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

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Message-ID: <90e5657a-d9f1-4d45-edfd-07e51d98731e@csgroup.eu>
Date:   Fri, 19 May 2023 05:40:26 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        David Laight <David.Laight@...LAB.COM>
CC:     'Michael Ellerman' <mpe@...erman.id.au>,
        "glider@...gle.com" <glider@...gle.com>,
        "elver@...gle.com" <elver@...gle.com>,
        "zhangpeng.00@...edance.com" <zhangpeng.00@...edance.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] mm: kfence: Fix false positives on big endian



Le 18/05/2023 à 00:20, Andrew Morton a écrit :
> On Fri, 5 May 2023 16:02:17 +0000 David Laight <David.Laight@...LAB.COM> 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@...erman.id.au>
>>> ---
>>>   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@intel.com.
> 
> 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