tests: fail the percpu instance by its id, prove the dispatch reaches the receiving CPU

tests/README.md

@@ -367,3 +367,7 @@ BTF, or `bpftool` or `clang` is unavailable.
   drops on rejection, so a working guard blocks the ping, proving the
   kfunc ran and returned without crashing.
 
+- **xdp percpu**: with the ping pinned to the last online CPU, which is where
+  the veth runs the receive softirq, the callback of a percpu script reports
+  that CPU as its instance id, and no other; skipped on a single CPU.
+

tests/runtime/percpu_fail.lua

@@ -6,11 +6,7 @@
 
 local lunatik = require("lunatik")
 
-local env = lunatik._ENV
-local count = (env.percpu_fail_count or 0) + 1
-env.percpu_fail_count = count
-if count == 2 then
-	env.percpu_fail_count = nil
+if lunatik.cpu() == 1 then
 	error("intentional error on the second instance")
 end
 

tests/xdp/Makefile

@@ -1,7 +1,7 @@
 # SPDX-FileCopyrightText: (c) 2026 Ashwani Kumar Kamal <ashwanikamal.im421@gmail.com>
 # SPDX-License-Identifier: MIT OR GPL-2.0-only
 
-all: xdp_pass.bpf.o xdp_drop.bpf.o xdp_detach.bpf.o xdp_zerokey.bpf.o
+all: xdp_pass.bpf.o xdp_drop.bpf.o xdp_detach.bpf.o xdp_zerokey.bpf.o xdp_percpu.bpf.o
 
 vmlinux.h:
 	bpftool btf dump file /sys/kernel/btf/vmlinux format c > $@
@@ -18,6 +18,9 @@ xdp_detach.bpf.o: xdp_detach.bpf.c vmlinux.h
 xdp_zerokey.bpf.o: xdp_zerokey.bpf.c vmlinux.h
 	clang -target bpf -Wall -O2 -c -g $<
 
+xdp_percpu.bpf.o: xdp_percpu.bpf.c vmlinux.h
+	clang -target bpf -Wall -O2 -c -g $<
+
 clean:
-	rm -f vmlinux.h xdp_pass.bpf.o xdp_drop.bpf.o xdp_detach.bpf.o xdp_zerokey.bpf.o
+	rm -f vmlinux.h xdp_pass.bpf.o xdp_drop.bpf.o xdp_detach.bpf.o xdp_zerokey.bpf.o xdp_percpu.bpf.o
 

tests/xdp/percpu.lua

@@ -0,0 +1,29 @@
+--
+-- SPDX-FileCopyrightText: (c) 2026 Ring Zero Desenvolvimento de Software LTDA
+-- SPDX-License-Identifier: MIT OR GPL-2.0-only
+--
+-- Kernel-side script for the xdp percpu dispatch test (see test_xdp.sh).
+
+local lunatik = require("lunatik")
+local xdp     = require("xdp")
+local action  = require("linux.xdp")
+
+local ETH_HI  <const> = 12 -- ethertype, then IPv4 protocol
+local ETH_LO  <const> = 13
+local IPPROTO <const> = 23
+local ICMP    <const> = 1
+
+local cpu = lunatik.cpu()
+
+local function test_percpu(ctx)
+	local packet = ctx:packet()
+	-- the namespace also emits IPv6 autoconf traffic, from a context the test does not pin
+	if packet:getuint8(ETH_HI) == 0x08 and packet:getuint8(ETH_LO) == 0x00
+			and packet:getuint8(IPPROTO) == ICMP then
+		print("xdp percpu test hit: cpu " .. cpu)
+	end
+	ctx:action(action.PASS)
+end
+
+xdp.attach(test_percpu)
+

tests/xdp/test_xdp.sh

@@ -21,7 +21,7 @@ DIR="$(dirname "$(readlink -f "$0")")"
 source "$DIR/../lib.sh"
 
