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

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Message-ID: <CAFd5g44jmpDyeh--=Hd2fAexzik4gMYLMdNhAMqxWqnjSh4KeA@mail.gmail.com>
Date:   Thu, 25 Oct 2018 14:46:50 -0700
From:   Brendan Higgins <brendanhiggins@...gle.com>
To:     eric.dumazet@...il.com
Cc:     "David S . Miller" <davem@...emloft.net>, 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 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.

Cheers