Re: [PATCH 18/22] gpio: acpi: fix string overflow for large pin numbers

[Arnd Bergmann]

On Fri, Jul 14, 2017 at 2:52 PM, Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> On Fri, 2017-07-14 at 14:07 +0200, Arnd Bergmann wrote:
>> gcc-7 notices that the pin_table is an array of 16-bit numbers,
>> but we assume it can be printed as a two-character hexadecimal
>> string:
>>
>> drivers/gpio/gpiolib-acpi.c: In function
>> 'acpi_gpiochip_request_interrupt':
>> drivers/gpio/gpiolib-acpi.c:206:24: warning: '%02X' directive writing
>> between 2 and 4 bytes into a region of size 3 [-Wformat-overflow=]
>>    sprintf(ev_name, "_%c%02X",
>>                         ^~~~
>> drivers/gpio/gpiolib-acpi.c:206:20: note: directive argument in the
>> range [0, 65535]
>>    sprintf(ev_name, "_%c%02X",
>>                     ^~~~~~~~~
>> drivers/gpio/gpiolib-acpi.c:206:3: note: 'sprintf' output between 5
>> and 7 bytes into a destination of size 5
>>    sprintf(ev_name, "_%c%02X",
>>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
>>     agpio->triggering == ACPI_EDGE_SENSITIVE ? 'E' : 'L',
>>     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>     pin);
>>     ~~~~
>
>
> This is obviously a false positive warning.
>
> Here we have
> int pin = u16 pin_table[0] <= 255 (implying >= 0).
>
> I see few options how to make it more clear
> 1) your proposal;
> 2) use "%02hhX" instead;
> 3) use if (ret >= 0 && ret <= 255) condition.
>
> I would choose one of the 2-3.
>
> In case gcc will complain about 3), file a bug to gcc crazy warning.

Makes sense. I didn't remember the syntax for 2) and couldn't find
it in the man page when I first looked. This seems like a good solution
here.

I'm pretty sure I tried 3) a few times when the warning first showed
up last year, but couldn't get that to work. Filing a gcc bug also seems
like a good idea, but I should first see if it's already fixed. The version
I use for testing at the moment is from late April, and others may
have complained about that already.

      Arnd