selftests/bpf: Add test for redirect from qdisc qevent block

Add a regression test for the NULL current->bpf_net_context deref hit
when a BPF classifier attached to a qdisc qevent block asks for a
redirect. The classifier runs from tcf_qevent_handle() on the qdisc
enqueue path, outside any bpf_net_context.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t qevent
  [...]
  + /etc/rcS.d/S50-startup
  ./test_progs -t qevent
  #496/1   tc_qevent/redirect_verdict:OK
  #496/2   tc_qevent/redirect_helper:OK
  #496     tc_qevent:OK
  Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260706185609.330006-4-daniel@iogearbox.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/bpf/config

@@ -82,6 +82,7 @@ CONFIG_NET_SCH_BPF=y
 CONFIG_NET_SCH_FQ=y
 CONFIG_NET_SCH_INGRESS=y
 CONFIG_NET_SCH_HTB=y
+CONFIG_NET_SCH_RED=y
 CONFIG_NET_SCHED=y
 CONFIG_NETDEVSIM=y
 CONFIG_NETFILTER=y

tools/testing/selftests/bpf/prog_tests/tc_qevent.c

@@ -0,0 +1,113 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include <network_helpers.h>
+#include <sys/socket.h>
+#include <netinet/in.h>
+#include <arpa/inet.h>
+#include <unistd.h>
+#include <string.h>
+
+#include "test_tc_qevent.skel.h"
+
+#define NS_TX		"tc_qevent_tx"
+#define NS_RX		"tc_qevent_rx"
+#define IP_TX		"10.255.0.1"
+#define IP_RX		"10.255.0.2"
+#define PIN_PATH	"/sys/fs/bpf/tc_qevent_redirect"
+
+static void blast_udp(void)
+{
+	struct sockaddr_in dst = {};
+	char buf[1400] = {};
+	int fd, i;
+
+	fd = socket(AF_INET, SOCK_DGRAM, 0);
+	if (!ASSERT_GE(fd, 0, "udp socket"))
+		return;
+
+	dst.sin_family = AF_INET;
+	dst.sin_port = htons(12345);
+	inet_pton(AF_INET, IP_RX, &dst.sin_addr);
+
+	/*
+	 * Push far more than the RED queue can hold. Once qavg crosses qth_min
+	 * every further packet hits the congestion_drop / early_drop qevent.
+	 */
+	for (i = 0; i < 50000; i++)
+		sendto(fd, buf, sizeof(buf), MSG_DONTWAIT,
+		       (struct sockaddr *)&dst, sizeof(dst));
+
+	close(fd);
+}
+
+static void run_qevent_redirect(struct bpf_program *prog, __u64 *counter)
+{
+	struct nstoken *tok = NULL;
+	int err;
+
+	SYS_NOFAIL("ip netns del %s", NS_TX);
+	SYS_NOFAIL("ip netns del %s", NS_RX);
+	unlink(PIN_PATH);
+
+	err = bpf_program__pin(prog, PIN_PATH);
+	if (!ASSERT_OK(err, "pin prog"))
+		return;
+
+	SYS(unpin,  "ip netns add %s", NS_TX);
+	SYS(del_tx, "ip netns add %s", NS_RX);
+	SYS(del_rx, "ip -n %s link add veth0 type veth peer name veth1 netns %s", NS_TX, NS_RX);
+	SYS(del_rx, "ip -n %s addr add %s/24 dev veth0", NS_TX, IP_TX);
+	SYS(del_rx, "ip -n %s link set veth0 up", NS_TX);
+	SYS(del_rx, "ip -n %s addr add %s/24 dev veth1", NS_RX, IP_RX);
+	SYS(del_rx, "ip -n %s link set veth1 up", NS_RX);
+
+	tok = open_netns(NS_TX);
+	if (!ASSERT_OK_PTR(tok, "open_netns"))
+		goto del_rx;
+
+	SYS(close_ns, "tc qdisc add dev veth0 root handle 1: htb default 1");
+	SYS(close_ns, "tc class add dev veth0 parent 1: classid 1:1 htb rate 1mbit ceil 1mbit");
+
+	if (system("tc qdisc add dev veth0 parent 1:1 handle 11: red "
+		   "limit 500000 avpkt 1000 probability 1 min 5000 max 6000 "
+		   "burst 6 qevent early_drop block 10 2>/dev/null")) {
+		test__skip();
+		goto close_ns;
+	}
+
+	if (system("tc filter add block 10 bpf da object-pinned "
+		   PIN_PATH " 2>/dev/null")) {
+		test__skip();
+		goto close_ns;
+	}
+
+	blast_udp();
+	ASSERT_GT(*counter, 0, "qevent classifier ran");
+close_ns:
+	close_netns(tok);
+del_rx:
+	SYS_NOFAIL("ip netns del %s", NS_RX);
+del_tx:
+	SYS_NOFAIL("ip netns del %s", NS_TX);
+unpin:
+	bpf_program__unpin(prog, PIN_PATH);
+}
+
+void test_tc_qevent(void)
+{
+	struct test_tc_qevent *skel;
+
+	skel = test_tc_qevent__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	if (test__start_subtest("redirect_verdict"))
+		run_qevent_redirect(skel->progs.qevent_redirect_verdict,
+				    &skel->bss->verdict_calls);
+	if (test__start_subtest("redirect_helper"))
+		run_qevent_redirect(skel->progs.qevent_redirect_helper,
+				    &skel->bss->helper_calls);
+
+	test_tc_qevent__destroy(skel);
+}

tools/testing/selftests/bpf/progs/test_tc_qevent.c

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+
+int redirect_ifindex = 1;
+__u64 verdict_calls = 0;
+__u64 helper_calls = 0;
+
+SEC("tc")
+int qevent_redirect_verdict(struct __sk_buff *skb)
+{
+	__sync_fetch_and_add(&verdict_calls, 1);
+	return TCX_REDIRECT;
+}
+
+SEC("tc")
+int qevent_redirect_helper(struct __sk_buff *skb)
+{
+	__sync_fetch_and_add(&helper_calls, 1);
+	return bpf_redirect(redirect_ifindex, 0);
+}
+
+char _license[] SEC("license") = "GPL";