[PATCH net 1/2] selftests: netconsole: Add race condition test for userdata corruption

[Gustavo Luiz Duarte <gustavold@gmail.com>]

Add a test to verify that netconsole userdata handling is properly
synchronized under concurrent read/write operations. The test creates
two competing loops: one continuously sending netconsole messages
(which read userdata), and another rapidly alternating userdata values
between two distinct 198-byte patterns filled with 'A' and 'B'
characters.

Without proper synchronization, concurrent reads and writes could result
in torn reads where a message contains mixed userdata (e.g., starting
with 'A' but containing 'B', or vice versa). The test monitors 10,000
messages and fails if it detects any such corruption, ensuring that the
netconsole implementation maintains data consistency through proper
locking mechanisms.

This test validates the fix for potential race conditions in the
netconsole userdata path and serves as a regression test to prevent
similar issues in the future.

Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
---
 .../selftests/drivers/net/netcons_race_userdata.sh | 87 ++++++++++++++++++++++
 1 file changed, 87 insertions(+)

diff --git a/tools/testing/selftests/drivers/net/netcons_race_userdata.sh b/tools/testing/selftests/drivers/net/netcons_race_userdata.sh
new file mode 100755
index 0000000000000..d6574f0364ead
--- /dev/null
+++ b/tools/testing/selftests/drivers/net/netcons_race_userdata.sh
@@ -0,0 +1,87 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: GPL-2.0
+
+# This test verifies that netconsole userdata remains consistent under concurrent
+# read/write operations. It creates two loops: one continuously writing netconsole
+# messages (which read userdata) and another rapidly alternating userdata values
+# between two distinct patterns. The test checks that no message contains corrupted
+# or mixed userdata, ensuring proper synchronization in the netconsole implementation.
+#
+# Author: Gustavo Luiz Duarte <gustavold@gmail.com>
+
+set -euo pipefail
+
+SCRIPTDIR=$(dirname "$(readlink -e "${BASH_SOURCE[0]}")")
+
+source "${SCRIPTDIR}"/lib/sh/lib_netcons.sh
+
+function loop_set_userdata() {
+	MSGA=$(printf 'A%.0s' {1..198})
+	MSGB=$(printf 'B%.0s' {1..198})
+
+	while true; do
+		echo "$MSGA" > "${NETCONS_PATH}/userdata/${USERDATA_KEY}/value"
+		echo "$MSGB" > "${NETCONS_PATH}/userdata/${USERDATA_KEY}/value"
+	done
+}
+
+function loop_print_msg() {
+	while true; do
+		echo "test msg" > /dev/kmsg
+	done
+}
+
+cleanup_children() {
+	pkill_socat
+	# Remove the namespace, interfaces and netconsole target
+	cleanup
+	kill $child1 $child2 2> /dev/null || true
+	wait $child1 $child2 2> /dev/null || true
+}
+
+modprobe netdevsim 2> /dev/null || true
+modprobe netconsole 2> /dev/null || true
+
+OUTPUT_FILE="stdout"
+# Check for basic system dependency and exit if not found
+check_for_dependencies
+# Set current loglevel to KERN_INFO(6), and default to KERN_NOTICE(5)
+echo "6 5" > /proc/sys/kernel/printk
+# kill child processes and remove interfaces on exit
+trap cleanup_children EXIT
+
+# Create one namespace and two interfaces
+set_network
+# Create a dynamic target for netconsole
+create_dynamic_target
+# Set userdata "key" with the "value" value
+set_user_data
+
+# Start userdata read loop (printk)
+loop_print_msg &
+child1=$!
+
+# Start userdata write loop
+loop_set_userdata &
+child2=$!
+
+# Start socat to listen for netconsole messages and check for corrupted userdata.
+MAX_COUNT=10000
+i=0
+while read line; do
+	if [ $i -ge $MAX_COUNT ]; then
+		echo "Test passed."
+		exit ${ksft_pass}
+	fi
+
+	if [[ "$line" == "key=A"* && "$line" == *"B"* ||
+	      "$line" == "key=B"* && "$line" == *"A"* ]]; then
+		echo "Test failed. Found corrupted userdata: $line"
+		exit ${ksft_fail}
+	fi
+
+	i=$((i + 1))
+done < <(listen_port_and_save_to ${OUTPUT_FILE} 2> /dev/null)
+
+echo "socat died before we could check $MAX_COUNT messages. Skipping test. ${ksft_skip}"
+exit ${ksft_skip}

-- 
2.47.3