netdev - Re: [PATCH v2] net/cls_cgroup: Fix task_get

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Message-ID: <87303e90-3c74-4e4f-8fac-2001d82b90d8@blackwall.org>
Date: Fri, 22 Aug 2025 10:26:41 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Yafang Shao <laoar.shao@...il.com>, davem@...emloft.net,
 edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, horms@...nel.org,
 daniel@...earbox.net, bigeasy@...utronix.de, tgraf@...g.ch,
 paulmck@...nel.org
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v2] net/cls_cgroup: Fix task_get_classid() during qdisc
 run

On 8/22/25 09:42, Yafang Shao wrote:
> During recent testing with the netem qdisc to inject delays into TCP
> traffic, we observed that our CLS BPF program failed to function correctly
> due to incorrect classid retrieval from task_get_classid(). The issue
> manifests in the following call stack:
> 
>         bpf_get_cgroup_classid+5
>         cls_bpf_classify+507
>         __tcf_classify+90
>         tcf_classify+217
>         __dev_queue_xmit+798
>         bond_dev_queue_xmit+43
>         __bond_start_xmit+211
>         bond_start_xmit+70
>         dev_hard_start_xmit+142
>         sch_direct_xmit+161
>         __qdisc_run+102             <<<<< Issue location
>         __dev_xmit_skb+1015
>         __dev_queue_xmit+637
>         neigh_hh_output+159
>         ip_finish_output2+461
>         __ip_finish_output+183
>         ip_finish_output+41
>         ip_output+120
>         ip_local_out+94
>         __ip_queue_xmit+394
>         ip_queue_xmit+21
>         __tcp_transmit_skb+2169
>         tcp_write_xmit+959
>         __tcp_push_pending_frames+55
>         tcp_push+264
>         tcp_sendmsg_locked+661
>         tcp_sendmsg+45
>         inet_sendmsg+67
>         sock_sendmsg+98
>         sock_write_iter+147
>         vfs_write+786
>         ksys_write+181
>         __x64_sys_write+25
>         do_syscall_64+56
>         entry_SYSCALL_64_after_hwframe+100
> 
> The problem occurs when multiple tasks share a single qdisc. In such cases,
> __qdisc_run() may transmit skbs created by different tasks. Consequently,
> task_get_classid() retrieves an incorrect classid since it references the
> current task's context rather than the skb's originating task.
> 
> Given that dev_queue_xmit() always executes with bh disabled, we can safely
> use in_softirq() instead of in_serving_softirq() to properly identify the
> softirq context and obtain the correct classid.
>
 
nit: you are no longer using in_softirq() in v2, you should update the
commit message as well.

[snip]

Cheers,
 Nik