linux-kernel - Re: [PATCH] net: sxgbe: fix unintended sign extension

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Message-Id: <20190206.115241.1549120165262106425.davem@davemloft.net>
Date:   Wed, 06 Feb 2019 11:52:41 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     colin.king@...onical.com
Cc:     bh74.an@...sung.com, ks.giri@...sung.com, vipul.pandya@...sung.com,
        netdev@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: sxgbe: fix unintended sign extension

From: Colin King <colin.king@...onical.com>
Date: Wed,  6 Feb 2019 10:25:03 +0000

> From: Colin Ian King <colin.king@...onical.com>
> 
> Shifting a u8 by 24 will cause the value to be promoted to an integer. If
> the top bit of the u8 is set then the following conversion to an unsigned
> long will sign extend the value causing the upper 32 bits to be set in
> the result.

We feed this into a writel() which truncates to a 32-bit value, so nothing
bad can happen here.

This is a very canonical way to code up something like this, it works
properly in all situations, and therefore I'd rather not add all of
these fat ugly looking casts.

Thank you.