netdev - Re: [PATCH net] net: stmmac: prevent division by 0 in stmmac_init_tstamp

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Message-ID: <c3183a23-21da-435d-b599-7003ae7ba79b@lunn.ch>
Date: Mon, 8 Sep 2025 18:47:26 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Sergey Shtylyov <s.shtylyov@....ru>
Cc: Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH net] net: stmmac: prevent division by 0 in
 stmmac_init_tstamp_counter()

On Fri, Sep 05, 2025 at 07:06:50PM +0300, Sergey Shtylyov wrote:
> In stmmac_init_tstamp_counter(), the sec_inc variable is initialized to 0,
> and if stmmac_config_sub_second_increment() fails to set it to some non-0
> value, the following div_u64() call would cause a kernel oops (because of
> the divide error exception).  Let's check sec_inc for 0 before dividing by
> it and just return -EINVAL if so...
> 
> Found by Linux Verification Center (linuxtesting.org) with the Svace static
> analysis tool.
> 
> Fixes: df103170854e ("net: stmmac: Avoid sometimes uninitialized Clang warnings")
> Signed-off-by: Sergey Shtylyov <s.shtylyov@....ru>
> 
> ---
> The patch is against the master branch of Linus Torvalds' linux.git repo.

Wrong tree. Please see:

https://www.kernel.org/doc/html/latest/process/maintainer-netdev.html

This also needs reviewing by somebody who know the STMMAC
hardware. There is a comment:

	/* For GMAC3.x, 4.x versions, in "fine adjustement mode" set sub-second
	 * increment to twice the number of nanoseconds of a clock cycle.
	 * The calculation of the default_addend value by the caller will set it
	 * to mid-range = 2^31 when the remainder of this division is zero,
	 * which will make the accumulator overflow once every 2 ptp_clock
	 * cycles, adding twice the number of nanoseconds of a clock cycle :
	 * 2000000000ULL / ptp_clock.

So i'm wondering if the subsecond adjustment is sufficient, the
sec_inc might be zero, and rather than returning an error, the
hardware just needs programming differently?

    Andrew

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