Re: [PATCH net-next v2] ipv4: hold a consistent view of rt->dst.dev under RCU

[Ido Schimmel <idosch@nvidia.com>]

On Wed, Jul 01, 2026 at 11:24:34AM +0800, xuanqiang.luo@linux.dev wrote:
> From: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
> 
> rt_flush_dev() walks the per-CPU uncached route list and rewrites
> rt->dst.dev in-place to blackhole_netdev under spin_lock_bh().
> This lock does not exclude RCU readers, which may load rt->dst.dev
> multiple times within a single rcu_read_lock() region.
> 
> ip_rt_send_redirect() is a typical example: it reads rt->dst.dev
> three times to obtain in_dev, the L3 master ifindex, and net.
> A concurrent device unregistration can repoint rt->dst.dev to
> blackhole_netdev between those reads, making the reader combine
> state from two different net_devices — for instance, an in_dev
> from the real device but a netns and peer lookup from the blackhole
> device.  ip_rt_get_source() has the same problem: it reads
> rt->dst.dev four times to obtain the output ifindex, the netns,
> and the source address, so a concurrent flush can cause the source
> selection to mix state from different devices.

Why only change ip_rt_send_redirect() and ip_rt_get_source() when the
patch is titled "ipv4: hold a consistent view of rt->dst.dev under RCU"?
What is the criterion?

> 
> Take a single dst_dev_rcu() snapshot of rt->dst.dev at the start
> of each affected RCU reader and use that snapshot throughout, so
> concurrent flushes cannot cause mid-function inconsistency.
> Publish the in-place write in rt_flush_dev() with rcu_assign_pointer()
> to match the readers.

The rt_flush_dev() change should be a separate change. Note that
dst_dev_put() was already converted to use rcu_assign_pointer().

> 
> Fixes: caacf05e5ad1a ("ipv4: Properly purge netdev references on uncached routes.")

Please remove the Fixes tag given you are targeting net-next.

> Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
> ---
> v2:
> - Use dst_dev_rcu() and dev_net_rcu() for the RCU readers.
> - Use rcu_assign_pointer() when publishing the uncached route device
>   replacement.
> - Slightly adjust the commit message wording because this issue was found
>   by inspection, not from an observed user-visible failure.
> 
> v1: https://lore.kernel.org/all/20260630094250.29386-1-xuanqiang.luo@linux.dev/
> 
>  net/ipv4/route.c | 29 +++++++++++++++++------------
>  1 file changed, 17 insertions(+), 12 deletions(-)
> 
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 3f3de5164d6e5..57f38467e6d0c 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -873,6 +873,7 @@ static void ipv4_negative_advice(struct sock *sk,
>  void ip_rt_send_redirect(struct sk_buff *skb)
>  {
>  	struct rtable *rt = skb_rtable(skb);
> +	struct net_device *dev;

https://docs.kernel.org/process/maintainer-netdev.html#local-variable-ordering-reverse-xmas-tree-rcs

Same in other places.