Re: [PATCH v2] net/cls_cgroup: Fix task_get_classid() during qdisc run

[Yafang Shao <laoar.shao@gmail.com>]

On Fri, Aug 22, 2025 at 3:26 PM Nikolay Aleksandrov <razor@blackwall.org> wrote:
>
> 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.

Oh, my bad.
I will update it.

-- 
Regards
Yafang