linux-kernel - Re: [RFC PATCH 7/7] drivers: hwmon: add runtime format string checking

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Date:   Sat, 27 Oct 2018 10:44:43 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jean Delvare <jdelvare@...e.com>, linux-kernel@...r.kernel.org,
        linux-hwmon@...r.kernel.org
Subject: Re: [RFC PATCH 7/7] drivers: hwmon: add runtime format string
 checking

Hi,

On Sat, Oct 27, 2018 at 01:24:09AM +0200, Rasmus Villemoes wrote:
> With -Wformat-nonliteral, gcc complains
> 
> drivers/hwmon/hwmon.c: In function ‘hwmon_genattr’:
> drivers/hwmon/hwmon.c:282:6: warning: format not a string literal, argument types not checked [-Wformat-nonliteral]
>       index + hwmon_attr_base(type));
> 
> Add a runtime check to ensure that the template indeed has a single %d
> printf specifier. Using fmtcheck() also makes gcc verify that the
> expression 'index + hwmon_attr_base(type)' is suitable for %d.
> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>

I'll defer to others on this one; let me know if the series is otherwise
accepted. Personally I think that the compiler is at fault here
for not detecting that the format string can not be wrong (since it
is declared and used only in this file), and I find the fmtcheck()
confusing/obfuscating.

Guenter

> ---
>  drivers/hwmon/hwmon.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hwmon/hwmon.c b/drivers/hwmon/hwmon.c
> index 33d51281272b..ec6f5f36b5fc 100644
> --- a/drivers/hwmon/hwmon.c
> +++ b/drivers/hwmon/hwmon.c
> @@ -278,7 +278,8 @@ static struct attribute *hwmon_genattr(struct device *dev,
>  	if (type == hwmon_chip) {
>  		name = (char *)template;
>  	} else {
> -		scnprintf(hattr->name, sizeof(hattr->name), template,
> +		scnprintf(hattr->name, sizeof(hattr->name),
> +			  fmtcheck(template, "type%dwhat", 0),
>  			  index + hwmon_attr_base(type));
>  		name = hattr->name;
>  	}