Re: [PATCH] mm: kfence: Fix false positives on big endian

[Christophe Leroy]

Le 18/05/2023 à 00:20, Andrew Morton a écrit :
> On Fri, 5 May 2023 16:02:17 +0000 David Laight <David.Laight@xxxxxxxxxx> wrote:
> 
>> From: Michael Ellerman
>>> Sent: 05 May 2023 04:51
>>>
>>> Since commit 1ba3cbf3ec3b ("mm: kfence: improve the performance of
>>> __kfence_alloc() and __kfence_free()"), kfence reports failures in
>>> random places at boot on big endian machines.
>>>
>>> The problem is that the new KFENCE_CANARY_PATTERN_U64 encodes the
>>> address of each byte in its value, so it needs to be byte swapped on big
>>> endian machines.
>>>
>>> The compiler is smart enough to do the le64_to_cpu() at compile time, so
>>> there is no runtime overhead.
>>>
>>> Fixes: 1ba3cbf3ec3b ("mm: kfence: improve the performance of __kfence_alloc() and __kfence_free()")
>>> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
>>> ---
>>>   mm/kfence/kfence.h | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
>>> index 2aafc46a4aaf..392fb273e7bd 100644
>>> --- a/mm/kfence/kfence.h
>>> +++ b/mm/kfence/kfence.h
>>> @@ -29,7 +29,7 @@
>>>    * canary of every 8 bytes is the same. 64-bit memory can be filled and checked
>>>    * at a time instead of byte by byte to improve performance.
>>>    */
>>> -#define KFENCE_CANARY_PATTERN_U64 ((u64)0xaaaaaaaaaaaaaaaa ^ (u64)(0x0706050403020100))
>>> +#define KFENCE_CANARY_PATTERN_U64 ((u64)0xaaaaaaaaaaaaaaaa ^ (u64)(le64_to_cpu(0x0706050403020100)))
>>
>> What at the (u64) casts for?
>> The constants should probably have a ul (or ull) suffix.
>>
> 
> I tried that, didn't fix the sparse warnings described at
> https://lkml.kernel.org/r/202305132244.DwzBUcUd-lkp@xxxxxxxxx.
> 
> Michael, have you looked into this?
> 
> I'll merge it upstream - I guess we can live with the warnings for a while.
> 

sparse warning goes away with:

#define KFENCE_CANARY_PATTERN_U64 (0xaaaaaaaaaaaaaaaaULL ^ 
le64_to_cpu((__force __le64)0x0706050403020100))

Christophe