[BUG] net/sched: DualPI2 memory accounting underflows after TBF peek

[shaos <shawnq1@163.com>]

Hello,

A TBF parent peeks a DualPI2 child packet, causing DualPI2 to subtract its truesize while parking it in gso_skb. Reducing the DualPI2 memory limit drains the parked skb and subtracts its truesize again, wrapping the u32 memory_used counter.

HEAD commit:    c03c1a14507dfaf8b856c2608f9ac216021fc222
git tree:       30fd2ef82912c26cf3a58ad0ba6afc375fa851ac
source version: Linux 7.3.0, local master
compiler:       gcc 11.4.0, GNU ld 2.38

console output:
https://gist.githubusercontent.com/shaos26/6f3e3d3667401ad3325959e321928400/raw/c50aae5c021dee0b3ab97e76d260da585d1fde98/console-output.txt
kernel config:
https://gist.githubusercontent.com/shaos26/6f3e3d3667401ad3325959e321928400/raw/6b13332e308ed8ce37b28ea2d7c8fb50265a8b88/kernel-config.txt
reproducer:
https://gist.githubusercontent.com/shaos26/6f3e3d3667401ad3325959e321928400/raw/937e24e9b0dcfcd847a80e74ad2707f6b4781030/poc.c
auxiliary reproducer (init):
https://gist.githubusercontent.com/shaos26/6f3e3d3667401ad3325959e321928400/raw/17fb0deae08ba739b9e044cfd6c93be24eff2e6e/init

Build the tested kernel with the attached configuration and CONFIG_NET_SCH_DUALPI2 and CONFIG_NET_SCH_TBF enabled. Run poc.c as a static /init in the attached minimal initramfs inside disposable QEMU; the test creates a user/network namespace and reports the counter through rtnetlink.

Impact and prerequisites:
After the rejected/in-progress accounting state wraps, packets continue to fail with ENOBUFS even after restoring the original memory limit; replacing the qdisc is required. The test is reachable by a caller with CAP_NET_ADMIN in a user-created network namespace and is a namespace-local availability issue.

Validation: Two exact-revision fresh boots reproduced the same counter wrap and persistent enqueue failures; no-drain and close accounting controls passed.

Thanks,

shaos