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

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Message-ID: <CAKA6ep+EFNOYY8k8PFP9kf_F5GY+5g8qu_LphEAX6N7iEFTs9Q@mail.gmail.com>
Date:   Tue, 28 Jul 2020 17:52:44 -0700
From:   Ashutosh Grewal <ashutoshgrewal@...il.com>
To:     davem@...emloft.net, netdev@...r.kernel.org
Subject: Bug: ip utility fails to show routes with large # of multipath next-hops

Hello David and all,

I hope this is the correct way to report a bug.

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(...)

Thanks,
Ashutosh