mpi: introduce mpi_key_length()

hulk inclusion
category: feature
feature: IMA Digest Lists extension
bugzilla: 46797

-------------------------------------------------

Introduce the new function to get the number of bits and bytes from an MPI.
Signed-off-by: NRoberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: NTianxing Zhang <zhangtianxing3@huawei.com>
Reviewed-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

include/linux/mpi.h

@@ -90,6 +90,8 @@ enum gcry_mpi_format {
 };
 
 MPI mpi_read_raw_data(const void *xbuffer, size_t nbytes);
+int mpi_key_length(const void *xbuffer, unsigned int ret_nread,
+		   unsigned int *nbits_arg, unsigned int *nbytes_arg);
 MPI mpi_read_from_buffer(const void *buffer, unsigned *ret_nread);
 int mpi_fromstr(MPI val, const char *str);
 MPI mpi_scanval(const char *string);

lib/mpi/mpicoder.c

@@ -79,22 +79,41 @@ MPI mpi_read_raw_data(const void *xbuffer, size_t nbytes)
 }
 EXPORT_SYMBOL_GPL(mpi_read_raw_data);
 
-MPI mpi_read_from_buffer(const void *xbuffer, unsigned *ret_nread)
+int mpi_key_length(const void *xbuffer, unsigned int ret_nread,
+		   unsigned int *nbits_arg, unsigned int *nbytes_arg)
 {
 	const uint8_t *buffer = xbuffer;
-	unsigned int nbits, nbytes;
-	MPI val;
+	unsigned int nbits;
 
-	if (*ret_nread < 2)
-		return ERR_PTR(-EINVAL);
+	if (ret_nread < 2)
+		return -EINVAL;
 	nbits = buffer[0] << 8 | buffer[1];
 
 	if (nbits > MAX_EXTERN_MPI_BITS) {
 		pr_info("MPI: mpi too large (%u bits)\n", nbits);
-		return ERR_PTR(-EINVAL);
+		return -EINVAL;
 	}
 
-	nbytes = DIV_ROUND_UP(nbits, 8);
+	if (nbits_arg)
+		*nbits_arg = nbits;
+	if (nbytes_arg)
+		*nbytes_arg = DIV_ROUND_UP(nbits, 8);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(mpi_key_length);
+
+MPI mpi_read_from_buffer(const void *xbuffer, unsigned *ret_nread)
+{
+	const uint8_t *buffer = xbuffer;
+	unsigned int nbytes;
+	MPI val;
+	int ret;
+
+	ret = mpi_key_length(xbuffer, *ret_nread, NULL, &nbytes);
+	if (ret < 0)
+		return ERR_PTR(ret);
+
 	if (nbytes + 2 > *ret_nread) {
 		pr_info("MPI: mpi larger than buffer nbytes=%u ret_nread=%u\n",
 				nbytes, *ret_nread);