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