linux-kernel - Re: [PATCH v2] wifi: wl18xx: replace deprecated strncpy with strscpy

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Message-ID: <202310182229.7B4E7394@keescook>
Date:   Wed, 18 Oct 2023 22:30:00 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Kalle Valo <kvalo@...nel.org>, linux-wireless@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] wifi: wl18xx: replace deprecated strncpy with strscpy

On Wed, Oct 18, 2023 at 09:18:24PM +0000, Justin Stitt wrote:
> 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>

Yup, a clear case for strscpy.

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook