[RFC PATCH 3/5] kasan: allow architectures to provide an outline readiness check

[Christophe Leroy]

Hi Daniel,

Le 18/02/2019 à 07:13, Daniel Axtens a écrit :
> christophe leroy <christophe.leroy at c-s.fr> writes:
> 
>> Le 15/02/2019 à 01:04, Daniel Axtens a écrit :
>>> In powerpc (as I understand it), we spend a lot of time in boot
>>> running in real mode before MMU paging is initalised. During
>>> this time we call a lot of generic code, including printk(). If
>>> we try to access the shadow region during this time, things fail.
>>>
>>> My attempts to move early init before the first printk have not
>>> been successful. (Both previous RFCs for ppc64 - by 2 different
>>> people - have needed this trick too!)
>>>
>>> So, allow architectures to define a check_return_arch_not_ready()
>>> hook that bails out of check_memory_region_inline() unless the
>>> arch has done all of the init.
>>>
>>> Link: https://lore.kernel.org/patchwork/patch/592820/ # ppc64 hash series
>>> Link: https://patchwork.ozlabs.org/patch/795211/      # ppc radix series
>>> Originally-by: Balbir Singh <bsingharora at gmail.com>
>>> Cc: Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com>
>>> Signed-off-by: Daniel Axtens <dja at axtens.net>
>>> ---
>>>    include/linux/kasan.h | 4 ++++
>>>    mm/kasan/generic.c    | 2 ++
>>>    2 files changed, 6 insertions(+)
>>>
>>> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
>>> index f6261840f94c..83edc5e2b6a0 100644
>>> --- a/include/linux/kasan.h
>>> +++ b/include/linux/kasan.h
>>> @@ -14,6 +14,10 @@ struct task_struct;
>>>    #include <asm/kasan.h>
>>>    #include <asm/pgtable.h>
>>>    
>>> +#ifndef check_return_arch_not_ready
>>> +#define check_return_arch_not_ready()	do { } while (0)
>>> +#endif
>>
>> A static inline would be better I believe.
>>
>> Something like
>>
>> #ifndef kasan_arch_is_ready
>> static inline bool kasan_arch_is_ready {return true;}
>> #endif
>>
>>> +
>>>    extern unsigned char kasan_early_shadow_page[PAGE_SIZE];
>>>    extern pte_t kasan_early_shadow_pte[PTRS_PER_PTE];
>>>    extern pmd_t kasan_early_shadow_pmd[PTRS_PER_PMD];
>>> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
>>> index bafa2f986660..4c18bbd09a20 100644
>>> --- a/mm/kasan/generic.c
>>> +++ b/mm/kasan/generic.c
>>> @@ -170,6 +170,8 @@ static __always_inline void check_memory_region_inline(unsigned long addr,
>>>    						size_t size, bool write,
>>>    						unsigned long ret_ip)
>>>    {
>>> +	check_return_arch_not_ready();
>>> +
>>
>> Not good for readibility that the above macro embeds a return, something
>> like below would be better I think:
>>
>> 	if (!kasan_arch_is_ready())
>> 		return;
>>
>> Unless somebody minds, I'll do the change and take this patch in my
>> series in order to handle the case of book3s/32 hash.
> 
> Please do; feel free to take as many of the patches as you would like
> and I'll rebase whatever is left on the next version of your series.

I have now done a big step with v7: works on both nohash and hash ppc32 
without any special feature in the core of kasan. Have to do more tests 
on the hash version, but it seems promissing.

I have kept your patches on sync on top of it (allthough totally 
untested), you can find them in 
https://github.com/chleroy/linux/commits/kasan

> 
> The idea with the macro magic was to take advantage of the speed of
> static keys (I think, I borrowed it from Balbir's patch). Perhaps an
> inline function will achieve this anyway, but given that KASAN with
> outline instrumentation is inevitably slow, I guess it doesn't matter
> much either way.

You'll see in the modifications I've done to your patches, we can still 
use static keys while using static inline functions.

Christophe