linux-kernel - [PATCH] wl18xx: replace deprecated strncpy with strscpy

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Message-ID: <20231017-strncpy-drivers-net-wireless-ti-wl18xx-main-c-v1-1-ed5322ec8068@google.com>
Date:   Tue, 17 Oct 2023 23:39:36 +0000
From:   Justin Stitt <justinstitt@...gle.com>
To:     Kalle Valo <kvalo@...nel.org>
Cc:     linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org,
        Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] wl18xx: replace deprecated strncpy with strscpy

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

wl->chip.phy_fw_ver_str is obviously intended to be NUL-terminated by
the deliberate comment telling us as much. Furthermore, its only use is
drivers/net/wireless/ti/wlcore/debugfs.c shows us it should be
NUL-terminated since its used in scnprintf:
492 | DRIVER_STATE_PRINT_STR(chip.phy_fw_ver_str);
which is defined as:
| #define DRIVER_STATE_PRINT_STR(x)  DRIVER_STATE_PRINT(x, "%s")
...
| #define DRIVER_STATE_PRINT(x, fmt)   \
| 	(res += scnprintf(buf + res, DRIVER_STATE_BUF_LEN - res,\
| 			  #x " = " fmt "\n", wl->x))

We can also see that NUL-padding is not required.

Considering the above, a suitable replacement is `strscpy` [2] due to
the fact that it guarantees NUL-termination on the destination buffer
without unnecessarily NUL-padding.

The very fact that a plain-english comment had to be made alongside a
manual NUL-byte assignment for such a simple purpose shows why strncpy
is faulty. It has non-obvious behavior that has to be clarified every
time it is used (and if it isn't then the reader suffers).

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@...r.kernel.org
Signed-off-by: Justin Stitt <justinstitt@...gle.com>
---
Note: build-tested only.

Found with: $ rg "strncpy\("
---
 drivers/net/wireless/ti/wl18xx/main.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/net/wireless/ti/wl18xx/main.c b/drivers/net/wireless/ti/wl18xx/main.c
index 0b3cf8477c6c..1e4a430d1543 100644
--- a/drivers/net/wireless/ti/wl18xx/main.c
+++ b/drivers/net/wireless/ti/wl18xx/main.c
@@ -1516,12 +1516,9 @@ static int wl18xx_handle_static_data(struct wl1271 *wl,
 	struct wl18xx_static_data_priv *static_data_priv =
 		(struct wl18xx_static_data_priv *) static_data->priv;

-	strncpy(wl->chip.phy_fw_ver_str, static_data_priv->phy_version,
+	strscpy(wl->chip.phy_fw_ver_str, static_data_priv->phy_version,
 		sizeof(wl->chip.phy_fw_ver_str));

-	/* make sure the string is NULL-terminated */
-	wl->chip.phy_fw_ver_str[sizeof(wl->chip.phy_fw_ver_str) - 1] = '\0';
-
 	wl1271_info("PHY firmware version: %s", static_data_priv->phy_version);

 	return 0;

---
base-commit: 58720809f52779dc0f08e53e54b014209d13eebb
change-id: 20231017-strncpy-drivers-net-wireless-ti-wl18xx-main-c-356a0272bab4

Best regards,
--
Justin Stitt <justinstitt@...gle.com>