linux-kernel - Re: [PATCH] iio: sx9324: replace deprecated strncpy

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Date:   Sat, 23 Sep 2023 18:47:51 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Lars-Peter Clausen <lars@...afoo.de>, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] iio: sx9324: replace deprecated strncpy

On Thu, 21 Sep 2023 07:01:01 +0000
Justin Stitt <justinstitt@...gle.com> wrote:

> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> 
> We should prefer more robust and less ambiguous string interfaces.
> 
> `prop` is defined as this string literal with size 30 (including null):
> |       #define SX9324_PROXRAW_DEF "semtech,ph01-proxraw-strength"
> |      	char prop[] = SX9324_PROXRAW_DEF;
> 
> Each of the strncpy->strscpy replacements involve string literals with a
> size less than 30 which means there are no current problems with how
> strncpy is used. However, let's move away from using strncpy entirely.
> 
> A suitable replacement is `strscpy` [2] due to the fact that it
> guarantees NUL-termination on the destination buffer without
> unnecessarily NUL-padding.
> 
> Moreover, let's opt for the more conventional `sizeof()` as opposed to
> `ARRAY_SIZE` for these simple strings.
> 
> 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>
> ---
> FWIW: It seems fragile to base future `prop` stores on the
> size of it's default string.

Agreed - can we just get rid of the copying?  Just set a const char *
to point to appropriate string instead and use that?

The dance is just about reasonable for the case where there is
a string format being used to fill in the numbers, here I can't see any
advantage at all. (for the other case, a kasprintf() or similar is probably
more appropriate anyway) given this isn't a particular hot path.

Jonathan
> 
> Note: build-tested
> ---
>  drivers/iio/proximity/sx9324.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/iio/proximity/sx9324.c b/drivers/iio/proximity/sx9324.c
> index 438f9c9aba6e..25ac2733bcef 100644
> --- a/drivers/iio/proximity/sx9324.c
> +++ b/drivers/iio/proximity/sx9324.c
> @@ -937,11 +937,9 @@ sx9324_get_default_reg(struct device *dev, int idx,
>  	case SX9324_REG_AFE_CTRL4:
>  	case SX9324_REG_AFE_CTRL7:
>  		if (reg_def->reg == SX9324_REG_AFE_CTRL4)
> -			strncpy(prop, "semtech,ph01-resolution",
> -				ARRAY_SIZE(prop));
> +			strscpy(prop, "semtech,ph01-resolution", sizeof(prop));
>  		else
> -			strncpy(prop, "semtech,ph23-resolution",
> -				ARRAY_SIZE(prop));
> +			strscpy(prop, "semtech,ph23-resolution", sizeof(prop));
>  
>  		ret = device_property_read_u32(dev, prop, &raw);
>  		if (ret)
> @@ -1012,11 +1010,9 @@ sx9324_get_default_reg(struct device *dev, int idx,
>  	case SX9324_REG_PROX_CTRL0:
>  	case SX9324_REG_PROX_CTRL1:
>  		if (reg_def->reg == SX9324_REG_PROX_CTRL0)
> -			strncpy(prop, "semtech,ph01-proxraw-strength",
> -				ARRAY_SIZE(prop));
> +			strscpy(prop, "semtech,ph01-proxraw-strength", sizeof(prop));
>  		else
> -			strncpy(prop, "semtech,ph23-proxraw-strength",
> -				ARRAY_SIZE(prop));
> +			strscpy(prop, "semtech,ph23-proxraw-strength", sizeof(prop));
>  		ret = device_property_read_u32(dev, prop, &raw);
>  		if (ret)
>  			break;
> 
> ---
> base-commit: 2cf0f715623872823a72e451243bbf555d10d032
> change-id: 20230921-strncpy-drivers-iio-proximity-sx9324-c-8c3437676039
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
>