Re: [PATCH 6.12 189/220] ipv4: start using dst_dev_rcu()

On Fri, Aug 21, 2026 at 11:45:50AM +0200, Miguel Gazquez wrote:
> > +	dev = skb->dev ? : skb_dst_dev_rcu(skb);
> > +	vif = l3mdev_master_ifindex_rcu(dev);
>
> Here, the upstream version has a `rcu_read_lock();` that I think should
> be added. I will send an updated version.

Agreed, and confirmed here. Upstream that hunk lands after the
rcu_read_lock() introduced by ca0359df45a5 ("inet: frags: save a pair of
atomic operations in reassembly", v6.15), which isn't in 6.12 - so as
backported, ip_defrag() reaches skb_dst_dev_rcu() -> dst_dev_rcu() ->
rcu_dereference() with no RCU read-side critical section of its own,
where the pre-patch 6.12 code used skb_dst_dev() -> READ_ONCE() and had
no such requirement.

In practice every in-tree ip_defrag() caller already runs under RCU
(netfilter hooks or the NAPI receive path), so this is a lockdep /
correctness defect rather than a guaranteed CONFIG_PROVE_RCU splat, but
it does defeat the point of the conversion.

Dropped from the 6.12 queue, thanks - please do send the v2.

-- 
Thanks,
Sasha