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

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Message-ID: <CAFd5g47U0Y4FHaxz5PCzpvCfnzDmGWQze364wA+hxmb3oKo8Sg@mail.gmail.com>
Date:   Thu, 1 Nov 2018 09:56:54 -0700
From:   Brendan Higgins <brendanhiggins@...gle.com>
To:     eric.dumazet@...il.com
Cc:     "David S . Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        yoshfuji@...ux-ipv6.org, netdev@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] net: ipv6: fix racey clock check in route cache aging logic

On Thu, Oct 25, 2018 at 3:15 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 10/25/2018 02:46 PM, Brendan Higgins wrote:
> > On Thu, Oct 25, 2018 at 2:40 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
> >> On 10/25/2018 02:13 PM, Brendan Higgins wrote:
> > <snip>
> >>>
> >>> 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.
> >
> > Then why do we pass the `now` parameter in at all and use it at all,
> > like here: https://elixir.bootlin.com/linux/latest/source/net/ipv6/route.c#L1764
> > ?
> >
> > I am still skeptical that we should check jiffies in each check, but
> > we should at least be consistent.
>
> Well, this is a case where we do not really care.
>
> When a bug is fixed (you added a Fixes: tag which is good), we want
> to understand the real problem that needs to be fixed on stable kernels.

So by bug you mean user visible bug?
>
> Since this does not seem to be a real issue, I would suggest you send a cleanup
> patch when net-next is open (few days after linux-4.20-rc1 is release)

Sounds good, I will resend shortly.

Thanks!