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Subject: Re: [PATCH] net: 8390: Fix possible data races in __ei_get_stats
To:     Eric Dumazet <eric.dumazet@gmail.com>, davem@davemloft.net,
        fthain@telegraphics.com.au, joe@perches.com
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20180507140809.28847-1-baijiaju1990@gmail.com>
 <e48345b4-fe9f-d267-777b-6a51baba0307@gmail.com>
From:   Jia-Ju Bai <baijiaju1990@gmail.com>
Message-ID: <d4de4f6c-c697-fde5-9d31-d8792621bfb5@gmail.com>
Date:   Tue, 8 May 2018 08:51:12 +0800
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On 2018/5/7 22:15, Eric Dumazet wrote:
>
> On 05/07/2018 07:08 AM, Jia-Ju Bai wrote:
>> The write operations to "dev->stats" are protected by
>> the spinlock on line 862-864, but the read operations to
>> this data on line 858 and 867 are not protected by the spinlock.
>> Thus, there may exist data races for "dev->stats".
>>
>> To fix the data races, the read operations to "dev->stats" are
>> protected by the spinlock, and a local variable is used for return.
>>
>> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
>> ---
>>   drivers/net/ethernet/8390/lib8390.c | 14 ++++++++++----
>>   1 file changed, 10 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/8390/lib8390.c b/drivers/net/ethernet/8390/lib8390.c
>> index c9c55c9eab9f..198952247d30 100644
>> --- a/drivers/net/ethernet/8390/lib8390.c
>> +++ b/drivers/net/ethernet/8390/lib8390.c
>> @@ -852,19 +852,25 @@ static struct net_device_stats *__ei_get_stats(struct net_device *dev)
>>   	unsigned long ioaddr = dev->base_addr;
>>   	struct ei_device *ei_local = netdev_priv(dev);
>>   	unsigned long flags;
>> +	struct net_device_stats *stats;
>> +
>> +	spin_lock_irqsave(&ei_local->page_lock, flags);
>>   
>>   	/* If the card is stopped, just return the present stats. */
>> -	if (!netif_running(dev))
>> -		return &dev->stats;
>> +	if (!netif_running(dev)) {
>> +		stats = &dev->stats;
>> +		spin_unlock_irqrestore(&ei_local->page_lock, flags);
>> +		return stats;
>> +	}
>>   
>> -	spin_lock_irqsave(&ei_local->page_lock, flags);
>>   	/* Read the counter registers, assuming we are in page 0. */
>>   	dev->stats.rx_frame_errors  += ei_inb_p(ioaddr + EN0_COUNTER0);
>>   	dev->stats.rx_crc_errors    += ei_inb_p(ioaddr + EN0_COUNTER1);
>>   	dev->stats.rx_missed_errors += ei_inb_p(ioaddr + EN0_COUNTER2);
>> +	stats = &dev->stats;
>>   	spin_unlock_irqrestore(&ei_local->page_lock, flags);
>>   
>> -	return &dev->stats;
>> +	return stats;
>>   }
>>   
>>   /*
>>
> dev->stats is not a pointer, it is an array embedded in the
> struct net_device
>
> So this patch is not needed, since dev->stats can not change.

Thanks for your reply :)

I do not understand that why "dev->stats can not change".
Its data is indeed changed by the code:
      dev->stats.rx_frame_errors  += ei_inb_p(ioaddr + EN0_COUNTER0);
      dev->stats.rx_crc_errors    += ei_inb_p(ioaddr + EN0_COUNTER1);
      dev->stats.rx_missed_errors += ei_inb_p(ioaddr + EN0_COUNTER2);

So I think a data race may occur when returning "dev->stats" without 
lock protection.

By the way, I find this possible data race is similar to the previous 
commit 7b31b4deda76 for the tg3 driver.


Best wishes,
Jia-Ju Bai