lib: modem_key_mgmt: Return size of key

In modem_key_mgmt_read(), if *len parameter is too small to fit
the certificate read, return -ENOMEM and set *len to the size
needed.

This is helpful to check the size of a certificate without
actually needing the contents.

Signed-off-by: Pete Skeggs <[email protected]>

Author: plskeggs

include/modem/modem_key_mgmt.h

@@ -106,7 +106,8 @@ int modem_key_mgmt_clear(nrf_sec_tag_t sec_tag);
  *
  * @retval 0		On success.
  * @retval -ENOBUFS	Internal buffer is too small.
- * @retval -ENOMEM	Credential does not fit in @p buf.
+ * @retval -ENOMEM	Credential does not fit in @p buf. Check *len for
+ *			required size.
  * @retval -ENOENT	No credential associated with the given
  *			@p sec_tag and @p cred_type.
  * @retval -EACCES	Access to credential not allowed.

lib/modem_key_mgmt/modem_key_mgmt.c

@@ -179,6 +179,7 @@ int modem_key_mgmt_read(nrf_sec_tag_t sec_tag,
 	}
 
 	if (end - begin > *len) {
+		*len = end - begin; /* Let caller know how large their buffer should be. */
 		err = -ENOMEM;
 		goto end;
 	}