[PATCH net] selftests: drv-net: rss_ctx: fix persist tests skip on IPv6 setups

[Gal Pressman <gal@nvidia.com>]

test_rss_context_persist_ifupdown() cycles the interface down/up and
pings the remote host before verifying the contexts survived.

When using IPv6, the down operation deletes the configured IPv6 address
(default keep_addr_on_down=0) and nothing restores it after the up, so
all ping attempts fail and both tests skip with "Cannot reach remote
host after interface up".

Set the interface's keep_addr_on_down sysctl around the flap to retain
the address.

Fixes: ce5a0f4612db ("selftests: drv-net: rss_ctx: test RSS contexts persist after ifdown/up")
Reviewed-by: Nimrod Oren <noren@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
---
Disclaimer: I'm not sure if this should go to net or net-next.
---
 tools/testing/selftests/drivers/net/hw/rss_ctx.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/testing/selftests/drivers/net/hw/rss_ctx.py b/tools/testing/selftests/drivers/net/hw/rss_ctx.py
index f36f76d6ca59..eeea2fb292ac 100755
--- a/tools/testing/selftests/drivers/net/hw/rss_ctx.py
+++ b/tools/testing/selftests/drivers/net/hw/rss_ctx.py
@@ -938,6 +938,13 @@ def test_rss_context_persist_ifupdown(cfg, pre_down=False):
     ethtool(f"-X {cfg.ifname} equal 2")
     defer(ethtool, f"-X {cfg.ifname} default")
 
+    # IPv6 addresses are deleted on admin down and not restored on up
+    knob = f"net/ipv6/conf/{cfg.ifname}/keep_addr_on_down"
+    probe = cmd(f"sysctl -n {knob}", fail=False)
+    if probe.ret == 0:
+        cmd(f"sysctl -qw {knob}=1")
+        defer(cmd, f"sysctl -qw {knob}={probe.stdout.strip()}")
+
     ifup = defer(ip, f"link set dev {cfg.ifname} up")
     if pre_down:
         ip(f"link set dev {cfg.ifname} down")
-- 
2.54.0