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

[Paolo Abeni <pabeni@redhat.com>]

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