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

[luoxuanqiang <xuanqiang.luo@linux.dev>]

在 2026/7/7 17:02, Paolo Abeni 写道:
> On 7/7/26 3:26 AM, luoxuanqiang wrote:
>> 在 2026/7/6 00:46, Ido Schimmel 写道:
>>> 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?
>> Thanks! You are right, the subject is too broad. I will make them
>> more accurate in the next version.
>>
>>>> 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().
>>>
>> I will split the rt_flush_dev() change into a separate patch.
>>
>>>> Fixes: caacf05e5ad1a ("ipv4: Properly purge netdev references on uncached routes.")
>>> Please remove the Fixes tag given you are targeting net-next.
>> Just to clarify: is the suggestion to drop the Fixes tag here solely
>> because this patch is targeted at net-next? Or are there any other
>> reasons?
> Generally speaking, yes: net-next patches should not include a fixes tag
> unless the blamed commit is on net-next only.
>
> More specifically, this patch is really a behavior improvement and not a
> vertical fix, as such we want to avoid it propagating on stable trees,
> as the fixes tag sometimes does.
>
> /P
>
Many thanks for the detailed explanation!