[PATCH v3 1/2] net/sched: dualpi2: fix GSO backlog accounting

[PATCH v3 1/2] net/sched: dualpi2: fix GSO backlog accounting

[Xingquan Liu]

When DualPI2 splits a GSO skb into N segments, it propagates N
additional packets to its parent before returning NET_XMIT_SUCCESS.
The parent then accounts for the original skb once more, leaving its
qlen one larger than the number of packets actually queued.

With QFQ as the parent, after all real packets are dequeued, QFQ still
has a non-zero qlen while its in-service aggregate has no active
classes. qfq_choose_next_agg() returns NULL and qfq_dequeue() passes
the result to qfq_peek_skb(), causing a NULL pointer dereference.

Follow the same pattern used by tbf_segment() and taprio: count only
successfully queued segments, propagate the difference between the
original skb and those segments, and return NET_XMIT_SUCCESS whenever
at least one segment was queued.

Fixes: 8f9516daedd6 ("sched: Add enqueue/dequeue of dualpi2 qdisc")
Cc: stable@vger.kernel.org
Signed-off-by: Xingquan Liu <b1n@b1n.io>
---
v3:
- Move the UDP GSO sender into tdc_gso.py.

v2:
- Change patch commit message.
- Add tdc test.

 net/sched/sch_dualpi2.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/net/sched/sch_dualpi2.c b/net/sched/sch_dualpi2.c
index d7c3254ef800..5434df6ca8ef 100644
--- a/net/sched/sch_dualpi2.c
+++ b/net/sched/sch_dualpi2.c
@@ -461,7 +461,7 @@ static int dualpi2_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		if (IS_ERR_OR_NULL(nskb))
 			return qdisc_drop(skb, sch, to_free);
 
-		cnt = 1;
+		cnt = 0;
 		byte_len = 0;
 		orig_len = qdisc_pkt_len(skb);
 		skb_list_walk_safe(nskb, nskb, next) {
@@ -488,16 +488,15 @@ static int dualpi2_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 				byte_len += nskb->len;
 			}
 		}
-		if (cnt > 1) {
+		if (cnt > 0) {
 			/* The caller will add the original skb stats to its
 			 * backlog, compensate this if any nskb is enqueued.
 			 */
-			--cnt;
-			byte_len -= orig_len;
+			qdisc_tree_reduce_backlog(sch, 1 - cnt,
+						  orig_len - byte_len);
 		}
-		qdisc_tree_reduce_backlog(sch, -cnt, -byte_len);
 		consume_skb(skb);
-		return err;
+		return cnt > 0 ? NET_XMIT_SUCCESS : err;
 	}
 	return dualpi2_enqueue_skb(skb, sch, to_free);
 }

base-commit: 96e7f9122aae0ed000ee321f324b812a447906d9
-- 
Xingquan Liu

[PATCH v3 2/2] selftests/tc-testing: Add DualPI2 GSO backlog accounting test

[Xingquan Liu]

Add a regression test for DualPI2 GSO backlog accounting when it is
used as a child qdisc of QFQ.

The test sends one UDP GSO datagram through a QFQ class with DualPI2 as
the leaf qdisc. DualPI2 splits the skb into two segments. After the
traffic drains, both QFQ and DualPI2 must report zero backlog and zero
qlen.

On kernels with the broken accounting, QFQ can keep a stale non-zero
qlen after all real packets have been dequeued.

