[PATCH net-next v8 0/4] net/sched: cap the accounted backlog before it can wrap

[Jamal Hadi Salim <jhs@mojatatu.com>]

This series bounds the accounted backlog of the AQM qdiscs so that it
cannot wrap mod 2^32.

v1 widened the counters to u64 and was rejected; from v2 on the fix drops
at enqueue instead. The v5 posting was a single patch; at the reviewer's
request it is split into four:

  1/4  cake: classifier refactor: cake_tcf_classify() returns its result
       to the caller.
  2/4  cake: use a dedicated split-GSO list iterator.
  3/4  the fix: drop at enqueue once the stored backlog cannot
       represent the packet's accounted length -- the standing
       QDISC_MAX_BACKLOG ceiling for ordinary packets, a per-packet
       representability check for the larger wire estimates. Covers
       fq_codel, cake, codel, pie, fq_pie, dualpi2, RED, pfifo, plug
       and gred.
  4/4  cake: split cake_hash() into side-effect-free key extraction (on
       the unsegmented skb) and set-associative resolution (after
       admission), move the split-GSO path ahead of the resolution, and
       tag every deferred split segment.

The commit logs and the in-code comments are written for readability.

The guard half is a net fix, but the CAKE hash split is new-code net-next
material, so the series targets net-next with no Fixes: (the aggregate
wrap predates every touched qdisc).

Changes in v8:
  - 1/4 is cake-only. fq_codel_classify() is line-for-line the same as
    fq_pie_classify(), and fq_pie is not changed by this series, so the
    two stay in lockstep: fq_codel keeps its original classify shape and
    the fq_codel backlog guard is placed inline in its enqueue path
    (3/4) instead. (Toke Høiland-Jørgensen)
  - 3/4: the fq_codel guard sits between idx-- and
    codel_set_enqueue_time(), so a dropped packet cannot touch flow
    state. The commit message is reworded as suggested. (Toke
    Høiland-Jørgensen)
  - 4/4: the original skb is consumed unconditionally right after a
    successful split -- that is the caller's reference, since a fraglist
    GSO takes the list's own reference on the same skb in
    skb_segment_list() -- so the reject branch tags every deferred
    segment and drops the whole list with qdisc_drop_all(), and the
    admitted path no longer consumes the skb after the walk. (Toke
    Høiland-Jørgensen)

Changes in v7:
  - 1/4: complete the fq_codel_classify() comment (Sashiko nipa v6
    main-2-1, Low). The comment listed only the terminal-action and
    no-class-ID cases; the function also returns true when no filter
    matched or when the selected class minor is above q->flows_cnt,
    and those packets are BYPASS drops, not a fallback to hashing as
    in cake. Comment-only, no code change; folded into this resend
    since the series is revised anyway.
  - 3/4: do not clamp the shared qdisc_pkt_len() (Sashiko nipa v6
    main-2-6, Medium). The clamp silently undercharged BPF
    __sk_buff.wire_len, act_police and the HTB/TBF shapers for a valid
    large GSO estimate. The initializer now stays truthful for every
    input and the overflow protection is enqueue admission: each
    affected qdisc drops a packet once the stored backlog cannot
    represent its accounted length -- the standing QDISC_MAX_BACKLOG
    ceiling covers ordinary packets, and a per-packet representability
    check covers the larger wire estimates, which a supported
    small-MSS BIG TCP aggregate can legitimately carry above 1 MiB
    while its raw length stays within GSO_MAX_SIZE. The consumers read
    the real length, and sch_handle_ingress() keeps complete pkt_segs
    metadata.
  - 4/4: tag every deferred split-GSO segment with QDISC_DROP_OVERLIMIT
    (Sashiko nipa v6 gpt-6-astra-2-5, Medium). __skb_gso_segment() stores
    SKB_GSO_CB at cb offset 32, aliasing tc_skb_cb.drop_reason, and the cb
    is copied into every segment, so tagging only the head left the rest
    with GSO scratch as their drop reason.
  - 4/4: snapshot q->config->flow_mode once with READ_ONCE() and pass it
    to both hash helpers (Sashiko nipa v6 gpt-6-astra-5-9, Low). The split
    introduced two loads with the segmentation work between them.
  - Not folded: the 3/4 intermediate-commit split-GSO limitation (4/4
    closes it; the pre-series path was already unbounded) and the
    Fixes:/stable suggestions (intentional net-next target). The gemini
    v6 TC_ACT_CONSUMED High is the v5 finding already rebutted as
    unreachable from a qdisc classify switch.

v6  rewrite as a four-patch series per Toke Høiland-Jørgensen's review of
    v5 (split the classifier refactor, the split-GSO iterator, the
    overflow guard and the cake hash split; drop the machine-written
    prose). Considered the Sashiko gemini v5 TC_ACT_CONSUMED report and
    did not fold it (unreachable; see below). Reworded the cake block
    comment that had an embedded "/*" (Simon Horman's W=1 warning) and
    reworded a comment that overstated the classifier's return cases. The
    TC_ACT_CONSUMED switch case was dropped from the final 0001 because it
    still reached __qdisc_drop(); no in-tree producer can deliver
    TC_ACT_CONSUMED to a qdisc classify switch (act_ct returns it only on
    a clsact/ingress qdisc, act_mirred only for a non-cloning ingress
    redirect, and cls_bpf/act_bpf remap an unknown verdict to UNSPEC).

v5/v4/v3/v2 history: see the v6 posting. In short, v3 adopted the stored
    backlog compare qdisc_backlog_at_max() (no double-count of the 1 MiB
    headroom, no per-enqueue 64-bit add), moved the classifier and the
    ceiling check ahead of the hash/shaper state, and added pfifo; v4
    split the hash keys from the set-associative resolution and handled
    the fraglist-GSO self-link; v2 replaced the rejected u64 widening with
    the enqueue drop; v1 widened the counters.

Jamal Hadi Salim (4):
  net/sched: cake: return the classifier result to the caller
  net/sched: cake: use a dedicated split-GSO list iterator
  net/sched: cap the accounted backlog before it can wrap
  net/sched: cake: hash before segmenting and reject an overlong split

 include/net/pkt_sched.h   |   1 -
 include/net/sch_generic.h |  20 +++++
 net/sched/sch_cake.c      | 177 +++++++++++++++++++++++++++-----------
 net/sched/sch_codel.c     |  16 ++--
 net/sched/sch_dualpi2.c   |   1 +
 net/sched/sch_fifo.c      |   6 ++
 net/sched/sch_fq_codel.c  |   9 ++
 net/sched/sch_fq_pie.c    |   3 +-
 net/sched/sch_gred.c      |   9 ++
 net/sched/sch_pie.c       |   3 +-
 net/sched/sch_plug.c      |   9 +-
 net/sched/sch_red.c       |  23 +++--
 12 files changed, 209 insertions(+), 68 deletions(-)

-- 
2.43.0