linux-kernel - Re: [Cocci] [PATCH] usb: atm: don't use snprintf() for sysfs attrs

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Date:   Fri, 28 Aug 2020 00:01:34 +0300
From:   Denis Efremov <efremov@...ux.com>
To:     Julia Lawall <julia.lawall@...ia.fr>, Joe Perches <joe@...ches.com>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, cocci <cocci@...teme.lip6.fr>,
        accessrunner-general@...ts.sourceforge.net,
        Alex Dewar <alex.dewar90@...il.com>
Subject: Re: [Cocci] [PATCH] usb: atm: don't use snprintf() for sysfs attrs

Hi all,

On 8/27/20 10:42 PM, Julia Lawall wrote:
> 
> 
> On Thu, 27 Aug 2020, Joe Perches wrote:
> 
>> On Thu, 2020-08-27 at 15:48 +0100, Alex Dewar wrote:
>>> On Thu, Aug 27, 2020 at 03:41:06PM +0200, Rasmus Villemoes wrote:
>>>> On 27/08/2020 15.18, Alex Dewar wrote:
>>>>> On Thu, Aug 27, 2020 at 09:15:37AM +0200, Greg Kroah-Hartman wrote:
>>>>>> On Thu, Aug 27, 2020 at 08:42:06AM +0200, Rasmus Villemoes wrote:
>>>>>>> On 25/08/2020 00.23, Alex Dewar wrote:
>>>>>>>> kernel/cpu.c: don't use snprintf() for sysfs attrs
>>>>>>>>
>>>>>>>> As per the documentation (Documentation/filesystems/sysfs.rst),
>>>>>>>> snprintf() should not be used for formatting values returned by sysfs.

Just FYI, I've send an addition to the device_attr_show.cocci script[1] to turn
simple cases of snprintf (e.g. "%i") to sprintf. Looks like many developers would
like it more than changing snprintf to scnprintf. As for me, I don't like the idea
of automated altering of the original logic from bounded snprint to unbouded one
with sprintf.

[1] https://lkml.org/lkml/2020/8/13/786

Regarding current device_attr_show.cocci implementation, it detects the functions
by declaration:
ssize_t any_name(struct device *dev, struct device_attribute *attr, char *buf)

and I limited the check to:
"return snprintf"
pattern because there are already too many warnings.

Actually, it looks more correct to check for:
ssize_t show(struct device *dev, struct device_attribute *attr, char *buf)
{
        <...
*       snprintf@p(...);
        ...>
}

This pattern should also highlight the snprintf calls there we save returned
value in a var, e.g.:

ret += snprintf(...);
...
ret += snprintf(...);
...
ret += snprintf(...);

return ret;

> 
> Something like
> 
> identifier f;
> fresh identifier = "sysfs" ## f;
> 
> may be useful.  Let me know if further help is needed.

Initially, I wrote the rule to search for DEVICE_ATTR(..., ..., func_name, ...)
functions. However, it looks like matching function prototype is enough. At least,
I failed to find false positives. I rejected the initial DEVICE_ATTR() searching
because I thought that it's impossible to handle DEVICE_ATTR_RO()/DEVICE_ATTR_RW()
macroses with coccinelle as they "generate" function names internally with
"##". "fresh identifier" should really help here, but now I doubt it's required in
device_attr_show.cocci, function prototype is enough.

Thanks,
Denis