Signed-off-by: Xingquan Liu <b1n@b1n.io>
---
 .../tc-testing/tc-tests/qdiscs/dualpi2.json   | 44 +++++++++++++++++++
 tools/testing/selftests/tc-testing/tdc_gso.py | 43 ++++++++++++++++++
 2 files changed, 87 insertions(+)
 create mode 100755 tools/testing/selftests/tc-testing/tdc_gso.py

diff --git a/tools/testing/selftests/tc-testing/tc-tests/qdiscs/dualpi2.json b/tools/testing/selftests/tc-testing/tc-tests/qdiscs/dualpi2.json
index cd1f2ee8f354..ed6a900bb568 100644
--- a/tools/testing/selftests/tc-testing/tc-tests/qdiscs/dualpi2.json
+++ b/tools/testing/selftests/tc-testing/tc-tests/qdiscs/dualpi2.json
@@ -250,5 +250,49 @@
         "teardown": [
             "$TC qdisc del dev $DUMMY handle 1: root"
         ]
+    },
+    {
+        "id": "891f",
+        "name": "Verify DualPI2 GSO backlog accounting with QFQ parent",
+        "category": [
+            "qdisc",
+            "dualpi2",
+            "qfq",
+            "gso"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "$IP link set dev $DUMMY up || true",
+            "$IP addr add 10.10.10.10/24 dev $DUMMY || true",
+            "$TC qdisc add dev $DUMMY root handle 1: qfq",
+            "$TC class add dev $DUMMY parent 1: classid 1:1 qfq weight 1 maxpkt 4096",
+            "$TC qdisc add dev $DUMMY parent 1:1 handle 2: dualpi2",
+            "$TC filter add dev $DUMMY parent 1: matchall classid 1:1"
+        ],
+        "cmdUnderTest": "./tdc_gso.py 10.10.10.10 10.10.10.1 9000 1200 2400",
+        "expExitCode": "0",
+        "verifyCmd": "$TC -j -s qdisc ls dev $DUMMY",
+        "matchJSON": [
+            {
+                "kind": "qfq",
+                "handle": "1:",
+                "packets": 2,
+                "backlog": 0,
+                "qlen": 0
+            },
+            {
+                "kind": "dualpi2",
+                "handle": "2:",
+                "packets": 2,
+                "backlog": 0,
+                "qlen": 0
+            }
+        ],
+        "teardown": [
+            "$TC qdisc del dev $DUMMY root",
+            "$IP addr del 10.10.10.10/24 dev $DUMMY || true"
+        ]
     }
 ]
diff --git a/tools/testing/selftests/tc-testing/tdc_gso.py b/tools/testing/selftests/tc-testing/tdc_gso.py
new file mode 100755
index 000000000000..b66528ea4b68
--- /dev/null
+++ b/tools/testing/selftests/tc-testing/tdc_gso.py
@@ -0,0 +1,43 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+
+"""
+tdc_gso.py - send a UDP GSO datagram
+
+Copyright (C) 2026 Xingquan Liu <b1n@b1n.io>
+"""
+
+import argparse
+import socket
+import struct
+import sys
+
+UDP_MAX_SEGMENTS = 1 << 7
+
+
+parser = argparse.ArgumentParser(description="UDP GSO datagram sender")
+parser.add_argument("src", help="source IPv4 address")
+parser.add_argument("dst", help="destination IPv4 address")
+parser.add_argument("port", type=int, help="destination UDP port")
+parser.add_argument("gso_size", type=int, help="UDP GSO segment payload size")
+parser.add_argument("payload_len", type=int, help="total UDP payload length")
+args = parser.parse_args()
+
+if args.gso_size <= 0 or args.gso_size > 0xFFFF:
+    parser.error("gso_size must fit in an unsigned 16-bit integer")
+if args.payload_len <= args.gso_size:
+    parser.error("payload_len must be larger than gso_size")
+if args.payload_len > args.gso_size * UDP_MAX_SEGMENTS:
+    parser.error("payload_len exceeds UDP_MAX_SEGMENTS")
+
+SOL_UDP = getattr(socket, "SOL_UDP", socket.IPPROTO_UDP)
+UDP_SEGMENT = getattr(socket, "UDP_SEGMENT", 103)
+
+sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+sock.bind((args.src, 0))
+
+payload = b"b" * args.payload_len
+cmsg = [(SOL_UDP, UDP_SEGMENT, struct.pack("=H", args.gso_size))]
+
+sent = sock.sendmsg([payload], cmsg, 0, (args.dst, args.port))
+sys.exit(sent != len(payload))
-- 
Xingquan Liu

