linux-kernel - Re: [PATCH] asm-generic: fix __get_unaligned

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Date:   Mon, 11 Apr 2022 20:00:49 +0000
From:   Alexander Lobakin <alobakin@...me>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Alexander Lobakin <alobakin@...me>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Bart Van Assche <bvanassche@....org>,
        Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>,
        Chaitanya Kulkarni <kch@...dia.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] asm-generic: fix __get_unaligned_be48() on 32 bit platforms

From: Alexander Lobakin <alobakin@...me>
Date: Wed, 06 Apr 2022 23:46:04 +0000

> While testing the new macros for working with 48 bit containers,
> I faced a weird problem:
>
> 32 + 16: 0x2ef6e8da 0x79e60000
> 48: 0xffffe8da + 0x79e60000
>
> All the bits starting from the 32nd were getting 1d in 9/10 cases.
> The debug showed:
>
> p[0]: 0x00002e0000000000
> p[1]: 0x00002ef600000000
> p[2]: 0xffffffffe8000000
> p[3]: 0xffffffffe8da0000
> p[4]: 0xffffffffe8da7900
> p[5]: 0xffffffffe8da79e6
>
> that the value becomes a garbage after the third OR, i.e. on
> `p[2] << 24`.
> When the 31st bit is 1 and there's no explicit cast to an unsigned,
> it's being considered as a signed int and getting sign-extended on
> OR, so `e8000000` becomes `ffffffffe8000000` and messes up the
> result.
> Cast the @p[2] to u64 as well to avoid this. Now:
>
> 32 + 16: 0x7ef6a490 0xddc10000
> 48: 0x7ef6a490 + 0xddc10000
>
> p[0]: 0x00007e0000000000
> p[1]: 0x00007ef600000000
> p[2]: 0x00007ef6a4000000
> p[3]: 0x00007ef6a4900000
> p[4]: 0x00007ef6a490dd00
> p[5]: 0x00007ef6a490ddc1
>
> Fixes: c2ea5fcf53d5 ("asm-generic: introduce be48 unaligned accessors")
> Signed-off-by: Alexander Lobakin <alobakin@...me>

Uhm, ping?

> ---
>  include/asm-generic/unaligned.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/asm-generic/unaligned.h b/include/asm-generic/unalign=
> ed.h
> index 8fc637379899..df30f11b4a46 100644
> --- a/include/asm-generic/unaligned.h
> +++ b/include/asm-generic/unaligned.h
> @@ -143,7 +143,7 @@ static inline void put_unaligned_be48(const u64 val, v=
> oid *p)
>
>  static inline u64 __get_unaligned_be48(const u8 *p)
>  {
> -	return (u64)p[0] << 40 | (u64)p[1] << 32 | p[2] << 24 |
> +	return (u64)p[0] << 40 | (u64)p[1] << 32 | (u64)p[2] << 24 |
>  		p[3] << 16 | p[4] << 8 | p[5];
>  }
>
> --
> 2.35.1

Thanks,
Al