[PATCH 10/11] staging: greybus: change strncpy() to strscpy()

[Arnd Bergmann]

From: Arnd Bergmann <arnd@xxxxxxxx>

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, so I'm not
sure what's going on. Changing it to strspy() avoids the warning. In this
case the existing check for zero-termination would fail but can be replaced
with an error check.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
This is from randconfig testing with random gcc versions, a .config to
reproduce is at https://pastebin.com/r13yezkU
---
 drivers/staging/greybus/fw-management.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/greybus/fw-management.c b/drivers/staging/greybus/fw-management.c
index 3054f084d777..35bfdd5f32d2 100644
--- a/drivers/staging/greybus/fw-management.c
+++ b/drivers/staging/greybus/fw-management.c
@@ -303,13 +303,13 @@ 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(request.firmware_tag, tag, GB_FIRMWARE_TAG_MAX_SIZE);
 
 	/*
 	 * 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;
 	}
-- 
2.39.2