staging: greybus: change strncpy() to strscpy_pad()

gcc-10 warns about a strncpy() that does not enforce zero-termination:

In file included from include/linux/string.h:369,
                 from drivers/staging/greybus/fw-management.c:9:
In function 'strncpy',
    inlined from 'fw_mgmt_backend_fw_update_operation' at drivers/staging/greybus/fw-management.c:306:2:
include/linux/fortify-string.h:108:30: error: '__builtin_strncpy' specified bound 10 equals destination size [-Werror=stringop-truncation]
  108 | #define __underlying_strncpy __builtin_strncpy
      |                              ^
include/linux/fortify-string.h:187:9: note: in expansion of macro '__underlying_strncpy'
  187 |  return __underlying_strncpy(p, q, size);
      |         ^~~~~~~~~~~~~~~~~~~~

For some reason, I cannot reproduce this with gcc-9 or gcc-11, and I only
get a warning for one of the four related strncpy()s, so I'm not
sure what's going on.

Change all four to strscpy_pad(), which is the safest replacement here,
as it avoids ending up with uninitialized stack data in the tag name.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Link: https://github.com/KSPP/linux/issues/90 [1]
Link: https://lore.kernel.org/r/20240408194821.3183462-3-arnd@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/greybus/fw-management.c

@@ -123,8 +123,7 @@ static int fw_mgmt_interface_fw_version_operation(struct fw_mgmt *fw_mgmt,
 	fw_info->major = le16_to_cpu(response.major);
 	fw_info->minor = le16_to_cpu(response.minor);
 
-	strncpy(fw_info->firmware_tag, response.firmware_tag,
-		GB_FIRMWARE_TAG_MAX_SIZE);
+	strscpy_pad(fw_info->firmware_tag, response.firmware_tag);
 
 	/*
 	 * The firmware-tag should be NULL terminated, otherwise throw error but
@@ -153,7 +152,7 @@ static int fw_mgmt_load_and_validate_operation(struct fw_mgmt *fw_mgmt,
 	}
 
 	request.load_method = load_method;
-	strncpy(request.firmware_tag, tag, GB_FIRMWARE_TAG_MAX_SIZE);
+	strscpy_pad(request.firmware_tag, tag);
 
 	/*
 	 * The firmware-tag should be NULL terminated, otherwise throw error and
@@ -249,8 +248,7 @@ static int fw_mgmt_backend_fw_version_operation(struct fw_mgmt *fw_mgmt,
 	struct gb_fw_mgmt_backend_fw_version_response response;
 	int ret;
 
-	strncpy(request.firmware_tag, fw_info->firmware_tag,
-		GB_FIRMWARE_TAG_MAX_SIZE);
+	strscpy_pad(request.firmware_tag, fw_info->firmware_tag);
 
 	/*
 	 * The firmware-tag should be NULL terminated, otherwise throw error and
@@ -303,13 +301,13 @@ static int fw_mgmt_backend_fw_update_operation(struct fw_mgmt *fw_mgmt,
 	struct gb_fw_mgmt_backend_fw_update_request request;
 	int ret;
 
-	strncpy(request.firmware_tag, tag, GB_FIRMWARE_TAG_MAX_SIZE);
+	ret = strscpy_pad(request.firmware_tag, tag);
 
 	/*
 	 * The firmware-tag should be NULL terminated, otherwise throw error and
 	 * fail.
 	 */
-	if (request.firmware_tag[GB_FIRMWARE_TAG_MAX_SIZE - 1] != '\0') {
+	if (ret == -E2BIG) {
 		dev_err(fw_mgmt->parent, "backend-update: firmware-tag is not NULL terminated\n");
 		return -EINVAL;
 	}