Re: [PATCH v3 1/2] net/sched: dualpi2: fix GSO backlog accounting

[Jamal Hadi Salim]

On Fri, Jun 19, 2026 at 11:15 AM Xingquan Liu <b1n@b1n.io> wrote:
>
> When DualPI2 splits a GSO skb into N segments, it propagates N
> additional packets to its parent before returning NET_XMIT_SUCCESS.
> The parent then accounts for the original skb once more, leaving its
> qlen one larger than the number of packets actually queued.
>
> With QFQ as the parent, after all real packets are dequeued, QFQ still
> has a non-zero qlen while its in-service aggregate has no active
> classes. qfq_choose_next_agg() returns NULL and qfq_dequeue() passes
> the result to qfq_peek_skb(), causing a NULL pointer dereference.
>
> Follow the same pattern used by tbf_segment() and taprio: count only
> successfully queued segments, propagate the difference between the
> original skb and those segments, and return NET_XMIT_SUCCESS whenever
> at least one segment was queued.
>
> Fixes: 8f9516daedd6 ("sched: Add enqueue/dequeue of dualpi2 qdisc")
> Cc: stable@vger.kernel.org
> Signed-off-by: Xingquan Liu <b1n@b1n.io>

Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>

cheers,
jamal

> ---
> v3:
> - Move the UDP GSO sender into tdc_gso.py.
>
> v2:
> - Change patch commit message.
> - Add tdc test.
>
>  net/sched/sch_dualpi2.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/net/sched/sch_dualpi2.c b/net/sched/sch_dualpi2.c
> index d7c3254ef800..5434df6ca8ef 100644
> --- a/net/sched/sch_dualpi2.c
> +++ b/net/sched/sch_dualpi2.c
> @@ -461,7 +461,7 @@ static int dualpi2_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>                 if (IS_ERR_OR_NULL(nskb))
>                         return qdisc_drop(skb, sch, to_free);
>
> -               cnt = 1;
> +               cnt = 0;
>                 byte_len = 0;
>                 orig_len = qdisc_pkt_len(skb);
>                 skb_list_walk_safe(nskb, nskb, next) {
> @@ -488,16 +488,15 @@ static int dualpi2_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>                                 byte_len += nskb->len;
>                         }
>                 }
> -               if (cnt > 1) {
> +               if (cnt > 0) {
>                         /* The caller will add the original skb stats to its
>                          * backlog, compensate this if any nskb is enqueued.
>                          */
> -                       --cnt;
> -                       byte_len -= orig_len;
> +                       qdisc_tree_reduce_backlog(sch, 1 - cnt,
> +                                                 orig_len - byte_len);
>                 }
> -               qdisc_tree_reduce_backlog(sch, -cnt, -byte_len);
>                 consume_skb(skb);
> -               return err;
> +               return cnt > 0 ? NET_XMIT_SUCCESS : err;
>         }
>         return dualpi2_enqueue_skb(skb, sch, to_free);
>  }
>
> base-commit: 96e7f9122aae0ed000ee321f324b812a447906d9
> --
> Xingquan Liu
>

Re: [PATCH v3 2/2] selftests/tc-testing: Add DualPI2 GSO backlog accounting test

[Jamal Hadi Salim]

On Fri, Jun 19, 2026 at 11:16 AM Xingquan Liu <b1n@b1n.io> wrote:
>
> Add a regression test for DualPI2 GSO backlog accounting when it is
> used as a child qdisc of QFQ.
>
> The test sends one UDP GSO datagram through a QFQ class with DualPI2 as
> the leaf qdisc. DualPI2 splits the skb into two segments. After the
> traffic drains, both QFQ and DualPI2 must report zero backlog and zero
> qlen.
>
> On kernels with the broken accounting, QFQ can keep a stale non-zero
> qlen after all real packets have been dequeued.
>
> Signed-off-by: Xingquan Liu <b1n@b1n.io>

Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>

cheers,
jamal

> ---
>  .../tc-testing/tc-tests/qdiscs/dualpi2.json   | 44 +++++++++++++++++++
>  tools/testing/selftests/tc-testing/tdc_gso.py | 43 ++++++++++++++++++
>  2 files changed, 87 insertions(+)
>  create mode 100755 tools/testing/selftests/tc-testing/tdc_gso.py
>
> diff --git a/tools/testing/selftests/tc-testing/tc-tests/qdiscs/dualpi2.json b/tools/testing/selftests/tc-testing/tc-tests/qdiscs/dualpi2.json
> index cd1f2ee8f354..ed6a900bb568 100644
> --- a/tools/testing/selftests/tc-testing/tc-tests/qdiscs/dualpi2.json
> +++ b/tools/testing/selftests/tc-testing/tc-tests/qdiscs/dualpi2.json
> @@ -250,5 +250,49 @@
>          "teardown": [
>              "$TC qdisc del dev $DUMMY handle 1: root"
>          ]
> +    },
> +    {
> +        "id": "891f",
> +        "name": "Verify DualPI2 GSO backlog accounting with QFQ parent",
> +        "category": [
> +            "qdisc",
> +            "dualpi2",
> +            "qfq",
> +            "gso"
> +        ],
> +        "plugins": {
> +            "requires": "nsPlugin"
> +        },
> +        "setup": [
> +            "$IP link set dev $DUMMY up || true",
> +            "$IP addr add 10.10.10.10/24 dev $DUMMY || true",
> +            "$TC qdisc add dev $DUMMY root handle 1: qfq",
> +            "$TC class add dev $DUMMY parent 1: classid 1:1 qfq weight 1 maxpkt 4096",
> +            "$TC qdisc add dev $DUMMY parent 1:1 handle 2: dualpi2",
> +            "$TC filter add dev $DUMMY parent 1: matchall classid 1:1"
> +        ],
> +        "cmdUnderTest": "./tdc_gso.py 10.10.10.10 10.10.10.1 9000 1200 2400",
> +        "expExitCode": "0",
> +        "verifyCmd": "$TC -j -s qdisc ls dev $DUMMY",
> +        "matchJSON": [
> +            {
> +                "kind": "qfq",
> +                "handle": "1:",
> +                "packets": 2,
> +                "backlog": 0,
> +                "qlen": 0
> +            },
> +            {
> +                "kind": "dualpi2",
> +                "handle": "2:",
> +                "packets": 2,
> +                "backlog": 0,
> +                "qlen": 0
> +            }
> +        ],
> +        "teardown": [
> +            "$TC qdisc del dev $DUMMY root",
> +            "$IP addr del 10.10.10.10/24 dev $DUMMY || true"
> +        ]
>      }
>  ]
> diff --git a/tools/testing/selftests/tc-testing/tdc_gso.py b/tools/testing/selftests/tc-testing/tdc_gso.py
> new file mode 100755
> index 000000000000..b66528ea4b68
> --- /dev/null
> +++ b/tools/testing/selftests/tc-testing/tdc_gso.py
> @@ -0,0 +1,43 @@
> +#!/usr/bin/env python3
> +# SPDX-License-Identifier: GPL-2.0
> +
> +"""
> +tdc_gso.py - send a UDP GSO datagram
> +
> +Copyright (C) 2026 Xingquan Liu <b1n@b1n.io>
> +"""
> +
> +import argparse
> +import socket
> +import struct
> +import sys
> +
> +UDP_MAX_SEGMENTS = 1 << 7
> +
> +
> +parser = argparse.ArgumentParser(description="UDP GSO datagram sender")
> +parser.add_argument("src", help="source IPv4 address")
> +parser.add_argument("dst", help="destination IPv4 address")
> +parser.add_argument("port", type=int, help="destination UDP port")
> +parser.add_argument("gso_size", type=int, help="UDP GSO segment payload size")
> +parser.add_argument("payload_len", type=int, help="total UDP payload length")
> +args = parser.parse_args()
> +
> +if args.gso_size <= 0 or args.gso_size > 0xFFFF:
> +    parser.error("gso_size must fit in an unsigned 16-bit integer")
> +if args.payload_len <= args.gso_size:
> +    parser.error("payload_len must be larger than gso_size")
> +if args.payload_len > args.gso_size * UDP_MAX_SEGMENTS:
> +    parser.error("payload_len exceeds UDP_MAX_SEGMENTS")
> +
> +SOL_UDP = getattr(socket, "SOL_UDP", socket.IPPROTO_UDP)
> +UDP_SEGMENT = getattr(socket, "UDP_SEGMENT", 103)
> +
> +sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
> +sock.bind((args.src, 0))
> +
> +payload = b"b" * args.payload_len
> +cmsg = [(SOL_UDP, UDP_SEGMENT, struct.pack("=H", args.gso_size))]
> +
> +sent = sock.sendmsg([payload], cmsg, 0, (args.dst, args.port))
> +sys.exit(sent != len(payload))
> --
> Xingquan Liu
>

