Subject: Fw: [Bug 220774] New: netem is broken in 6.18

From: Stephen Hemminger <[email-masked]>
Date: Mon, 10 Nov 2025 12:38:07 -0800

Regression caused by:

commit ec8e0e3d7adef940cdf9475e2352c0680189d14e
Author: William Liu <[email-masked]>
Date:   Tue Jul 8 16:43:26 2025 +0000

    net/sched: Restrict conditions for adding duplicating netems to qdisc tree
    
    netem_enqueue's duplication prevention logic breaks when a netem
    resides in a qdisc tree with other netems - this can lead to a
    soft lockup and OOM loop in netem_dequeue, as seen in [1].
    Ensure that a duplicating netem cannot exist in a tree with other
    netems.
    
    Previous approaches suggested in discussions in chronological order:
    
    1) Track duplication status or ttl in the sk_buff struct. Considered
    too specific a use case to extend such a struct, though this would
    be a resilient fix and address other previous and potential future
    DOS bugs like the one described in loopy fun [2].
    
    2) Restrict netem_enqueue recursion depth like in act_mirred with a
    per cpu variable. However, netem_dequeue can call enqueue on its
    child, and the depth restriction could be bypassed if the child is a
    netem.
    
    3) Use the same approach as in 2, but add metadata in netem_skb_cb
    to handle the netem_dequeue case and track a packet's involvement
    in duplication. This is an overly complex approach, and Jamal
    notes that the skb cb can be overwritten to circumvent this
    safeguard.
    
    4) Prevent the addition of a netem to a qdisc tree if its ancestral
    path contains a netem. However, filters and actions can cause a
    packet to change paths when re-enqueued to the root from netem
    duplication, leading us to the current solution: prevent a
    duplicating netem from inhabiting the same tree as other netems.
    
    [1] https://lore.kernel.org/netdev/8DuRWwfqjoRDLDmBMlIfbrsZg9Gx50DHJc1ilxsEBNe2D6NMoigR_eIRIG0LOjMc3r10nUUZtArXx4oZBIdUfZQrwjcQhdinnMis_0G7VEk=@willsroot.io/
    [2] https://lwn.net/Articles/719297/
    
    Fixes: 0afb51e72855 ("[PKT_SCHED]: netem: reinsert for duplication")
    Reported-by: William Liu <[email-masked]>
    Reported-by: Savino Dicanosa <[email-masked]>
    Signed-off-by: William Liu <[email-masked]>
    Signed-off-by: Savino Dicanosa <[email-masked]>
    Acked-by: Jamal Hadi Salim <[email-masked]>
    Link: https://patch.msgid.link/[email-masked]
    Signed-off-by: Jakub Kicinski <[email-masked]>

Begin forwarded message:

Date: Mon, 10 Nov 2025 19:13:57 +0000
From: [email-masked]
To: [email-masked]
Subject: [Bug 220774] New: netem is broken in 6.18

https://bugzilla.kernel.org/show_bug.cgi?id=220774

            Bug ID: 220774
           Summary: netem is broken in 6.18
           Product: Networking
           Version: 2.5
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: high
          Priority: P3
         Component: Other
          Assignee: [email-masked]
          Reporter: [email-masked]
        Regression: No

[jschung@localhost ~]$ cat test.sh 
#!/bin/bash

DEV="eth0"
NUM_QUEUES=32
DUPLICATE_PERCENT="5%"

tc qdisc del dev $DEV root > /dev/null 2>&1
tc qdisc add dev $DEV root handle 1: mq

for i in $(seq 1 $NUM_QUEUES); do
    HANDLE_ID=$((i * 10))
    PARENT_ID="1:$i"
    tc qdisc add dev $DEV parent $PARENT_ID handle ${HANDLE_ID}: netem
duplicate $DUPLICATE_PERCENT
done

[jschung@localhost ~]$ sudo ./test.sh 
[  2976.073299] netem: change failed
Error: netem: cannot mix duplicating netems with other netems in tree.

[jschung@localhost ~]$ uname -r
6.18.0-rc4

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