linux-kernel - Re: [net-next] ipv6: fix routing cache overflow for raw sockets

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Date:   Wed, 21 Dec 2022 08:55:38 +1100
From:   Jonathan Maxwell <jmaxwell37@...il.com>
To:     David Ahern <dsahern@...nel.org>
Cc:     Paolo Abeni <pabeni@...hat.com>, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, yoshfuji@...ux-ipv6.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [net-next] ipv6: fix routing cache overflow for raw sockets

On Wed, Dec 21, 2022 at 2:10 AM David Ahern <dsahern@...nel.org> wrote:
>
> On 12/20/22 5:35 AM, Paolo Abeni wrote:
> > On Mon, 2022-12-19 at 10:48 +1100, Jon Maxwell wrote:
> >> Sending Ipv6 packets in a loop via a raw socket triggers an issue where a
> >> route is cloned by ip6_rt_cache_alloc() for each packet sent. This quickly
> >> consumes the Ipv6 max_size threshold which defaults to 4096 resulting in
> >> these warnings:
> >>
> >> [1]   99.187805] dst_alloc: 7728 callbacks suppressed
> >> [2] Route cache is full: consider increasing sysctl net.ipv6.route.max_size.
> >> .
> >> .
> >> [300] Route cache is full: consider increasing sysctl net.ipv6.route.max_size.
> >
> > If I read correctly, the maximum number of dst that the raw socket can
> > use this way is limited by the number of packets it allows via the
> > sndbuf limit, right?
> >
> > Are other FLOWI_FLAG_KNOWN_NH users affected, too? e.g. nf_dup_ipv6,
> > ipvs, seg6?
> >
> > @DavidA: why do we need to create RTF_CACHE clones for KNOWN_NH flows?
> >
> > Thanks,
> >
> > Paolo
> >
>
> If I recall the details correctly: that sysctl limit was added back when
> ipv6 routes were managed as dst_entries and there was a desire to allow
> an admin to limit the memory consumed. At this point in time, IPv6 is
> more inline with IPv4 - a separate struct for fib entries from dst
> entries. That "Route cache is full" message is now out of date since
> this is dst_entries which have a gc mechanism.
>
> IPv4 does not limit the number of dst_entries that can be allocated
> (ip_rt_max_size is the sysctl variable behind the ipv4 version of
> max_size and it is a no-op). IPv6 can probably do the same here?
>

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index dbc224023977..701aba7feaf5 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -6470,7 +6470,7 @@ static int __net_init ip6_route_net_init(struct net *net)
 #endif

        net->ipv6.sysctl.flush_delay = 0;
-       net->ipv6.sysctl.ip6_rt_max_size = 4096;
+       net->ipv6.sysctl.ip6_rt_max_size = INT_MAX;
        net->ipv6.sysctl.ip6_rt_gc_min_interval = HZ / 2;
        net->ipv6.sysctl.ip6_rt_gc_timeout = 60*HZ;
        net->ipv6.sysctl.ip6_rt_gc_interval = 30*HZ;

The above patch resolved it for the Ipv6 reproducer.

Would that be sufficient?

> I do not believe the suggested flag is the right change.

Regards

Jon