netdev - Re: [PATCH] net: mscc: fix in frame extraction

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Date:   Mon, 17 Feb 2020 10:20:06 +0100
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Horatiu Vultur <horatiu.vultur@...rochip.com>
Cc:     UNGLinuxDriver@...rochip.com, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: mscc: fix in frame extraction

On 17/02/2020 09:31:33+0100, Horatiu Vultur wrote:
> Each extracted frame on Ocelot has an IFH. The frame and IFH are extracted
> by reading chuncks of 4 bytes from a register.
> 
> In case the IFH and frames were read corretly it would try to read the next
> frame. In case there are no more frames in the queue, it checks if there
> were any previous errors and in that case clear the queue. But this check
> will always succeed also when there are no errors. Because when extracting
> the IFH the error is checked against 4(number of bytes read) and then the
> error is set only if the extraction of the frame failed. So in a happy case
> where there are no errors the err variable is still 4. So it could be
> a case where after the check that there are no more frames in the queue, a
> frame will arrive in the queue but because the error is not reseted, it
> would try to flush the queue. So the frame will be lost.
> 
> The fix consist in resetting the error after reading the IFH.
> 
> Signed-off-by: Horatiu Vultur <horatiu.vultur@...rochip.com>
Acked-by: Alexandre Belloni <alexandre.belloni@...tlin.com>

> ---
>  drivers/net/ethernet/mscc/ocelot_board.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/net/ethernet/mscc/ocelot_board.c b/drivers/net/ethernet/mscc/ocelot_board.c
> index b38820849faa..1135a18019c7 100644
> --- a/drivers/net/ethernet/mscc/ocelot_board.c
> +++ b/drivers/net/ethernet/mscc/ocelot_board.c
> @@ -114,6 +114,14 @@ static irqreturn_t ocelot_xtr_irq_handler(int irq, void *arg)
>  		if (err != 4)
>  			break;
>  
> +		/* At this point the IFH was read correctly, so it is safe to
> +		 * presume that there is no error. The err needs to be reset
> +		 * otherwise a frame could come in CPU queue between the while
> +		 * condition and the check for error later on. And in that case
> +		 * the new frame is just removed and not processed.
> +		 */
> +		err = 0;
> +
>  		ocelot_parse_ifh(ifh, &info);
>  
>  		ocelot_port = ocelot->ports[info.port];
> -- 
> 2.17.1
> 

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com