From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753880AbeEHB4g (ORCPT ); Mon, 7 May 2018 21:56:36 -0400 Received: from mail-pl0-f68.google.com ([209.85.160.68]:36081 "EHLO mail-pl0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753737AbeEHB4c (ORCPT ); Mon, 7 May 2018 21:56:32 -0400 X-Google-Smtp-Source: AB8JxZqoXrLVe8iVhkTezpcPcWaepCkU6G2hQbb+jmnnV0LB7VjvqMcXTkXt7CZ1anhlzcAMkACxQw== Subject: Re: [PATCH] net: 8390: Fix possible data races in __ei_get_stats To: Jia-Ju Bai , Eric Dumazet , 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> From: Eric Dumazet Message-ID: Date: Mon, 7 May 2018 18:56:29 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >>> --- >>>   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.