linux-kernel - Re: Calculating array sizes in C - was: Re: Build regressions/improvements in v6.2-rc1

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Message-ID: <1732342f-49fe-c20e-b877-bc0a340e1a50@fu-berlin.de>
Date:   Thu, 19 Jan 2023 23:11:09 +0100
From:   "Michael.Karcher" <Michael.Karcher@...berlin.de>
To:     John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        linux-wireless@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linuxppc-dev@...ts.ozlabs.org, kasan-dev@...glegroups.com,
        linux-xtensa@...ux-xtensa.org,
        Michael Karcher <kernel@...rcher.dialup.fu-berlin.de>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: Calculating array sizes in C - was: Re: Build
 regressions/improvements in v6.2-rc1

Isn't this supposed to be caught by this check:
>>>
>>>          a, __same_type(a, NULL)
>>>
>>> ?
>>
>> Yeah, but gcc thinks it is smarter than us...
>> Probably it drops the test, assuming UB cannot happen.
> Hmm, sounds like a GGC bug to me then. Not sure how to fix this then.

I don't see a clear bug at this point. We are talking about the C expression

   __same_type((void*)0, (void*)0)? 0 : sizeof((void*)0)/sizeof(*((void*0))

This expression is valid (assuming __same_type works, which is a GCC 
extension), and should return 0. As of now, I have no indication that 
this expression does not return 0. Also, it is true that this expression 
contains the suspicious pattern "sizeof(void*)/sizeof(void)", which is 
does not calculate the size of any array. GCC is free to emit as much 
warnings is it wants for any kind of expressions. From a C standard 
point of view, it's just a "quality of implementation" issue, and an 
implementation that emits useless warnings is of low quality, but not 
non-conforming.

In this case, we requested that gcc refuses to compile if it emits any 
kind of warning, which instructs gcc to reject programs that would be 
valid according to the C standard, but are deemed to be "likely incorrect".

I suggest to file a bug against gcc complaining about a "spurious 
warning", and using "-Werror -Wno-error-sizeof-pointer-div" until gcc is 
adapted to not emit the warning about the pointer division if the result 
is not used.

Regards,
   Michael Karcher