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

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Date:   Tue, 19 Sep 2017 15:47:08 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Victor Chibotaru <tchibo@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] kcov: remove #ifdef CONFIG_RANDOMIZE_BASE

On Tue, Sep 19, 2017 at 3:30 PM, Mark Rutland <mark.rutland@....com> wrote:
> 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.

Sounds reasonable to me.