[PATCH net 2/2] selftests: net: packetdrill: add test for negative sk_forward_alloc in tcp_wmem_schedule()

[Octavian Purdila <tavip@google.com>]

Add a packetdrill test that drives sk->sk_forward_alloc negative via
tcp_collapse() on the receive queue while sk->sk_wmem_queued exceeds
sysctl_tcp_wmem[0], and verifies that a subsequent non-blocking write()
under TCP memory pressure fails with EAGAIN rather than returning a
negative copy length from tcp_wmem_schedule() and failing with EFAULT.

Assisted-by: LLM
Signed-off-by: Octavian Purdila <tavip@google.com>
---
 .../testing/selftests/net/packetdrill/config  |   1 +
 .../tcp_wmem_schedule_negative_fwd_alloc.pkt  | 116 ++++++++++++++++++
 2 files changed, 117 insertions(+)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_wmem_schedule_negative_fwd_alloc.pkt

diff --git a/tools/testing/selftests/net/packetdrill/config b/tools/testing/selftests/net/packetdrill/config
index b7df8bf309200..dc3cd72a8da8c 100644
--- a/tools/testing/selftests/net/packetdrill/config
+++ b/tools/testing/selftests/net/packetdrill/config
@@ -1,6 +1,7 @@
 CONFIG_HZ=1000
 CONFIG_HZ_1000=y
 CONFIG_IPV6=y
+CONFIG_MEMCG=y
 CONFIG_NET_NS=y
 CONFIG_NET_SCH_FIFO=y
 CONFIG_NET_SCH_FQ=y
diff --git a/tools/testing/selftests/net/packetdrill/tcp_wmem_schedule_negative_fwd_alloc.pkt b/tools/testing/selftests/net/packetdrill/tcp_wmem_schedule_negative_fwd_alloc.pkt
new file mode 100644
index 0000000000000..db232b54c554e
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_wmem_schedule_negative_fwd_alloc.pkt
@@ -0,0 +1,116 @@
+// SPDX-License-Identifier: GPL-2.0
+// Test that tcp_wmem_schedule() clamps negative sk->sk_forward_alloc to 0
+// when sk_wmem_schedule() fails under memory pressure and
+// sk->sk_wmem_queued >= sysctl_tcp_wmem[0], returning EAGAIN rather than EFAULT.
+
+--mtu=65535
+--tolerance_usecs=100000
+
+`set -e
+./defaults.sh
+ip link set dev tun0 mtu 65535
+sysctl -q net.ipv4.tcp_rmem="4096 131072 6291456"
+sysctl -q net.ipv4.tcp_wmem="4096 16384 4194304"
+cgroot=$(mount -t cgroup2 | head -1 | awk '{print $3}')
+[ -n "$cgroot" ]
+echo +memory > "${cgroot}/cgroup.subtree_control"
+mkdir -p "${cgroot}/pkt_${PPID}"
+echo $PPID > "${cgroot}/pkt_${PPID}/cgroup.procs"
+`
+
+// Create listening socket in pkt_${PPID} memcg and establish connection
+0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
++0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
++0.000 bind(3, ..., ...) = 0
++0.000 listen(3, 1) = 0
+
++0.000 < S 0:0(0) win 65535 <mss 36000,nop,wscale 7>
++0.000 > S. 0:0(0) ack 1 <...>
++0.000 < . 1:1(0) ack 1 win 512
++0.000 accept(3, ..., ...) = 4
+
+// Move packetdrill back to its original cgroup so only the sockets remain charged to pkt_${PPID}
++0.000 `cgroot=$(mount -t cgroup2 | head -1 | awk '{print $3}')
+runner_pid=$(awk '/^PPid:/ {print $2}' /proc/$PPID/status)
+orig_cg=$(sed -n 's/^0:://p' /proc/$runner_pid/cgroup)
+echo $PPID > "${cgroot}${orig_cg}/cgroup.procs"`
+
++0.000 setsockopt(4, SOL_SOCKET, SO_SNDBUF, [2097152], 4) = 0
++0.000 setsockopt(4, SOL_SOCKET, SO_RCVBUF, [145536], 4) = 0
+
+// 1. Warm up rcv_ssthresh to full window (~256 KB)
++0.000 < P. 1:36001(36000) ack 1 win 512
++0.000 > . 1:1(0) ack 36001
++0.000 read(4, ..., 36000) = 36000
