linux-kernel - Re: [PATCH 1/3] kcov: remove #ifdef CONFIG_RANDOMIZE

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Message-ID: <CACT4Y+Yi4S8fofsYui48HUb=+jP-sAXBwjowcsne4wzDV9Rm4g@mail.gmail.com>
Date:   Tue, 19 Sep 2017 15:03:30 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Victor Chibotaru <tchibo@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>,
        Mark Rutland <mark.rutland@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] kcov: remove #ifdef CONFIG_RANDOMIZE_BASE

On Tue, Sep 19, 2017 at 2:46 PM, Andrey Ryabinin
<aryabinin@...tuozzo.com> wrote:
> There is no need to surround kaslr_offset() with CONFIG_RANDOMIZE_BASE ifdef.
> kaslr_offset() will just return 0 if CONFIG_RANDOMIZE_BASE isn't set.
>
> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>


As you said this is a very hot function, but this changes code from:

  2e:   48 8b 75 08             mov    0x8(%rbp),%rsi

to:

  2b:   48 c7 c2 00 00 00 81    mov    $0xffffffff81000000,%rdx
  32:   48 81 ea 00 00 00 00    sub    $0x0,%rdx
  3c:   48 03 55 08             add    0x8(%rbp),%rdx

This also changes semantics, as returned PCs won't match nm/objdump output.


> ---
>  kernel/kcov.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 3f693a0f6f3e..2f0e7a7c7afc 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -69,9 +69,7 @@ void notrace __sanitizer_cov_trace_pc(void)
>                 unsigned long pos;
>                 unsigned long ip = _RET_IP_;
>
> -#ifdef CONFIG_RANDOMIZE_BASE
>                 ip -= kaslr_offset();
> -#endif
>
>                 /*
>                  * There is some code that runs in interrupts but for which
> --
> 2.13.5
>