linux-kernel - Re: [PATCH net v3 2/3] net: stmmac: Consider Tx VLAN offload tag length for maxSDU

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Message-ID: <aPoKOIfCGvDEIWS7@shell.armlinux.org.uk>
Date: Thu, 23 Oct 2025 11:58:00 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: "G Thomas, Rohan" <rohan.g.thomas@...era.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>,
	Jose Abreu <Jose.Abreu@...opsys.com>,
	Rohan G Thomas <rohan.g.thomas@...el.com>,
	Boon Khai Ng <boon.khai.ng@...era.com>, netdev@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Matthew Gerlach <matthew.gerlach@...era.com>
Subject: Re: [PATCH net v3 2/3] net: stmmac: Consider Tx VLAN offload tag
 length for maxSDU

On Sat, Oct 18, 2025 at 07:36:26AM +0530, G Thomas, Rohan wrote:
> Hi Russell,
> 
> Thanks, I'll update the commit message.
> 
> On 10/17/2025 6:14 PM, Russell King (Oracle) wrote:
> > On Fri, Oct 17, 2025 at 02:11:20PM +0800, Rohan G Thomas via B4 Relay wrote:
> > > From: Rohan G Thomas <rohan.g.thomas@...era.com>
> > > 
> > > On hardware with Tx VLAN offload enabled, add the VLAN tag length to
> > > the skb length before checking the Qbv maxSDU if Tx VLAN offload is
> > > requested for the packet. Add 4 bytes for 802.1Q tag.
> > 
> > This needs to say _why_. Please describe the problem that the current
> > code suffers from. (e.g. the packet becomes too long for the queue to
> > handle, which causes it to be dropped - which is my guess.)
> > 
> > We shouldn't be guessing the reasons behind changes.
> > 
> 
> Queue maxSDU requirement of 802.1 Qbv standard requires mac to drop
> packets that exceeds maxSDU length and maxSDU doesn't include preamble,
> destination and source address, or FCS but includes ethernet type and VLAN
> header.
> 
> On hardware with Tx VLAN offload enabled, VLAN header length is not
> included in the skb->len, when Tx VLAN offload is requested. This leads
> to incorrect length checks and allows transmission of oversized packets.
> Add the VLAN_HLEN to the skb->len before checking the Qbv maxSDU if Tx
> VLAN offload is requested for the packet.
> 
> This patch ensures that the VLAN header length (`VLAN_HLEN`) is
> accounted for in the SDU length check when VLAN offload is requested.

Please put that in the commit message, thanks.

-- 
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