linux-kernel - RE: [PATCH net 1/8] net: enetc: fix the off-by-one issue in enetc_map_tx

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Message-ID:
 <PAXPR04MB85105B5C71672A828E8D6F6988FA2@PAXPR04MB8510.eurprd04.prod.outlook.com>
Date: Tue, 18 Feb 2025 02:02:46 +0000
From: Wei Fang <wei.fang@....com>
To: Claudiu Manoil <claudiu.manoil@....com>, Vladimir Oltean
	<vladimir.oltean@....com>, Clark Wang <xiaoning.wang@....com>,
	"andrew+netdev@...n.ch" <andrew+netdev@...n.ch>, "davem@...emloft.net"
	<davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>
CC: Ioana Ciornei <ioana.ciornei@....com>, "Y.B. Lu" <yangbo.lu@....com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"imx@...ts.linux.dev" <imx@...ts.linux.dev>, "stable@...r.kernel.org"
	<stable@...r.kernel.org>
Subject: RE: [PATCH net 1/8] net: enetc: fix the off-by-one issue in
 enetc_map_tx_buffs()

> > -----Original Message-----
> > From: Wei Fang <wei.fang@....com>
> > Sent: Monday, February 17, 2025 11:39 AM
> [...]
> > Subject: [PATCH net 1/8] net: enetc: fix the off-by-one issue in
> > enetc_map_tx_buffs()
> >
> > When the DMA mapping error occurs, it will attempt to free 'count + 1'
> 
> Hi Wei,
> Are you sure?
> 
> dma_err occurs before count is incremented, at least that's the design.
> 
> First step already contradicts your claim:
> ```
> static int enetc_map_tx_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb)
> { [...]
> 	int i, count = 0;
> [...]
> 	dma = dma_map_single(tx_ring->dev, skb->data, len, DMA_TO_DEVICE);
> 	if (unlikely(dma_mapping_error(tx_ring->dev, dma)))
> 		goto dma_err;
> 
> ===> count is 0 on goto path!
> [...]
> 	count++;
> ```
> 

Hi Claudiu,

I think it's fine in your case, the count is 0, and the tx_swbd is not set,
so it's unnecessary to call enetc_free_tx_frame() in the dma_err path.

But if the count is not zero, then that is the problem, for example, count
is 1, and then goto dma_err path, the it will attempt to free 2 tx_swbd.