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

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Message-ID: <f094c6c2-640b-73e4-95cb-19fd74d4fdd7@virtuozzo.com>
Date:   Fri, 29 Sep 2017 19:20:46 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] kcov: remove #ifdef CONFIG_RANDOMIZE_BASE

On 09/19/2017 04:47 PM, Dmitry Vyukov wrote:
> 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.
> 

As kbuilt robot pointed out, we would also need to introduce generic kaslr_offset() for arches
that don't have it. I seem like too much trouble for very little gain, so I'll keep current code as is.