[PATCH net-next] net: dev_addr_lists: don't WARN on GFP_ATOMIC kmalloc failure in netif_rx_mode_run

[Zijing Yin]

netif_rx_mode_run() fires netdev_WARN() when netif_addr_lists_snapshot()
returns non-zero. The only path to a non-zero return is -ENOMEM from
__hw_addr_create() failing its kmalloc(GFP_ATOMIC) -- syzkaller hits it
via failslab, and it can also be reached under real memory pressure.

This is the only allocator-failure site in this file that WARNs.
__hw_addr_create() itself, and every other caller of it in this file
(__hw_addr_add_ex(), dev_uc_add_excl(), dev_uc_add(), dev_mc_add(), ...)
just propagate the -ENOMEM silently. GFP_ATOMIC is a may-fail allocator;
callers are required to handle NULL, so a returned -ENOMEM here is an
expected runtime condition, not an invariant violation.

The miss is self-healing: any subsequent change to dev->uc / dev->mc
(every dev_{uc,mc}_{add,del}, IFF_PROMISC flip, etc.) calls
__dev_set_rx_mode() -> netif_rx_mode_queue(), which re-queues the device
and retries the sync. The only cost of a failed attempt is one stale
rx-mode window until the next update; nothing in the kernel relies on
this attempt succeeding.

Demote to net_err_ratelimited() so the condition stays observable in
dmesg without tripping panic_on_warn.

Reproducer (syzkaller .prog format with setup notes): 
https://pastebin.com/t7AQKx9v

Fixes: 3554b4345d85 ("net: introduce ndo_set_rx_mode_async and netdev_rx_mode_work")
Signed-off-by: Zijing Yin <yzjaurora@xxxxxxxxx>
---
 net/core/dev_addr_lists.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/dev_addr_lists.c b/net/core/dev_addr_lists.c
index d73fcb0c6..c6fdcac74 100644
--- a/net/core/dev_addr_lists.c
+++ b/net/core/dev_addr_lists.c
@@ -1275,7 +1275,8 @@ static void netif_rx_mode_run(struct net_device *dev)
 		err = netif_addr_lists_snapshot(dev, &uc_snap, &mc_snap,
 						&uc_ref, &mc_ref);
 		if (err) {
-			netdev_WARN(dev, "failed to sync uc/mc addresses\n");
+			net_err_ratelimited("%s: failed to sync uc/mc addresses\n",
+					    netdev_name(dev));
 			netif_addr_unlock_bh(dev);
 			return;
 		}
-- 
2.43.0