Messages in this thread | | | Subject | Re: [PATCH] net: 8390: Fix possible data races in __ei_get_stats | From | Eric Dumazet <> | Date | Mon, 7 May 2018 18:56:29 -0700 |
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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.
> > By the way, I find this possible data race is similar to the previous commit 7b31b4deda76 for the tg3 driver.
Very different things really.
This does a copy of the whole stats, not the pointer :
*stats = tp->net_stats_prev;
I guess you are confusing simple C semantics about returning the address of a structure, instead of returning a whole structure.
If __ei_get_stats(struct net_device *dev) prototype was :
struct net_device_stats __ei_get_stats(struct net_device *dev)
instead of :
struct net_device_stats *__ei_get_stats(struct net_device *dev)
Then sure, your patch might been needed.
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