linux-kernel - Re: [PATCH 07/14] proc/kcore: hide a harmless warning

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Date:   Tue, 18 Jul 2017 20:55:44 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Guenter Roeck <linux@...ck-us.net>,
        IDE-ML <linux-ide@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Laura Abbott <labbott@...hat.com>,
        Pratyush Anand <panand@...hat.com>
Subject: Re: [PATCH 07/14] proc/kcore: hide a harmless warning

On 18 July 2017 at 20:53, Arnd Bergmann <arnd@...db.de> wrote:
> On Fri, Jul 14, 2017 at 2:28 PM, Ard Biesheuvel
> <ard.biesheuvel@...aro.org> wrote:
>> On 14 July 2017 at 10:25, Arnd Bergmann <arnd@...db.de> wrote:
>>> gcc warns when MODULES_VADDR/END is defined to the same value as
>>> VMALLOC_START/VMALLOC_END, e.g. on x86-32:
>>>
>>> fs/proc/kcore.c: In function ‘add_modules_range’:
>>> fs/proc/kcore.c:622:161: error: self-comparison always evaluates to false [-Werror=tautological-compare]
>>>   if (/*MODULES_VADDR != VMALLOC_START && */MODULES_END != VMALLOC_END) {
>>>
>>
>> Does it occur for subtraction as well? Or only for comparison?
>
> This replacement patch would also address the warning:
>
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index 45629f4b5402..35824e986c2c 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -623,7 +623,7 @@ static void __init proc_kcore_text_init(void)
>  struct kcore_list kcore_modules;
>  static void __init add_modules_range(void)
>  {
> -       if (MODULES_VADDR != VMALLOC_START && MODULES_END != VMALLOC_END) {
> +       if (MODULES_VADDR - VMALLOC_START && MODULES_END - VMALLOC_END) {
>                 kclist_add(&kcore_modules, (void *)MODULES_VADDR,
>                         MODULES_END - MODULES_VADDR, KCORE_VMALLOC);
>         }
>
> I have also verified that four of the 14 patches are not needed when building
> without ccache, this is one of them:
>
>  acpi: thermal: fix gcc-6/ccache warning
>  proc/kcore: hide a harmless warning
>  SFI: fix tautological-compare warning
>  [media] fix warning on v4l2_subdev_call() result interpreted as bool
>
> Not sure what to do with those, we could either ignore them all and
> not care about ccache, or we try to address them all in some way.
>

Any idea why ccache makes a difference here? It is not obvious (not to
me at least)