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

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Date:   Wed, 17 May 2023 15:20:28 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     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

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.