+
++0.000 < P. 36001:72001(36000) ack 1 win 512
++0.000 > . 1:1(0) ack 72001
++0.000 read(4, ..., 36000) = 36000
+
++0.000 < P. 72001:108001(36000) ack 1 win 512
++0.000 > . 1:1(0) ack 108001
++0.000 read(4, ..., 36000) = 36000
+
++0.000 < P. 108001:144001(36000) ack 1 win 512
++0.000 > . 1:1(0) ack 144001
++0.000 read(4, ..., 36000) = 36000
+
+// 2. Queue 500 KB on TX so sk_wmem_queued >= sysctl_tcp_wmem[0]
++0.000 write(4, ..., 500000) = 500000
+
+// 3a. Inject 6 high-density 36 KB segments (216 KB payload, 222.7 KB truesize)
+//     and read 100 bytes from the head SKB so before(skb->seq, tp->copied_seq) is true.
++0.000 < P. 144001:180001(36000) ack 1 win 512
++0.000 read(4, ..., 100) = 100
+
++0.000 < P. 180001:216001(36000) ack 1 win 512
++0.000 < P. 216001:252001(36000) ack 1 win 512
++0.000 < P. 252001:288001(36000) ack 1 win 512
++0.000 < P. 288001:324001(36000) ack 1 win 512
++0.000 < P. 324001:360001(36000) ack 1 win 512
+
+// 3b. Inject 22 smaller 1000-byte segments into the remaining advertised rcv_wnd.
+//     Their higher truesize pushes sk_rmem_alloc > sk_rcvbuf (291072),
+//     triggering tcp_prune_queue -> tcp_collapse on the 216 KB of 36 KB SKBs
+//     and driving sk->sk_forward_alloc negative (-32768).
++0.000 < P. 360001:361001(1000) ack 1 win 512
++0.000 < P. 361001:362001(1000) ack 1 win 512
++0.000 < P. 362001:363001(1000) ack 1 win 512
++0.000 < P. 363001:364001(1000) ack 1 win 512
++0.000 < P. 364001:365001(1000) ack 1 win 512
++0.000 < P. 365001:366001(1000) ack 1 win 512
++0.000 < P. 366001:367001(1000) ack 1 win 512
++0.000 < P. 367001:368001(1000) ack 1 win 512
++0.000 < P. 368001:369001(1000) ack 1 win 512
++0.000 < P. 369001:370001(1000) ack 1 win 512
++0.000 < P. 370001:371001(1000) ack 1 win 512
++0.000 < P. 371001:372001(1000) ack 1 win 512
++0.000 < P. 372001:373001(1000) ack 1 win 512
++0.000 < P. 373001:374001(1000) ack 1 win 512
++0.000 < P. 374001:375001(1000) ack 1 win 512
++0.000 < P. 375001:376001(1000) ack 1 win 512
++0.000 < P. 376001:377001(1000) ack 1 win 512
++0.000 < P. 377001:378001(1000) ack 1 win 512
++0.000 < P. 378001:379001(1000) ack 1 win 512
++0.000 < P. 379001:380001(1000) ack 1 win 512
++0.000 < P. 380001:381001(1000) ack 1 win 512
++0.000 < P. 381001:382001(1000) ack 1 win 512
+
+// 4. Cap the socket memcg at its current usage and expect -EAGAIN on non-blocking write.
++0.010 `cgroot=$(mount -t cgroup2 | head -1 | awk '{print $3}')
+echo 0 > "${cgroot}/pkt_${PPID}/memory.high"
+cat "${cgroot}/pkt_${PPID}/memory.current" > "${cgroot}/pkt_${PPID}/memory.max"`
++0.000 fcntl(4, F_SETFL, O_RDWR|O_NONBLOCK) = 0
++0.000 write(4, ..., 16384) = -1 EAGAIN (Resource temporarily unavailable)
+
++0.000 close(4) = 0
++0.000 close(3) = 0
+
+// Untimed cleanup command executed by packetdrill on both pass and early failure.
+`cgroot=$(mount -t cgroup2 | head -1 | awk '{print $3}')
+[ -n "$cgroot" ] || exit 0
+runner_pid=$(awk '/^PPid:/ {print $2}' /proc/$PPID/status)
+orig_cg=$(sed -n 's/^0:://p' /proc/$runner_pid/cgroup)
+echo $PPID > "${cgroot}${orig_cg}/cgroup.procs" 2>/dev/null || true
+echo $$ > "${cgroot}${orig_cg}/cgroup.procs" 2>/dev/null || true
+rmdir "${cgroot}/pkt_${PPID}"`
-- 
2.56.0.176.g3575e65682-goog