linux-kernel - Re: [PATCH v2] ubsan: don't handle misaligned address when support unaligned access

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Message-ID: <022fdf6b-6e00-fe70-3124-80b8ff50a859@virtuozzo.com>
Date:   Thu, 7 Dec 2017 16:31:23 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Ding Tianhong <dingtianhong@...wei.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        LinuxArm <linuxarm@...wei.com>,
        David Laight <David.Laight@...LAB.COM>
Subject: Re: [PATCH v2] ubsan: don't handle misaligned address when support
 unaligned access

On 12/07/2017 03:49 AM, Andrew Morton wrote:
> (correcting Andrey's email address)
> 
> 
> From: Ding Tianhong <dingtianhong@...wei.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.