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

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Message-ID: <a3ce5632-e06a-827e-7f60-e5a4b01301ad@canonical.com>
Date:   Thu, 11 Mar 2021 17:22:15 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Doug Anderson <dianders@...omium.org>
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

On 11/03/2021 17:12, Doug Anderson wrote:
> 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.

yep, that's correct, I forgot to mention that issue too :-/

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