Re: [PATCH 2/2] of: fix side effect in macro expansion

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

On 4/29/20 10:41 AM, Sascha Hauer wrote:
> On Wed, Apr 29, 2020 at 10:26:27AM +0200, Ahmad Fatoum wrote:
>> On 4/29/20 10:00 AM, Sascha Hauer wrote:
>>> On Wed, Apr 29, 2020 at 08:40:40AM +0200, Ahmad Fatoum wrote:
>>>> On little endian platforms that don't define __arch_swab32p, be32_to_cpu
>>>> ultimately expands to an expression that evaluates its argument multiple
>>>> times. Because of the increment, this is wrong (and UB because of
>>>> unsequenced modification).
>>>> Hoist the side effect out of the macro argument.
>>>
>>> I'm not sure if the reasoning is correct. That would mean that
>>> of_read_number() doesn't work on ARM, but I can assure it actually does.
>>
>> Typo, sorry. arch needs to define __arch_swab32 (no p at the end).
>>
>> of_read_number -> be32_to_cpu -> __be32_to_cpu -> __swab32 -> __fswab32
>>
>> then
>>
>> static inline __attribute_const__ __u32 __fswab32(__u32 val)
>> {
>> #ifdef __arch_swab32
>>         return __arch_swab32(val);
>> #else
>>         return ___constant_swab32(val);
>> #endif
>> }
>>
>> where ___constant_swab32 is the macro that evaluates the argument
>> multiple times.
> 
> __fswab32 is a function, for the caller it doesn't matter how often
> ___constant_swab32 evaluates its argument.

Oh. Looks like a false positive. cppcheck thinks we could reach
__constant_swab32, which is a macro, but it's only reachable for
constant expressions, which are always side-effect free.

Please dismiss. I'll send a v2 with corrected reasoning in the commit
message.

Cheers
Ahmad

> 
> Sascha
> 

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