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

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Date:   Tue, 18 Jul 2017 21:53:40 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
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 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.

        Arnd