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Date: Thu, 7 Dec 2017 15:24:53 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Ding Tianhong <dingtianhong@huawei.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        LinuxArm <linuxarm@huawei.com>, David Laight <David.Laight@ACULAB.COM>
Subject: Re: [PATCH v2] ubsan: don't handle misaligned address when support
 unaligned access
Message-Id: <20171207152453.7aaf8f420ebf69d13c29a761@linux-foundation.org>
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        <20171206164916.687d5006b7a49ef1149e7154@linux-foundation.org>
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On Thu, 7 Dec 2017 16:31:23 +0300 Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:

> On 12/07/2017 03:49 AM, Andrew Morton wrote:
> > (correcting Andrey's email address)
> > 
> > 
> > From: Ding Tianhong <dingtianhong@huawei.com>
> > Subject: lib/ubsan.c: don't handle misaligned address when kernel supports unaligned access
> > 
> > ubsan reports a warning like:
> > 
> > UBSAN: Undefined behaviour in ../include/linux/etherdevice.h:386:9
> > load of misaligned address ffffffc069ba0482 for type 'long unsigned int'
> > which requires 8 byte alignment
> > CPU: 0 PID: 901 Comm: sshd Not tainted 4.xx+ #1
> > Hardware name: linux,dummy-virt (DT)
> > Call trace:
> > [<ffffffc000093600>] dump_backtrace+0x0/0x348
> > [<ffffffc000093968>] show_stack+0x20/0x30
> > [<ffffffc001651664>] dump_stack+0x144/0x1b4
> > [<ffffffc0016519b0>] ubsan_epilogue+0x18/0x74
> > [<ffffffc001651bac>] __ubsan_handle_type_mismatch+0x1a0/0x25c
> > [<ffffffc00125d8a0>] dev_gro_receive+0x17d8/0x1830
> > [<ffffffc00125d928>] napi_gro_receive+0x30/0x158
> > [<ffffffc000f4f93c>] virtnet_receive+0xad4/0x1fa8
> > 
> > The reason is that when enabling the CONFIG_UBSAN_ALIGNMENT, ubsan will
> > report the unaligned access even if the system supports it
> > (CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y).  This produces a lot of noise
> > in the log and causes confusion.
> > 
> 
> NACK. This doesn't make sense. If you don't want to see misaligned access reports
> you simply shouldn't enable CONFIG_UBSAN_ALIGNMENT.

So should UBSAN Kconfig disable CONFIG_UBSAN_ALIGNMENT when
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y?