selftests: net: add netconsole test for cmdline configuration

Add a new selftest to verify netconsole module loading with command
line arguments. This test exercises the init_netconsole() path and
validates proper parsing of the netconsole= parameter format.

The test:
- Loads netconsole module with cmdline configuration instead of
  dynamic reconfiguration
- Validates message transmission through the configured target
- Adds helper functions for cmdline string generation and module
  validation

This complements existing netconsole selftests by covering the
module initialization code path that processes boot-time parameters.
This test is useful to test issues like the one described in [1].

Link: https://lore.kernel.org/netdev/Z36TlACdNMwFD7wv@dev-ushankar.dev.purestorage.com/ [1]
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250613-rework-v3-8-0752bf2e6912@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/drivers/net/Makefile

@@ -12,6 +12,7 @@ TEST_GEN_FILES := \
 TEST_PROGS := \
 	napi_id.py \
 	netcons_basic.sh \
+	netcons_cmdline.sh \
 	netcons_fragmented_msg.sh \
 	netcons_overflow.sh \
 	netcons_sysdata.sh \

tools/testing/selftests/drivers/net/lib/sh/lib_netcons.sh

@@ -121,6 +121,17 @@ function create_dynamic_target() {
 	echo 1 > "${NETCONS_PATH}"/enabled
 }
 
+# Generate the command line argument for netconsole following:
+#  netconsole=[+][src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
+function create_cmdline_str() {
+	DSTMAC=$(ip netns exec "${NAMESPACE}" \
+		 ip link show "${DSTIF}" | awk '/ether/ {print $2}')
+	SRCPORT="1514"
+	TGTPORT="6666"
+
+	echo "netconsole=\"+${SRCPORT}@${SRCIP}/${SRCIF},${TGTPORT}@${DSTIP}/${DSTMAC}\""
+}
+
 # Do not append the release to the header of the message
 function disable_release_append() {
 	echo 0 > "${NETCONS_PATH}"/enabled
@@ -173,13 +184,9 @@ function listen_port_and_save_to() {
 		socat UDP-LISTEN:"${PORT}",fork "${OUTPUT}"
 }
 
-function validate_result() {
+# Only validate that the message arrived properly
+function validate_msg() {
 	local TMPFILENAME="$1"
-	local FORMAT=${2:-"extended"}
-
-	# TMPFILENAME will contain something like:
-	# 6.11.1-0_fbk0_rc13_509_g30d75cea12f7,13,1822,115075213798,-;netconsole selftest: netcons_gtJHM
-	#  key=value
 
 	# Check if the file exists
 	if [ ! -f "$TMPFILENAME" ]; then
@@ -192,6 +199,17 @@ function validate_result() {
 		cat "${TMPFILENAME}" >&2
 		exit "${ksft_fail}"
 	fi
+}
+
+# Validate the message and userdata
+function validate_result() {
+	local TMPFILENAME="$1"
+
+	# TMPFILENAME will contain something like:
+	# 6.11.1-0_fbk0_rc13_509_g30d75cea12f7,13,1822,115075213798,-;netconsole selftest: netcons_gtJHM
+	#  key=value
+
+	validate_msg "${TMPFILENAME}"
 
 	# userdata is not supported on basic format target,
 	# thus, do not validate it.
@@ -267,3 +285,12 @@ function pkill_socat() {
 	pkill -f "${PROCESS_NAME}"
 	set -e
 }
+
+# Check if netconsole was compiled as a module, otherwise exit
+function check_netconsole_module() {
+	if modinfo netconsole | grep filename: | grep -q builtin
+	then
+		echo "SKIP: netconsole should be compiled as a module" >&2
+		exit "${ksft_skip}"
+	fi
+}

tools/testing/selftests/drivers/net/netcons_cmdline.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: GPL-2.0
+
+# This is a selftest to test cmdline arguments on netconsole.
+# It exercises loading of netconsole from cmdline instead of the dynamic
+# reconfiguration. This includes parsing the long netconsole= line and all the
+# flow through init_netconsole().
+#
+# Author: Breno Leitao <leitao@debian.org>
+
+set -euo pipefail
+
+SCRIPTDIR=$(dirname "$(readlink -e "${BASH_SOURCE[0]}")")
+
+source "${SCRIPTDIR}"/lib/sh/lib_netcons.sh
+
+check_netconsole_module
+
+modprobe netdevsim 2> /dev/null || true
+rmmod netconsole 2> /dev/null || true
+
+# The content of kmsg will be save to the following file
+OUTPUT_FILE="/tmp/${TARGET}"
+
+# Check for basic system dependency and exit if not found
+# check_for_dependencies
+# Set current loglevel to KERN_INFO(6), and default to KERN_NOTICE(5)
+echo "6 5" > /proc/sys/kernel/printk
+# Remove the namespace and network interfaces
+trap do_cleanup EXIT
+# Create one namespace and two interfaces
+set_network
+# Create the command line for netconsole, with the configuration from the
+# function above
+CMDLINE="$(create_cmdline_str)"
+
+# Load the module, with the cmdline set
+modprobe netconsole "${CMDLINE}"
+
+# Listed for netconsole port inside the namespace and destination interface
+listen_port_and_save_to "${OUTPUT_FILE}" &
+# Wait for socat to start and listen to the port.
+wait_local_port_listen "${NAMESPACE}" "${PORT}" udp
+# Send the message
+echo "${MSG}: ${TARGET}" > /dev/kmsg
+# Wait until socat saves the file to disk
+busywait "${BUSYWAIT_TIMEOUT}" test -s "${OUTPUT_FILE}"
+# Make sure the message was received in the dst part
+# and exit
+validate_msg "${OUTPUT_FILE}"
+
+exit "${ksft_pass}"