netdev - Re: [PATCH 14/22] [media] usbvision-i2c: fix format overflow warning

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Date:   Mon, 17 Jul 2017 14:57:50 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Networking <netdev@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        "James E . J . Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>
Subject: Re: [PATCH 14/22] [media] usbvision-i2c: fix format overflow warning

On Mon, Jul 17, 2017 at 2:53 PM, Hans Verkuil <hverkuil@...all.nl> wrote:
> On 14/07/17 14:07, Arnd Bergmann wrote:
>> gcc-7 notices that we copy a fixed length string into another
>> string of the same size, with additional characters:
>>
>> drivers/media/usb/usbvision/usbvision-i2c.c: In function 'usbvision_i2c_register':
>> drivers/media/usb/usbvision/usbvision-i2c.c:190:36: error: '%d' directive writing between 1 and 11 bytes into a region of size between 0 and 47 [-Werror=format-overflow=]
>>   sprintf(usbvision->i2c_adap.name, "%s-%d-%s", i2c_adap_template.name,
>>                                     ^~~~~~~~~~
>> drivers/media/usb/usbvision/usbvision-i2c.c:190:2: note: 'sprintf' output between 4 and 76 bytes into a destination of size 48
>>
>> We know this is fine as the template name is always "usbvision", so
>> we can easily avoid the warning by using this as the format string
>> directly.
>
> Hmm, how about replacing sprintf by snprintf? That feels a lot safer (this is very
> old code, it's not surprising it is still using sprintf).

With snprintf(), you will still get a -Wformat-truncation warning. One
of my patches
disables that warning by default, but Mauro likes build-testing with
"make W=1", so
it would still show up then.

However, we can do both: replace the string and use snprintf().

       Arnd