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[209.147.139.51]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-11c93de82c1sm20464204c88.3.2025.11.21.14.20.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 21 Nov 2025 14:20:01 -0800 (PST) From: Xiang Mei To: security@kernel.org Cc: netdev@vger.kernel.org, toke@toke.dk, xiyou.wangcong@gmail.com, cake@lists.bufferbloat.net, bestswngs@gmail.com, Xiang Mei Message-ID: <20251121221954.907033-1-xmei5@asu.edu> X-Mailer: git-send-email 2.43.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-MailFrom: xmei5@asu.edu X-Mailman-Rule-Hits: nonmember-moderation X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; loop; banned-address; emergency; member-moderation Message-ID-Hash: JNYIT6WPOHN3CL44PQW4WB43SCIONVHO X-Message-ID-Hash: JNYIT6WPOHN3CL44PQW4WB43SCIONVHO X-Mailman-Approved-At: Mon, 24 Nov 2025 11:20:18 +0100 X-Mailman-Version: 3.3.10 Precedence: list Subject: [Cake] [PATCH net v4] net/sched: sch_cake: Fix incorrect qlen reduction in cake_drop List-Id: Cake - FQ_codel the next generation Archived-At: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: Date: Fri, 21 Nov 2025 22:20:05 X-Original-Date: Fri, 21 Nov 2025 15:19:54 -0700 In cake_drop(), qdisc_tree_reduce_backlog() is used to update the qlen and backlog of the qdisc hierarchy. Its caller, cake_enqueue(), assumes that the parent qdisc will enqueue the current packet. However, this assumption breaks when cake_enqueue() returns NET_XMIT_CN: the parent qdisc stops enqueuing current packet, leaving the tree qlen/backlog accounting inconsistent. This mismatch can lead to a NULL dereference (e.g., when the parent Qdisc is qfq_qdisc). This patch computes the qlen/backlog delta in a more robust way by observing the difference before and after the series of cake_drop() calls, and then compensates the qdisc tree accounting if cake_enqueue() returns NET_XMIT_CN. To ensure correct compensation when ACK thinning is enabled, a new variable is introduced to keep qlen unchanged. Fixes: 15de71d06a40 ("net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit") Signed-off-by: Xiang Mei --- v2: add missing cc v3: move qdisc_tree_reduce_backlog out of cake_drop v4: remove redundant variable and handle ack branch correctly --- net/sched/sch_cake.c | 52 +++++++++++++++++++++++++------------------- 1 file changed, 30 insertions(+), 22 deletions(-) diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c index 32bacfc314c2..cf4d6454ca9c 100644 --- a/net/sched/sch_cake.c +++ b/net/sched/sch_cake.c @@ -1597,7 +1597,6 @@ static unsigned int cake_drop(struct Qdisc *sch, struct sk_buff **to_free) qdisc_drop_reason(skb, sch, to_free, SKB_DROP_REASON_QDISC_OVERLIMIT); sch->q.qlen--; - qdisc_tree_reduce_backlog(sch, 1, len); cake_heapify(q, 0); @@ -1750,7 +1749,8 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch, ktime_t now = ktime_get(); struct cake_tin_data *b; struct cake_flow *flow; - u32 idx, tin; + u32 idx, tin, prev_qlen, prev_backlog, drop_id; + bool same_flow = false; /* choose flow to insert into */ idx = cake_classify(sch, &b, skb, q->flow_mode, &ret); @@ -1823,6 +1823,8 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch, consume_skb(skb); } else { /* not splitting */ + int ack_pkt_len = 0; + cobalt_set_enqueue_time(skb, now); get_cobalt_cb(skb)->adjusted_len = cake_overhead(q, skb); flow_queue_add(flow, skb); @@ -1834,7 +1836,7 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch, b->ack_drops++; sch->qstats.drops++; b->bytes += qdisc_pkt_len(ack); - len -= qdisc_pkt_len(ack); + ack_pkt_len = qdisc_pkt_len(ack); q->buffer_used += skb->truesize - ack->truesize; if (q->rate_flags & CAKE_FLAG_INGRESS) cake_advance_shaper(q, b, ack, now, true); @@ -1848,11 +1850,11 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch, /* stats */ b->packets++; - b->bytes += len; - b->backlogs[idx] += len; - b->tin_backlog += len; - sch->qstats.backlog += len; - q->avg_window_bytes += len; + b->bytes += len - ack_pkt_len; + b->backlogs[idx] += len - ack_pkt_len; + b->tin_backlog += len - ack_pkt_len; + sch->qstats.backlog += len - ack_pkt_len; + q->avg_window_bytes += len - ack_pkt_len; } if (q->overflow_timeout) @@ -1927,24 +1929,30 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch, if (q->buffer_used > q->buffer_max_used) q->buffer_max_used = q->buffer_used; - if (q->buffer_used > q->buffer_limit) { - bool same_flow = false; - u32 dropped = 0; - u32 drop_id; + if (q->buffer_used <= q->buffer_limit) + return NET_XMIT_SUCCESS; - while (q->buffer_used > q->buffer_limit) { - dropped++; - drop_id = cake_drop(sch, to_free); + prev_qlen = sch->q.qlen; + prev_backlog = sch->qstats.backlog; - if ((drop_id >> 16) == tin && - (drop_id & 0xFFFF) == idx) - same_flow = true; - } - b->drop_overlimit += dropped; + while (q->buffer_used > q->buffer_limit) { + drop_id = cake_drop(sch, to_free); + if ((drop_id >> 16) == tin && + (drop_id & 0xFFFF) == idx) + same_flow = true; + } + + /* Compute the droppped qlen and pkt length */ + prev_qlen -= sch->q.qlen; + prev_backlog -= sch->qstats.backlog; + b->drop_overlimit += prev_backlog; - if (same_flow) - return NET_XMIT_CN; + if (same_flow) { + qdisc_tree_reduce_backlog(sch, prev_qlen - 1, + prev_backlog - len); + return NET_XMIT_CN; } + qdisc_tree_reduce_backlog(sch, prev_qlen, prev_backlog); return NET_XMIT_SUCCESS; } -- 2.43.0