linux-kernel - Re: [PATCH net-next] net: phy: mdio-bcm-unimac: Cast denominator to unsigned long to avoid overflow

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Date: Tue, 12 Mar 2024 19:42:16 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Doug Berger <opendmb@...il.com>
Cc: Florian Fainelli <f.fainelli@...il.com>,
	Daniil Dulov <d.dulov@...ddin.ru>, Jakub Kicinski <kuba@...nel.org>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Andrew Lunn <andrew@...n.ch>,
	Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	lvc-project@...uxtesting.org
Subject: Re: [PATCH net-next] net: phy: mdio-bcm-unimac: Cast denominator to
 unsigned long to avoid overflow

On Tue, Mar 12, 2024 at 12:23:20PM -0700, Doug Berger wrote:
> On 3/12/2024 10:23 AM, Florian Fainelli wrote:
> > On 3/12/24 10:18, Russell King (Oracle) wrote:
> > > On Tue, Mar 12, 2024 at 07:53:58PM +0300, Daniil Dulov wrote:
> > > > The expression priv->clk_freq * 2 can lead to overflow that will cause
> > > > a division by zero. So, let's cast it to unsigned long to avoid it.
> > > 
> > > How does casting this help? "unsigned long" can still be 32-bit.
> > > Maybe unimac_mdio_probe() should be validating the value it read from
> > > DT won't overflow? I suspect that a value of 2.1GHz is way too large
> > > for this property in any case.
> > > 
> > > https://en.wikipedia.org/wiki/Management_Data_Input/Output#Electrical_specification
> > > 
> > > (note, this driver is clause-22 only.)
> > > 
> > 
> > Had commented on the previous version (not sure why this was not
> > prefixed with v2) that the maximum clock frequency for this clock is
> > 250MHz, the driver could check that to prevent for an overflow, most
> > certainly.
> 
> Could also use:
> -	div = (rate / (2 * priv->clk_freq)) - 1;
> +	div = ((rate / priv->clk_freq) >> 1) - 1;
> which is mathematically equivalent without the risk of overflow.

What's the point when the maximum clock frequency that the driver should
allow fits within u32, nay u28?

-- 
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