Re: [PATCH v3 1/2] net/sched: dualpi2: fix GSO backlog accounting

[Jamal Hadi Salim]

On Fri, Jun 19, 2026 at 11:15 AM Xingquan Liu <b1n@b1n.io> wrote:
>
> When DualPI2 splits a GSO skb into N segments, it propagates N
> additional packets to its parent before returning NET_XMIT_SUCCESS.
> The parent then accounts for the original skb once more, leaving its
> qlen one larger than the number of packets actually queued.
>
> With QFQ as the parent, after all real packets are dequeued, QFQ still
> has a non-zero qlen while its in-service aggregate has no active
> classes. qfq_choose_next_agg() returns NULL and qfq_dequeue() passes
> the result to qfq_peek_skb(), causing a NULL pointer dereference.
>
> Follow the same pattern used by tbf_segment() and taprio: count only
> successfully queued segments, propagate the difference between the
> original skb and those segments, and return NET_XMIT_SUCCESS whenever
> at least one segment was queued.
>
> Fixes: 8f9516daedd6 ("sched: Add enqueue/dequeue of dualpi2 qdisc")
> Cc: stable@vger.kernel.org
> Signed-off-by: Xingquan Liu <b1n@b1n.io>

BTW, in the future make sure you wait at least 24 hours before you
send an updated version.
You didnt do that here..

cheers,
jamal
> ---
> v3:
> - Move the UDP GSO sender into tdc_gso.py.
>
> v2:
> - Change patch commit message.
> - Add tdc test.
>
>  net/sched/sch_dualpi2.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/net/sched/sch_dualpi2.c b/net/sched/sch_dualpi2.c
> index d7c3254ef800..5434df6ca8ef 100644
> --- a/net/sched/sch_dualpi2.c
> +++ b/net/sched/sch_dualpi2.c
> @@ -461,7 +461,7 @@ static int dualpi2_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>                 if (IS_ERR_OR_NULL(nskb))
>                         return qdisc_drop(skb, sch, to_free);
>
> -               cnt = 1;
> +               cnt = 0;
>                 byte_len = 0;
>                 orig_len = qdisc_pkt_len(skb);
>                 skb_list_walk_safe(nskb, nskb, next) {
> @@ -488,16 +488,15 @@ static int dualpi2_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>                                 byte_len += nskb->len;
>                         }
>                 }
> -               if (cnt > 1) {
> +               if (cnt > 0) {
>                         /* The caller will add the original skb stats to its
>                          * backlog, compensate this if any nskb is enqueued.
>                          */
> -                       --cnt;
> -                       byte_len -= orig_len;
> +                       qdisc_tree_reduce_backlog(sch, 1 - cnt,
> +                                                 orig_len - byte_len);
>                 }
> -               qdisc_tree_reduce_backlog(sch, -cnt, -byte_len);
>                 consume_skb(skb);
> -               return err;
> +               return cnt > 0 ? NET_XMIT_SUCCESS : err;
>         }
>         return dualpi2_enqueue_skb(skb, sch, to_free);
>  }
>
> base-commit: 96e7f9122aae0ed000ee321f324b812a447906d9
> --
> Xingquan Liu
>

