netconsole: selftest: Add test for fragmented messages

Add a new selftest to verify netconsole's handling of messages that
exceed the packet size limit and require fragmentation. The test sends
messages with varying sizes and userdata, validating that:

1. Large messages are correctly fragmented and reassembled
2. Userdata fields are properly preserved across fragments
3. Messages work correctly with and without kernel release version
   appending

The test creates a networking environment using netdevsim, sends
messages through /dev/kmsg, and verifies the received fragments maintain
message integrity.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250203-netcons_frag_msgs-v1-1-5bc6bedf2ac0@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -0,0 +1,122 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: GPL-2.0
+
+# Test netconsole's message fragmentation functionality.
+#
+# When a message exceeds the maximum packet size, netconsole splits it into
+# multiple fragments for transmission. This test verifies:
+#  - Correct fragmentation of large messages
+#  - Proper reassembly of fragments at the receiver
+#  - Preservation of userdata across fragments
+#  - Behavior with and without kernel release version appending
+#
+# Author: Breno Leitao <leitao@debian.org>
+
+set -euo pipefail
+
+SCRIPTDIR=$(dirname "$(readlink -e "${BASH_SOURCE[0]}")")
+
+source "${SCRIPTDIR}"/lib/sh/lib_netcons.sh
+
+modprobe netdevsim 2> /dev/null || true
+modprobe netconsole 2> /dev/null || true
+
+# The content of kmsg will be save to the following file
+OUTPUT_FILE="/tmp/${TARGET}"
+
+# set userdata to a long value. In this case, it is "1-2-3-4...50-"
+USERDATA_VALUE=$(printf -- '%.2s-' {1..60})
+
+# Convert the header string in a regexp, so, we can remove
+# the second header as well.
+# A header looks like "13,468,514729715,-,ncfrag=0/1135;". If
+# release is appended, you might find something like:L
+# "6.13.0-04048-g4f561a87745a,13,468,514729715,-,ncfrag=0/1135;"
+function header_to_regex() {
+	# header is everything before ;
+	local HEADER="${1}"
+	REGEX=$(echo "${HEADER}" | cut -d'=' -f1)
+	echo "${REGEX}=[0-9]*\/[0-9]*;"
+}
+
+# We have two headers in the message. Remove both to get the full message,
+# and extract the full message.
+function extract_msg() {
+	local MSGFILE="${1}"
+	# Extract the header, which is the very first thing that arrives in the
+	# first list.
+	HEADER=$(sed -n '1p' "${MSGFILE}" | cut -d';' -f1)
+	HEADER_REGEX=$(header_to_regex "${HEADER}")
+
+	# Remove the two headers from the received message
+	# This will return the message without any header, similarly to what
+	# was sent.
+	sed "s/""${HEADER_REGEX}""//g" "${MSGFILE}"
+}
+
+# Validate the message, which has two messages glued together.
+# unwrap them to make sure all the characters were transmitted.
+# File will look like the following:
+#  13,468,514729715,-,ncfrag=0/1135;<message>
+#   key=<part of key>-13,468,514729715,-,ncfrag=967/1135;<rest of the key>
+function validate_fragmented_result() {
+	# Discard the netconsole headers, and assemble the full message
+	RCVMSG=$(extract_msg "${1}")
+
+	# check for the main message
+	if ! echo "${RCVMSG}" | grep -q "${MSG}"; then
+		echo "Message body doesn't match." >&2
+		echo "msg received=" "${RCVMSG}" >&2
+		exit "${ksft_fail}"
+	fi
+
+	# check userdata
+	if ! echo "${RCVMSG}" | grep -q "${USERDATA_VALUE}"; then
+		echo "message userdata doesn't match" >&2
+		echo "msg received=" "${RCVMSG}" >&2
+		exit "${ksft_fail}"
+	fi
+	# test passed. hooray
+}
+
+# Check for basic system dependency and exit if not found
+check_for_dependencies
+# Set current loglevel to KERN_INFO(6), and default to KERN_NOTICE(5)
+echo "6 5" > /proc/sys/kernel/printk
+# Remove the namespace, interfaces and netconsole target on exit
+trap cleanup EXIT
+# Create one namespace and two interfaces
+set_network
+# Create a dynamic target for netconsole
+create_dynamic_target
+# Set userdata "key" with the "value" value
+set_user_data
+
+
+# TEST 1: Send message and userdata. They will fragment
+# =======
+MSG=$(printf -- 'MSG%.3s=' {1..150})
+
+# Listen 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 "${OUTPUT_FILE}"
+# Check if the message was not corrupted
+validate_fragmented_result "${OUTPUT_FILE}"
+
+# TEST 2: Test with smaller message, and without release appended
+# =======
+MSG=$(printf -- 'FOOBAR%.3s=' {1..100})
+# Let's disable release and test again.
+disable_release_append
+
+listen_port_and_save_to "${OUTPUT_FILE}" &
+wait_local_port_listen "${NAMESPACE}" "${PORT}" udp
+echo "${MSG}: ${TARGET}" > /dev/kmsg
+busywait "${BUSYWAIT_TIMEOUT}" test -s "${OUTPUT_FILE}"
+validate_fragmented_result "${OUTPUT_FILE}"
+exit "${ksft_pass}"