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

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Message-ID: <20170919133057.GE30715@leverpostej>
Date:   Tue, 19 Sep 2017 14:30:57 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>, akpm@...ux-foundation.org,
        Andrey Konovalov <andreyknvl@...gle.com>, tchibo@...gle.com,
        syzkaller@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] kcov: remove #ifdef CONFIG_RANDOMIZE_BASE

Hi,

On Tue, Sep 19, 2017 at 03:46:46PM +0300, Andrey Ryabinin 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>
> ---
>  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

I think this is sound, but as Dmitry points out it'll mean we do some
pointless work. For example on arm64 we have:

static inline unsigned long kaslr_offset(void)
{
	return kimage_vaddr - KIMAGE_VADDR;
}

... where kimage_vaddr is a global variable, and KIMAGE_VADDR is a
constant (and should be identical for !CONFIG_RANDOMIZE_BASE kernels).

I think it would be reasonable to make that:

static inline unsigned long kaslr_offset(void)
{
	if (!IS_ENABLED(CONFIG_RANDOMIZE_BASE))
		return 0;

	return kimage_vaddr - KIMAGE_VADDR;
}

... and simplify callers as above.

Thanks,
Mark.