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

[David Ahern <dsahern@gmail.com>]

On 11/30/21 5:47 PM, Peilin Ye wrote:
> From: Peilin Ye <peilin.ye@bytedance.com>
> 
> Currently rp_filter tests in fib_tests.sh:fib_rp_filter_test() are
> failing.  ping sockets are bound to dummy1 using the "-I" option
> (SO_BINDTODEVICE), but socket lookup is failing when receiving ping
> replies, since the routing table thinks they belong to dummy0.
> 
> For example, suppose ping is using a SOCK_RAW socket for ICMP messages.
> When receiving ping replies, in __raw_v4_lookup(), sk->sk_bound_dev_if
> is 3 (dummy1), but dif (skb_rtable(skb)->rt_iif) says 2 (dummy0), so the
> raw_sk_bound_dev_eq() check fails.  Similar things happen in
> ping_lookup() for SOCK_DGRAM sockets.
> 
> These tests used to pass due to a bug [1] in iputils, where "ping -I"
> actually did not bind ICMP message sockets to device.  The bug has been
> fixed by iputils commit f455fee41c07 ("ping: also bind the ICMP socket
> to the specific device") in 2016, which is why our rp_filter tests
> started to fail.  See [2] .
> 
> Fixing the tests while keeping everything in one netns turns out to be
> nontrivial.  Rework the tests and build the following topology:
> 
>  ┌─────────────────────────────┐    ┌─────────────────────────────┐
>  │  network namespace 1 (ns1)  │    │  network namespace 2 (ns2)  │
>  │                             │    │                             │
>  │  ┌────┐     ┌─────┐         │    │  ┌─────┐            ┌────┐  │
>  │  │ lo │<───>│veth1│<────────┼────┼─>│veth2│<──────────>│ lo │  │
>  │  └────┘     ├─────┴──────┐  │    │  ├─────┴──────┐     └────┘  │
>  │             │192.0.2.1/24│  │    │  │192.0.2.1/24│             │
>  │             └────────────┘  │    │  └────────────┘             │
>  └─────────────────────────────┘    └─────────────────────────────┘
> 
> Consider sending an ICMP_ECHO packet A in ns2.  Both source and
> destination IP addresses are 192.0.2.1, and we use strict mode rp_filter
> in both ns1 and ns2:
> 
>   1. A is routed to lo since its destination IP address is one of ns2's
>      local addresses (veth2);
>   2. A is redirected from lo's egress to veth2's egress using mirred;
>   3. A arrives at veth1's ingress in ns1;
>   4. A is redirected from veth1's ingress to lo's ingress, again, using
>      mirred;
>   5. In __fib_validate_source(), fib_info_nh_uses_dev() returns false,
>      since A was received on lo, but reverse path lookup says veth1;
>   6. However A is not dropped since we have relaxed this check for lo in
>      commit 66f8209547cc ("fib: relax source validation check for loopback
>      packets");
> 
> Making sure A is not dropped here in this corner case is the whole point
> of having this test.
> 
>   7. As A reaches the ICMP layer, an ICMP_ECHOREPLY packet, B, is
>      generated;
>   8. Similarly, B is redirected from lo's egress to veth1's egress (in
>      ns1), then redirected once again from veth2's ingress to lo's
>      ingress (in ns2), using mirred.
> 
> Also test "ping 127.0.0.1" from ns2.  It does not trigger the relaxed
> check in __fib_validate_source(), but just to make sure the topology
> works with loopback addresses.
> 
> Tested with ping from iputils 20210722-41-gf9fb573:
> 
> $ ./fib_tests.sh -t rp_filter
> 
> IPv4 rp_filter tests
>     TEST: rp_filter passes local packets		[ OK ]
>     TEST: rp_filter passes loopback packets		[ OK ]
> 
> [1] https://github.com/iputils/iputils/issues/55
> [2] https://github.com/iputils/iputils/commit/f455fee41c077d4b700a473b2f5b3487b8febc1d
> 
> Reported-by: Hangbin Liu <liuhangbin@gmail.com>
> Fixes: adb701d6cfa4 ("selftests: add a test case for rp_filter")
> Reviewed-by: Cong Wang <cong.wang@bytedance.com>
> Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
> ---
> Change in v3:
>     - "ping -I dummy0 198.51.100.1" always work (David Ahern
>       <dsahern@gmail.com>); use a different approach instead
> 
> Change in v2:
>     - s/SOCK_ICMP/SOCK_DGRAM/ in commit message
> 
>  tools/testing/selftests/net/fib_tests.sh | 59 ++++++++++++++++++++----
>  1 file changed, 49 insertions(+), 10 deletions(-)
> 

Acked-by: David Ahern <dsahern@kernel.org>