Re: [PATCH v3 2/2] selftests/tc-testing: Add DualPI2 GSO backlog accounting test

[Victor Nogueira]

On Fri, Jun 19, 2026 at 12:16 PM Xingquan Liu <b1n@b1n.io> wrote:
>
> Add a regression test for DualPI2 GSO backlog accounting when it is
> used as a child qdisc of QFQ.
>
> The test sends one UDP GSO datagram through a QFQ class with DualPI2 as
> the leaf qdisc. DualPI2 splits the skb into two segments. After the
> traffic drains, both QFQ and DualPI2 must report zero backlog and zero
> qlen.
>
> On kernels with the broken accounting, QFQ can keep a stale non-zero
> qlen after all real packets have been dequeued.
>
> Signed-off-by: Xingquan Liu <b1n@b1n.io>

Reviewed-by: Victor Nogueira <victor@mojatatu.com>

Re: [PATCH v3 1/2] net/sched: dualpi2: fix GSO backlog accounting

[Victor Nogueira]

On Fri, Jun 19, 2026 at 12:15 PM Xingquan Liu <b1n@b1n.io> wrote:
>
> When DualPI2 splits a GSO skb into N segments, it propagates N
> additional packets to its parent before returning NET_XMIT_SUCCESS.
> The parent then accounts for the original skb once more, leaving its
> qlen one larger than the number of packets actually queued.
>
> With QFQ as the parent, after all real packets are dequeued, QFQ still
> has a non-zero qlen while its in-service aggregate has no active
> classes. qfq_choose_next_agg() returns NULL and qfq_dequeue() passes
> the result to qfq_peek_skb(), causing a NULL pointer dereference.
>
> Follow the same pattern used by tbf_segment() and taprio: count only
> successfully queued segments, propagate the difference between the
> original skb and those segments, and return NET_XMIT_SUCCESS whenever
> at least one segment was queued.
>
> Fixes: 8f9516daedd6 ("sched: Add enqueue/dequeue of dualpi2 qdisc")
> Cc: stable@vger.kernel.org
> Signed-off-by: Xingquan Liu <b1n@b1n.io>

Reviewed-by: Victor Nogueira <victor@mojatatu.com>

Re: [PATCH v3 1/2] net/sched: dualpi2: fix GSO backlog accounting

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Fri, 19 Jun 2026 11:13:47 -0400 you wrote:
> When DualPI2 splits a GSO skb into N segments, it propagates N
> additional packets to its parent before returning NET_XMIT_SUCCESS.
> The parent then accounts for the original skb once more, leaving its
> qlen one larger than the number of packets actually queued.
> 
> With QFQ as the parent, after all real packets are dequeued, QFQ still
> has a non-zero qlen while its in-service aggregate has no active
> classes. qfq_choose_next_agg() returns NULL and qfq_dequeue() passes
> the result to qfq_peek_skb(), causing a NULL pointer dereference.
> 
> [...]

Here is the summary with links:
  - [v3,1/2] net/sched: dualpi2: fix GSO backlog accounting
    https://git.kernel.org/netdev/net/c/05ed733b65ab
  - [v3,2/2] selftests/tc-testing: Add DualPI2 GSO backlog accounting test
    https://git.kernel.org/netdev/net/c/54704b32b2ab

You are awesome, thank you!
-- 
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