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

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Date:   Fri, 5 May 2023 16:02:17 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Michael Ellerman' <mpe@...erman.id.au>,
        "glider@...gle.com" <glider@...gle.com>,
        "elver@...gle.com" <elver@...gle.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "zhangpeng.00@...edance.com" <zhangpeng.00@...edance.com>
CC:     "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

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.

	David

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