Re: [PATCH] tty/serial: digicolor: Fix bad usage of IS_ERR_VALUE

[Russell King - ARM Linux <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>]

On Tue, Feb 09, 2016 at 06:37:46PM -0800, Guenter Roeck wrote:
> On 02/09/2016 07:26 AM, Arnd Bergmann wrote:
> >On Tuesday 09 February 2016 07:08:59 Guenter Roeck wrote:
> >>IS_ERR_VALUE() assumes that its parameter is an unsigned long.
> >>It can not be used to check if an unsigned int reflects an error.
> >>Doing so can result in the following build warning.
> >>
> >>drivers/tty/serial/digicolor-usart.c: In function ‘digicolor_uart_probe’:
> >>include/linux/err.h:21:38: warning:
> >>         comparison is always false due to limited range of data type
> >>drivers/tty/serial/digicolor-usart.c:485:6: note:
> >>         in expansion of macro ‘IS_ERR_VALUE’
> >>
> >>If that warning is seen, an error return from platform_get_irq() is missed.
> >>
> >>
> >
> >The patch looks correct to me, but what compiler version and which kernel
> >tree is it that triggered the warning?
> >
> >Andrzej Hajda just modified the definition of IS_ERR_VALUE(), and the
> >changes are still under discussion, but I don't see that warning with
> >any of the versions.
> >
> I see it with gcc 5.1 and 5.2 (and W=1). I did not see / notice Andrzej's patch.
> 
> I agree that fixing the problem in IS_ERR_VALUE() is preferrable.

I disagree.  What happens if (eg) you decide to do this:

	u8 irq;

	irq = platform_get_irq(...);
	if (IS_ERR_VALUE(irq))
		...

is that the fault of IS_ERR_VALUE() that it will never be true?  No, it's
a programming error in the caller, because the caller is using the wrong
type here - in fact, you can't do anything in IS_ERR_VALUE() to correct
that.

The same is true if you assign an error value to an unsigned int: the
problem is in the caller, not in IS_ERR_VALUE().

What would be preferable is if there was some way IS_ERR_VALUE() could
detect if it's used on something that isn't at least a short, and is
signed, but I suspect coccinelle can do that much better than trying to
work out some way to get the C compiler to check that.

-- 
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