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SubjectRe: [PATCH] net: 8390: Fix possible data races in __ei_get_stats
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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

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