netdev - Re: [PATCH net-next v5 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance

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Message-ID: <a02bf61e-bb72-41fa-b8c3-0efbd3738aeb@linux.dev>
Date: Thu, 25 Sep 2025 09:51:59 +0800
From: luoxuanqiang <xuanqiang.luo@...ux.dev>
To: Jakub Kicinski <kuba@...nel.org>
Cc: edumazet@...gle.com, kuniyu@...gle.com, kerneljasonxing@...il.com,
 davem@...emloft.net, netdev@...r.kernel.org,
 Xuanqiang Luo <luoxuanqiang@...inos.cn>
Subject: Re: [PATCH net-next v5 3/3] inet: Avoid ehash lookup race in
 inet_twsk_hashdance_schedule()


在 2025/9/25 08:55, Jakub Kicinski 写道:
> On Wed, 24 Sep 2025 09:50:34 +0800xuanqiang.luo@...ux.dev wrote:
>> From: Xuanqiang Luo<luoxuanqiang@...inos.cn>
>>
>> Since ehash lookups are lockless, if another CPU is converting sk to tw
>> concurrently, fetching the newly inserted tw with tw->tw_refcnt == 0 cause
>> lookup failure.
>>
>> The call trace map is drawn as follows:
>>     CPU 0                                CPU 1
>>     -----                                -----
>> 				     inet_twsk_hashdance_schedule()
>> 				     spin_lock()
>> 				     inet_twsk_add_node_rcu(tw, ...)
>> __inet_lookup_established()
>> (find tw, failure due to tw_refcnt = 0)
>> 				     __sk_nulls_del_node_init_rcu(sk)
>> 				     refcount_set(&tw->tw_refcnt, 3)
>> 				     spin_unlock()
>>
>> By replacing sk with tw atomically via hlist_nulls_replace_init_rcu() after
>> setting tw_refcnt, we ensure that tw is either fully initialized or not
>> visible to other CPUs, eliminating the race.
> This one doesn't build cleanly
>
> net/ipv4/inet_timewait_sock.c:116:28: warning: unused variable 'ehead' [-Wunused-variable]
>    116 |         struct inet_ehash_bucket *ehead = inet_ehash_bucket(hashinfo, sk->sk_hash);
>        |                                   ^~~~~
> net/ipv4/inet_timewait_sock.c:91:13: warning: unused function 'inet_twsk_add_node_rcu' [-Wunused-function]
>     91 | static void inet_twsk_add_node_rcu(struct inet_timewait_sock *tw,
>        |             ^~~~~~~~~~~~~~~~~~~~~~

Oops, I introduced those build warnings in my changes and missed them.

Thanks for pointing this out—I'll fix them shortly.