[PATCH] powerpc/kasan/book3s_64: warn when running with hash MMU

[Christophe Leroy]

Le 10/10/2022 à 16:10, Nathan Lynch a écrit :
> Michael Ellerman <mpe at ellerman.id.au> writes:
>> Christophe Leroy <christophe.leroy at csgroup.eu> writes:
>>> + KASAN list
>>>
>>> Le 06/10/2022 à 06:10, Michael Ellerman a écrit :
>>>> Nathan Lynch <nathanl at linux.ibm.com> writes:
>>>>> kasan is known to crash at boot on book3s_64 with non-radix MMU. As
>>>>> noted in commit 41b7a347bf14 ("powerpc: Book3S 64-bit outline-only
>>>>> KASAN support"):
>>>>>
>>>>>     A kernel with CONFIG_KASAN=y will crash during boot on a machine
>>>>>     using HPT translation because not all the entry points to the
>>>>>     generic KASAN code are protected with a call to kasan_arch_is_ready().
>>>>
>>>> I guess I thought there was some plan to fix that.
>>>
>>> I was thinking the same.
>>>
>>> Do we have a list of the said entry points to the generic code that are
>>> lacking a call to kasan_arch_is_ready() ?
>>>
>>> Typically, the BUG dump below shows that kasan_byte_accessible() is
>>> lacking the check. It should be straight forward to add
>>> kasan_arch_is_ready() check to kasan_byte_accessible(), shouldn't it ?
>>
>> Yes :)
>>
>> And one other spot, but the patch below boots OK for me. I'll leave it
>> running for a while just in case there's a path I've missed.
> 
> It works for me too, thanks (p8 pseries qemu).
> 
> This avoids the boot-time oops, but kasan remains unimplemented for hash
> mmu. Raising the question: with the trivial crashes addressed, is the
> current message ('KASAN not enabled as it requires radix!') sufficient
> to notify developers (such as me, a week ago) who mean to use kasan on a
> book3s platform, unaware that it's radix-only? Would a WARN or something
> more prominent still be justified?
> 
> I guess people will figure it out as soon as they think to search the
> kernel log for 'KASAN'...

I don't think the big hammer WARN would be justified.
WARN is supposed to be used only with unexpected conditions.

KASAN not working with hash-MMU is expected. A pr_warn() should be enough.

Someone who has a kernel with KASAN built in but who is not interested 
by KASAN and who is booting it one a HASH-MMU will be terrified by a WARN.