 ktap_header
-ktap_plan 5
+ktap_plan 6
 
 skip_all()
 {
@@ -31,6 +31,7 @@ skip_all()
 	ktap_skip "xdp detach: callback stops firing and traffic resumes"
 	ktap_skip "xdp attach: refuses a sleepable runtime"
 	ktap_skip "xdp zero-key: a zero-sized key is rejected without a crash"
+	ktap_skip "xdp percpu: the callback runs on the instance of the receiving CPU"
 	ktap_totals
 	exit 0
 }
@@ -43,11 +44,12 @@ command -v clang > /dev/null 2>&1 || skip_all "clang not available"
 cleanup()
 {
 	bpftool net detach xdp dev "$IFACE" 2>/dev/null
-	rm -f "${PIN}_pass" "${PIN}_drop" "${PIN}_detach" "${PIN}_zerokey"
+	rm -f "${PIN}_pass" "${PIN}_drop" "${PIN}_detach" "${PIN}_zerokey" "${PIN}_percpu"
 	lunatik stop tests/xdp/pass > /dev/null 2>&1
 	lunatik stop tests/xdp/drop > /dev/null 2>&1
 	lunatik stop tests/xdp/detach > /dev/null 2>&1
 	lunatik stop tests/xdp/attach_sleepable > /dev/null 2>&1
+	lunatik stop tests/xdp/percpu > /dev/null 2>&1
 	ip netns del "$NETNS" 2>/dev/null
 	ip link del "$IFACE" 2>/dev/null
 }
@@ -150,6 +152,36 @@ zerokey_case()
 	ktap_pass "xdp zero-key: a zero-sized key is rejected without a crash"
 }
 
+# the veth runs the receive softirq on the sending CPU, so the pinned ping picks the instance
+percpu_case()
+{
+	local title="xdp percpu: the callback runs on the instance of the receiving CPU"
+	local cpu hits
+	cpu=$(sed 's/.*[-,]//' /sys/devices/system/cpu/online)
+	[ "$cpu" -ge 1 ] || { ktap_skip "$title (needs >1 CPU)"; return 0; }
+	command -v taskset > /dev/null 2>&1 || { ktap_skip "$title (taskset not available)"; return 0; }
+
+	bpftool prog load "$DIR/xdp_percpu.bpf.o" "${PIN}_percpu" type xdp ||
+		{ ktap_fail "xdp percpu: failed to load XDP program"; return 1; }
+	bpftool net attach xdp pinned "${PIN}_percpu" dev "$IFACE" ||
+		{ ktap_fail "xdp percpu: failed to attach XDP program"; bpftool prog unpin "${PIN}_percpu"; return 1; }
+
+	mark_dmesg
+	run_script "tests/xdp/percpu" softirq percpu
+	check_dmesg || { ktap_fail "xdp percpu: script raised an error"; return 1; }
+	taskset -c "$cpu" ip netns exec "$NETNS" ping -c 3 -W 2 "$TARGET" > /dev/null 2>&1
+
+	bpftool net detach xdp dev "$IFACE" 2>/dev/null
+	rm -f "${PIN}_percpu"
+	lunatik stop tests/xdp/percpu > /dev/null 2>&1
+
+	hits=$(dmesg_since | grep -o "xdp percpu test hit: cpu [0-9]*" | sort -u)
+	[ -n "$hits" ] || { ktap_fail "xdp percpu: the callback did not run"; return 1; }
+	[ "$hits" = "xdp percpu test hit: cpu $cpu" ] ||
+		{ ktap_fail "xdp percpu: expected the instance of CPU $cpu, got: $(echo $hits)"; return 1; }
+	ktap_pass "$title"
+}
+
 run_case xdp_pass.bpf.o "${PIN}_pass" pass.lua yes "xdp pass" \
 	"xdp pass: verdict enforced, packet and argument content verified" softirq
 run_case xdp_drop.bpf.o "${PIN}_drop" drop.lua no "xdp drop" \
@@ -163,6 +195,7 @@ lunatik stop tests/xdp/attach_sleepable > /dev/null 2>&1
 ktap_pass "xdp attach: refuses a sleepable runtime"
 
 zerokey_case
+percpu_case
 
 ktap_totals
 

tests/xdp/xdp_percpu.bpf.c

@@ -0,0 +1,23 @@
+/*
+* SPDX-FileCopyrightText: (c) 2026 Ring Zero Desenvolvimento de Software LTDA
+* SPDX-License-Identifier: MIT OR GPL-2.0-only
+*/
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+
+extern int bpf_luaxdp_run(char *key, size_t key__sz, struct xdp_md *ctx,
+		void *arg, size_t arg__sz) __ksym;
+
+static char runtime[] = "tests/xdp/percpu";
+
+SEC("xdp")
+int test_xdp_percpu(struct xdp_md *ctx)
+{
+	int ret;
+	ret = bpf_luaxdp_run(runtime, sizeof(runtime), ctx, NULL, 0);
+	return ret < 0 ? XDP_PASS : ret;
+}
+
+char _license[] SEC("license") = "Dual MIT/GPL";
+