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

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Message-ID: <20200729114317.GA2120829@shredder>
Date:   Wed, 29 Jul 2020 14:43:17 +0300
From:   Ido Schimmel <idosch@...sch.org>
To:     Ashutosh Grewal <ashutoshgrewal@...il.com>, dsahern@...il.com
Cc:     davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: Bug: ip utility fails to show routes with large # of multipath
 next-hops

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@...e8c892bb7:/# 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@...ntu-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.

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.

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.

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
:)

> 
> Thanks,
> Ashutosh