linux-kernel - Re: [PATCH net v2 2/2] net: stmmac: fix multiplication overflow when reading timestamp

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Message-ID: <20250423150233.38fb5437@device-40.home>
Date: Wed, 23 Apr 2025 15:02:33 +0200
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: Alexis Lothoré <alexis.lothore@...tlin.com>
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>, Richard Cochran <richardcochran@...il.com>,
 Daniel Machon <daniel.machon@...rochip.com>, Thomas Petazzoni
 <thomas.petazzoni@...tlin.com>, netdev@...r.kernel.org,
 linux-stm32@...md-mailman.stormreply.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 "Russell King (Oracle)" <linux@...linux.org.uk>
Subject: Re: [PATCH net v2 2/2] net: stmmac: fix multiplication overflow
 when reading timestamp

On Wed, 23 Apr 2025 09:12:10 +0200
Alexis Lothoré <alexis.lothore@...tlin.com> wrote:

> The current way of reading a timestamp snapshot in stmmac can lead to
> integer overflow, as the computation is done on 32 bits. The issue has
> been observed on a dwmac-socfpga platform returning chaotic timestamp
> values due to this overflow. The corresponding multiplication is done
> with a MUL instruction, which returns 32 bit values. Explicitly casting
> the value to 64 bits replaced the MUL with a UMLAL, which computes and
> returns the result on 64 bits, and so returns correctly the timestamps.
> 
> Prevent this overflow by explicitly casting the intermediate value to
> u64 to make sure that the whole computation is made on u64. While at it,
> apply the same cast on the other dwmac variant (GMAC4) method for
> snapshot retrieval.
> 
> Fixes: 477c3e1f6363 ("net: stmmac: Introduce dwmac1000 timestamping operations")
> Signed-off-by: Alexis Lothoré <alexis.lothore@...tlin.com>

Reviewed-by: Maxime Chevallier <maxime.chevallier@...tlin.com>

Thanks,

Maxime