Re: [PATCH net v3] selftests/fib_tests: Rework fib_rp_filter_test()

[Peilin Ye <yepeilin.cs@gmail.com>]

Hi David,

On Wed, Dec 01, 2021 at 11:00:26AM -0700, David Ahern wrote:
> On 11/30/21 5:47 PM, Peilin Ye wrote:
> >  ┌─────────────────────────────┐    ┌─────────────────────────────┐
> >  │  network namespace 1 (ns1)  │    │  network namespace 2 (ns2)  │
> >  │                             │    │                             │
> >  │  ┌────┐     ┌─────┐         │    │  ┌─────┐            ┌────┐  │
> >  │  │ lo │<───>│veth1│<────────┼────┼─>│veth2│<──────────>│ lo │  │
> >  │  └────┘     ├─────┴──────┐  │    │  ├─────┴──────┐     └────┘  │
> >  │             │192.0.2.1/24│  │    │  │192.0.2.1/24│             │
> >  │             └────────────┘  │    │  └────────────┘             │
> >  └─────────────────────────────┘    └─────────────────────────────┘
> 
> if the intention of the tests is to validate that rp_filter = 1 works as
> designed, then I suggest a simpler test. 2 namespaces, 2 veth pairs.
> Request goes through one interface, and the response comes in the other
> via routing in ns2. ns1 would see the response coming in the 'wrong'
> interface and drops it.

Quite the opposite - the goal is to make sure that commit 66f8209547cc
("fib: relax source validation check for loopback packets") _prevents_
packets from being dropped when rp_filter = 1 in this corner case, as I
mentioned in the commit message.

In order to test this corner case, I need a packet that:

  1. was received on lo;
  2. has a local source IP address (other than lo's 127.0.0.1/8, which
     is 192.0.2.1 in this case);
  3. has no dst attached to it (in this case since it was redirected
     from veth).

See __fib_validate_source():

+       dev_match = dev_match || (res.type == RTN_LOCAL &&
+                                 dev == net->loopback_dev);
					      ^^^^^^^^^^^^
This relaxed check only applies to lo, and I do need to redirect packets
from veth ingress to lo ingress in order to trigger this.

Thanks,
Peilin Ye