On 3/13/18 3:47 PM, Eric Dumazet wrote:



On 03/13/2018 03:37 PM, Yonghong Song wrote:
Adding additional cc's:
   Saeed Mahameed as this is most likely mlx5 driver related.
   Diptanu Gon Choudhury who initially reported the issue.


On 3/13/18 1:44 AM, Steffen Klassert wrote:
On Mon, Mar 12, 2018 at 11:25:09PM -0700, Eric Dumazet wrote:


On 03/12/2018 11:08 PM, Yonghong Song wrote:


On 3/12/18 11:04 PM, Eric Dumazet wrote:


On 03/12/2018 10:45 PM, Yonghong Song wrote:
...
Setup:
=====

The test will involve three machines:
    M_ipv6 <-> M_nat <-> M_ipv4

The M_nat will do ipv4<->ipv6 address translation and then
forward packet
to proper destination. The control plane will configure M_nat
properly
will understand virtual ipv4 address for machine M_ipv6, and
virtual ipv6 address for machine M_ipv4.

M_nat runs a bpf program, which is attached to clsact (ingress)
qdisc.
The program uses bpf_skb_change_proto to do protocol conversion.
bpf_skb_change_proto will adjust skb header_len and len properly
based on protocol change.
After the conversion, the program will make proper change on
ethhdr and ip4/6 header, recalculate checksum, and send the
packet out
through bpf_redirect.

Experiment:
===========

MTU: 1500B for all three machines.

The tso/lro/gro are enabled on the M_nat box.

ping works on both ways of M_ipv6 <-> M_ipv4.
It works for transfering a small file (4KB) between M_ipv6 and
M_ipv4 (both ways).
Transfering a large file (e.g., 4MB) from M_ipv6 to M_ipv4,
failed with the above BUG_ON, really fast.
Did not really test from M_ipv4 to M_ipv6 with large file.

The error path likely to be (also from the above call stack):
    nic -> lro/gro -> bpf_program -> gso (BUG_ON)

Just out of curiosity, are these packets created with LRO or GRO?
Usually LRO is disabled if forwarding is enabled on a machine,
because segmented LGO packets are likely corrupt.

In our experiments, LRO is disabled.
On mlx5, when GRO is on, the BUG_ON will happen, and
when GRO is off, the BUG_ON will not happen.


These packets take an alternative redirect path, so not sure what
happens here.


In one of experiments, I explicitly printed the skb->len and
skb->data_len. The values are below:
    skb_segment: len 2856, data_len 2686
They should be equal to avoid BUG.

In another experiment, I got:
    skb_segment: len 1428, data_len 1258

In both cases, the difference is 170 bytes. Not sure whether
this is just a coincidence or not.

Workaround:
===========

A workaround to avoid BUG_ON is to disable lro/gro. This way,
kernel will not receive big packets and hence gso is not really
called.

I am not familiar with gso code. Does anybody hit this BUG_ON
before?
Any suggestion on how to debug this?


skb_segment() works if incoming GRO packet is not modified in its
geometry.

In your case it seems you had to adjust gso_size (calling
skb_decrease_gso_size() or skb_increase_gso_size()), and this breaks
skb_segment() badly, because geometry changes, unless you had
specific MTU/MSS restrictions.

You will have to make skb_segment() more generic if you really
want this.

In net/core/filter.c function bpf_skb_change_proto, which is called
in the bpf program, does some GSO adjustment. Could you help check
whether it satisfies my above use case or not? Thanks!

As I said this  helper ends up modifying gso_size by +/- 20
(sizeof(ipv6
header) - sizeof(ipv4 header))

So it wont work if skb_segment() is called after this change.

Even HW TSO use gso_size to segment the packets. Would'nt this
result in broken packets too, if gso_size is modified on a
forwarding path?


Not clear why the GRO packet is not sent as is (as a TSO packet) since
mlx4/mlx5 NICs certainly support TSO.

This is a very good observation.
We did the same experiment on mlx4, the same kernel, the same
userspace apps, the same bpf program. The only difference is mlx4 vs.
mlx5.
The mlx4 works fine with LRO=off and GRO=on, while
mlx5 failed with the same LRO/GRO configuration.

On mlx4 box, we are able to see TSO packets are increasing as the
large file scp is in progress.
# ethtool -S eth0 | grep tso
      tso_packets: 45495
# ethtool -S eth0 | grep tso
      tso_packets: 45865
# ethtool -S eth0 | grep tso
      tso_packets: 46337
# ethtool -S eth0 | grep tso
      tso_packets: 46724

And use bcc tool to track to func call count for skb_segment
and find it is called 0 times. Clearly, mlx4 is able to take
the packet as TSO and hence the packet will not go to
the stack.

# funccount.py -i 3 'skb_segment'
Tracing 1 functions for "skb_segment"... Hit Ctrl-C to end.

FUNC                                    COUNT

FUNC                                    COUNT
...

CC Saeed Mahameed who may help debug and provide some insights
what is the problem.

There are many reasons why a GRO packet might need to be segmented by
software (skb_segment())

This is the step where you have crashes, so really mlx4/mlx5 difference
do not matter.

gso_size should not be dynamically changed. This needs to be fixed in
eBPF helpers eventually.

we have bpf_skb_proto_6_to_4() that was used by cilium for long time.
It's not clear why it's not crashing there, but we cannot just
reject changing proto in bpf programs now.
We have to fix whatever needs to be fixed in skb_segment
(if bug is there) or fix whatever necessary on mlx5 side.
In bpf helper we mark it as SKB_GSO_DODGY just like packets coming
through virtio would do, so if skb_segment() needs to do something
special with skb the SKB_GSO_DODGY flag is already there.

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