linux-kernel - Re: CONFIG_UBSAN_ALIGNMENT breaks x86-64 kernel with lockdep enabled

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Date:	Mon, 1 Feb 2016 18:10:38 +0300
From:	Andrey Ryabinin <aryabinin@...tuozzo.com>
To:	Mike Krinkin <krinkin.m.u@...il.com>
CC:	<akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>
Subject: Re: CONFIG_UBSAN_ALIGNMENT breaks x86-64 kernel with lockdep enabled

On 01/30/2016 03:36 AM, Mike Krinkin wrote:
> Hi,
> 
> option CONFIG_UBSAN_ALIGNMENT breaks x86-64 kernel with lockdep enabled,
> i. e kernel with CONFIG_UBSAN_ALIGNMENT fails to load without even any
> error message.
> 
> The problem is that ubsan callbacks use spinlocks and might be called
> before lockdep is initialized. Particularly this line in the
> reserve_ebda_region function causes problem:
> 
> lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);
> 
> If i put lockdep_init() before reserve_ebda_region call in
> x86_64_start_reservations kernel loads well. Since CONFIG_UBSAN_ALIGNMENT
> isn't useful for x86 anyway it might be better to disable this option for
> x86 arch?
> 


Alignment checks could be useful even on x86, because there are unaligned accesses in generic code.
I think we can disable alignment instrumentation for arch/x86 directory only.