linux-kernel - Re: [PATCH][next] nvmem: core: Fix unintentional sign extension issue

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Date:   Thu, 11 Mar 2021 09:12:40 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Colin King <colin.king@...onical.com>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH][next] nvmem: core: Fix unintentional sign extension issue

Hi,

On Thu, Mar 11, 2021 at 1:53 AM Colin King <colin.king@...onical.com> wrote:
>
> From: Colin Ian King <colin.king@...onical.com>
>
> The shifting of the u8 integer buf[3] by 24 bits to the left will
> be promoted to a 32 bit signed int and then sign-extended to a
> u64. In the event that the top bit of buf[3] is set then all
> then all the upper 32 bits of the u64 end up as also being set
> because of the sign-extension. Fix this by casting buf[i] to
> a u64 before the shift.
>
> Addresses-Coverity: ("Unintended sign extension")
> Fixes: 097eb1136ebb ("nvmem: core: Add functions to make number reading easy")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  drivers/nvmem/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Thanks! I had only tested the "u64" version to read smaller data and
store it in a u64. From my understanding of C rules, without your
patch it would have been even worse than just a sign extension though,
right? Shifting "buf[i]" by more than 32 bits would just not have
worked right.

In any case:

Reviewed-by: Douglas Anderson <dianders@...omium.org>