, size_t hdrlen, unsigned char tag,
 	if (-diff >= bufsize)
 		return -EINVAL;
 
-	if (diff) {
-		/* leading zeros not given in 'value' */
-		memset(rs, 0, -diff);
-	}
-
-	memcpy(&rs[-diff], d, vlen);
-
-	ecc_swap_digits((u64 *)rs, dest, ndigits);
+	ecc_digits_from_bytes(d, vlen, dest, ndigits);
 
 	return 0;
 }