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Subject: Re: [PATCH] lib/ubsan: default UBSAN_ALIGNMENT to not set
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Anders Roxell <anders.roxell@linaro.org>, arnd@arndb.de,
        linux-kernel@vger.kernel.org
References: <20181217150326.30933-1-anders.roxell@linaro.org>
 <8bc73bbf-6c12-7fd1-3253-676731bf5ae5@virtuozzo.com>
 <20190129173634.ee579795b227b141efc8bd30@linux-foundation.org>
From:   Andrey Ryabinin <aryabinin@virtuozzo.com>
Message-ID: <2370bfbc-762f-fb3f-79b1-483a095fdf7c@virtuozzo.com>
Date:   Wed, 30 Jan 2019 20:12:45 +0300
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On 1/30/19 4:36 AM, Andrew Morton wrote:
> On Fri, 11 Jan 2019 22:01:16 +0300 Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> 
>>
>>
>> On 12/17/18 6:03 PM, Anders Roxell wrote:
>>> When booting an allmodconfig kernel, there are a lot of false-positives.
>>> With a message like this 'UBSAN: Undefined behaviour in...' with a call
>>> trace that follows.
>>>
>>> Reworked so that when building a allmodconfig kernel that turns
>>> everything into '=m' or '=y' will turn off UBSAN_ALIGNMENT.
>>>
>>> Suggested-by: Arnd Bergmann <arnd@arndb.de>
>>> Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
>>> ---
>>
>> Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
>>
> 
> Confused.  Why does allmodconfig result in UBSAN warnings?
>  

UBSAN warnings is a result of enabling noisy CONFIG_UBSAN_ALIGNMENT which is disabled
by default if HAVE_EFFICIENT_UNALIGNED_ACCESS=y.
It's noisy even if don't have efficient unaligned access, e.g. people often add  __cacheline_aligned_in_smp
in structs, but forget to align allocations of such struct (kmalloc() give 8-byte alignment in worst case).