Re: Bug: ip utility fails to show routes with large # of multipath next-hops

[Ashutosh Grewal <ashutoshgrewal@gmail.com>]

Thanks Ido and David for your confirmation and insight.

-- Ashutosh

On Wed, Jul 29, 2020 at 8:17 AM David Ahern <dsahern@gmail.com> wrote:
>
> On 7/29/20 5:43 AM, Ido Schimmel wrote:
> > On Tue, Jul 28, 2020 at 05:52:44PM -0700, Ashutosh Grewal wrote:
> >> Hello David and all,
> >>
> >> I hope this is the correct way to report a bug.
> >
> > Sure
> >
> >>
> >> I observed this problem with 256 v4 next-hops or 128 v6 next-hops (or
> >> 128 or so # of v4 next-hops with labels).
> >>
> >> Here is an example -
> >>
> >> root@a6be8c892bb7:/# ip route show 2.2.2.2
> >> Error: Buffer too small for object.
> >> Dump terminated
> >>
> >> Kernel details (though I recall running into the same problem on 4.4*
> >> kernel as well) -
> >> root@ubuntu-vm:/# uname -a
> >> Linux ch1 5.4.0-33-generic #37-Ubuntu SMP Thu May 21 12:53:59 UTC 2020
> >> x86_64 x86_64 x86_64 GNU/Linux
> >>
> >> I think the problem may be to do with the size of the skbuf being
> >> allocated as part of servicing the netlink request.
> >>
> >> static int netlink_dump(struct sock *sk)
> >> {
> >>   <snip>
> >>
> >>                 skb = alloc_skb(...)
> >
> > Yes, I believe you are correct. You will get an skb of size 4K and it
> > can't fit the entire RTA_MULTIPATH attribute with all the nested
> > nexthops. Since it's a single attribute it cannot be split across
> > multiple messages.
>
> yep, well known problem.
>
> >
> > Looking at the code, I think a similar problem was already encountered
> > with IFLA_VFINFO_LIST. See commit c7ac8679bec9 ("rtnetlink: Compute and
> > store minimum ifinfo dump size").
> >
> > Maybe we can track the maximum number of IPv4/IPv6 nexthops during
> > insertion and then consult it to adjust 'min_dump_alloc' for
> > RTM_GETROUTE.
>
> That seems better than the current design for GETLINK which walks all
> devices to determine max dump size. Not sure how you will track that
> efficiently though - add is easy, delete is not.
>
> >
> > It's a bit complicated for IPv6 because you can append nexthops, but I
> > believe anyone using so many nexthops is already using RTA_MULTIPATH to
> > insert them, so we can simplify.
>
> I hope so.
>
> >
> > David, what do you think? You have a better / simpler idea? Maybe one
> > day everyone will be using the new nexthop API and this won't be needed
> > :)
>
> exactly. You won't have this problem with separate nexthops since each
> one is small (< 4k) and the group (multipath) is a set of ids, not the
> full set of attributes.