selftests/bpf: Fix fib_lookup VLAN tests on hosts with forwarding on

The VLAN tests assume the test namespace starts with IPv4 forwarding off,
but a new netns copies conf/all and conf/default from init_net
(devinet_init_net(), with net.core.devconf_inherit_init_net at its
default), so on a host with net.ipv4.conf.all.forwarding=1 the devices in
the netns come up with forwarding already enabled. IPv6 uses compiled
defaults at the same sysctl value, so only the IPv4 arms are affected.

Two arms break as a result.

The arms that expect BPF_FIB_LKUP_RET_FWD_DISABLED see the lookup pass the
forwarding check and return SUCCESS instead, so fib_lookup fails:

  test_fib_lookup:FAIL:fib_lookup_ret unexpected fib_lookup_ret: actual 0 != expected 5

Pin forwarding off in setup_netns() before the devices are created; the
existing per-device writes still enable it where the tests need it.

The netns arm has the opposite problem. It checks that a VLAN device in
another netns is not resolved and expects NOT_FWDED. The lookup runs
against the caller's FIB, which had no route to the destination, so a
kernel that resolved the moved device anyway also returned NOT_FWDED and
the arm passed regardless of the namespace check. On a forwarding-on host,
where the resolved device clears the forwarding gate, this makes the arm a
tautology. Add a route so a resolved device returns SUCCESS and the arm
can tell the two apart.

Verified by deleting the netns check from bpf_fib_vlan_input_dev(): with
the fix the arm fails on both a forwarding-off host (actual 5) and a
forwarding-on host (actual 0), where before it passed on the latter. The
real kernel passes the full suite on both.

Fixes: e54a87872e34 ("selftests/bpf: Add bpf_fib_lookup() VLAN flag tests")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260713163826.D70201F000E9@smtp.kernel.org/
Signed-off-by: Avinash Duduskar <avinash.duduskar@gmail.com>
Link: https://lore.kernel.org/bpf/20260715100349.2684391-1-avinash.duduskar@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

0 files changed, 0 insertions, 0 deletions