Re: CONFIG_UBSAN_ALIGNMENT breaks x86-64 kernel with lockdep enabled

Andrey Ryabinin
2016-02-01T15:09:37Z

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.