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

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Date:   Fri, 14 Jul 2017 13:28:10 +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>,
        linux-ide@...r.kernel.org, linux-media@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 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?

> The code is correct as it is required for most other configurations.
> The best workaround I found for shutting up that warning is to make
> it a little more complex by adding a temporary variable. The compiler
> will still optimize away the code as it finds the two to be identical,
> but it no longer warns because it doesn't condider the comparison
> "tautological" any more.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  fs/proc/kcore.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index 45629f4b5402..c503ad657c46 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -620,12 +620,14 @@ static void __init proc_kcore_text_init(void)
>  /*
>   * MODULES_VADDR has no intersection with VMALLOC_ADDR.
>   */
> -struct kcore_list kcore_modules;
> +static struct kcore_list kcore_modules;
>  static void __init add_modules_range(void)
>  {
> -       if (MODULES_VADDR != VMALLOC_START && MODULES_END != VMALLOC_END) {
> -               kclist_add(&kcore_modules, (void *)MODULES_VADDR,
> -                       MODULES_END - MODULES_VADDR, KCORE_VMALLOC);
> +       void *start = (void *)MODULES_VADDR;
> +       size_t len = MODULES_END - MODULES_VADDR;
> +
> +       if (start != (void *)VMALLOC_START && len != VMALLOC_END - VMALLOC_START) {
> +               kclist_add(&kcore_modules, start, len, KCORE_VMALLOC);
>         }
>  }
>  #else
> --
> 2.9.0
>