linux-kernel - Re: [PATCH v1] net: ipv6: fix racey clock check in route cache aging logic

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Message-ID: <d787249c-0679-065a-62cb-ee9f87b1fba3@gmail.com>
Date:   Thu, 25 Oct 2018 14:40:44 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Brendan Higgins <brendanhiggins@...gle.com>, davem@...emloft.net,
        kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] net: ipv6: fix racey clock check in route cache aging
 logic



On 10/25/2018 02:13 PM, Brendan Higgins wrote:
> Fix a bug where, with certain settings, the aging logic does not use the
> time passed in as the current time, but instead directly checks jiffies.
> 
> This bug can be reproduced with (and this fix verified with) the test
> at: https://kunit-review.googlesource.com/c/linux/+/1156
> 
> Fixes: 31afeb425f7f ("ipv6: change route cache aging logic")
> Discovered-by-KUnit: https://kunit-review.googlesource.com/c/linux/+/1156
> Signed-off-by: Brendan Higgins <brendanhiggins@...gle.com>
> ---
>  net/ipv6/route.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index 2a7423c394560..54d28b91fd840 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -1734,7 +1734,7 @@ static void rt6_age_examine_exception(struct rt6_exception_bucket *bucket,
>  			rt6_remove_exception(bucket, rt6_ex);
>  			return;
>  		}
> -	} else if (time_after(jiffies, rt->dst.expires)) {
> +	} else if (time_after(now, rt->dst.expires)) {
>  		RT6_TRACE("purging expired route %p\n", rt);
>  		rt6_remove_exception(bucket, rt6_ex);
>  		return;
> 


I do not think there is a bug here ?

As a matter of fact, using the latest value of jiffies is probably better,
since in some cases the @now variable could be quite in the past.