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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>
 <d4de4f6c-c697-fde5-9d31-d8792621bfb5@gmail.com>
 <c336fdb9-4865-4729-d34b-84dd0bfc9af5@gmail.com>
From:   Jia-Ju Bai <baijiaju1990@gmail.com>
Message-ID: <6fe51782-2169-53c8-9bac-1715262e716d@gmail.com>
Date:   Tue, 8 May 2018 10:16:49 +0800
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On 2018/5/8 9:56, Eric Dumazet wrote:
>
> On 05/07/2018 05:51 PM, Jia-Ju Bai wrote:
>>
>> 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 ?
>
>> So I think a data race may occur when returning "dev->stats" without lock protection.
> &dev->stats is a stable value.
>
> It wont change over the lifetime of net_device object.
>
> Adding a barrier before or after getting &dev->stats is useless, confusing and really not needed.
>

Yes, "&dev->stats" will not change, because it is a fixed address.
But the field data in "dev->stats" is changed (rx_frame_errors, 
rx_crc_errors and rx_missed_errors).
So if the driver returns "&dev->stats" without lock protection (like on 
line 858), the field data value of this return value can be the changed 
field data value or unchanged field data value.


Best wishes,
Jia-Ju Bai