net: lib: tls_credentials: return size required

If either no buffer is provided or the size of it
is too small, return the required length.

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

Author: plskeggs

include/zephyr/net/tls_credentials.h

@@ -107,6 +107,7 @@ int tls_credential_add(sec_tag_t tag, enum tls_credential_type type,
  * @retval -EACCES Access to the TLS credential subsystem was denied.
  * @retval -ENOENT Requested TLS credential was not found.
  * @retval -EFBIG Requested TLS credential does not fit in the buffer provided.
+ *                Check *credlen for size required.
  */
 int tls_credential_get(sec_tag_t tag, enum tls_credential_type type,
 		       void *cred, size_t *credlen);

subsys/net/lib/tls_credentials/tls_credentials.c

@@ -158,11 +158,13 @@ int tls_credential_get(sec_tag_t tag, enum tls_credential_type type,
 	credential = credential_get(tag, type);
 	if (credential == NULL) {
 		ret = -ENOENT;
+		*credlen = 0;
 		goto exit;
 	}
 
 	if (credential->len > *credlen) {
 		ret = -EFBIG;
+		*credlen = credential->len;
 		goto exit;
 	}