[PATCH net 2/2] selftests: net: test devinet inheritance after failed writes

[Yingjie Wang <x-professor@outlook.com>]

A failed per-device IPv4 sysctl write must not mark the setting as
explicitly configured. Otherwise, later conf/default updates stop
propagating to that device.

Add a test which rejects an invalid lo/arp_ignore write and then checks
that lo inherits a new default. Keep loopback unconfigured because adding
its address marks all devconf settings as explicit. Also verify the
complementary case: a valid same-value write marks the setting as explicit
and prevents inheritance.

Before each case, change the default to a value different from the current
lo value and require lo to inherit it. This avoids depending on init_net
defaults and skips the test if inherited state has already disabled
propagation.

The failed-write case fails on the base kernel. Both cases pass with the
preceding fix.

Signed-off-by: Yingjie Wang <x-professor@outlook.com>
---
 tools/testing/selftests/net/Makefile        |   1 +
 tools/testing/selftests/net/devinet_conf.sh | 101 ++++++++++++++++++++
 2 files changed, 102 insertions(+)
 create mode 100755 tools/testing/selftests/net/devinet_conf.sh

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index df215e1a62621..f551bd40bab80 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -26,6 +26,7 @@ TEST_PROGS := \
 	cmsg_so_priority.sh \
 	cmsg_time.sh \
 	cork_fragsize.py \
+	devinet_conf.sh \
 	double_udp_encap.sh \
 	drop_monitor_tests.sh \
 	ecmp_rehash.sh \
diff --git a/tools/testing/selftests/net/devinet_conf.sh b/tools/testing/selftests/net/devinet_conf.sh
new file mode 100755
index 0000000000000..54655dc2b0bd8
--- /dev/null
+++ b/tools/testing/selftests/net/devinet_conf.sh
@@ -0,0 +1,101 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+source lib.sh
+
+cleanup()
+{
+	cleanup_all_ns
+}
+
+trap cleanup EXIT
+
+prepare_inheritance()
+{
+	local ns=$1
+	local result=$2
+	local current
+	local probe
+	local value
+
+	if ! current=$(ip netns exec "$ns" \
+		sysctl -n net.ipv4.conf.lo.arp_ignore); then
+		echo "ERROR: failed to read lo/arp_ignore in $ns"
+		return "$ksft_fail"
+	fi
+	if ((current == 0)); then
+		probe=1
+	else
+		probe=0
+	fi
+
+	if ! ip netns exec "$ns" sysctl -qw \
+		net.ipv4.conf.default.arp_ignore="$probe"; then
+		echo "ERROR: failed to update default/arp_ignore in $ns"
+		return "$ksft_fail"
+	fi
+	if ! value=$(ip netns exec "$ns" \
+		sysctl -n net.ipv4.conf.lo.arp_ignore); then
+		echo "ERROR: failed to read lo/arp_ignore in $ns"
+		return "$ksft_fail"
+	fi
+	if ((value != probe)); then
+		echo "SKIP: lo/arp_ignore does not inherit conf/default in $ns"
+		return "$ksft_skip"
+	fi
+
+	printf -v "$result" '%d' "$probe"
+}
+
+# Adding the loopback address marks all devconf settings as explicit.
+# Leave it unconfigured so default inheritance remains active.
+failed_ns="failed-$(mktemp -u XXXXXX)"
+if ! ip netns add "$failed_ns"; then
+	echo "Failed to create network namespace $failed_ns"
+	exit "$ksft_skip"
+fi
+NS_LIST+=("$failed_ns")
+
+explicit_ns="explicit-$(mktemp -u XXXXXX)"
+if ! ip netns add "$explicit_ns"; then
+	echo "Failed to create network namespace $explicit_ns"
+	exit "$ksft_skip"
+fi
+NS_LIST+=("$explicit_ns")
+
+prepare_inheritance "$failed_ns" failed_value
+ret=$?
+((ret == 0)) || exit "$ret"
+prepare_inheritance "$explicit_ns" explicit_value
+ret=$?
+((ret == 0)) || exit "$ret"
+failed_next=$((!failed_value))
+explicit_next=$((!explicit_value))
+
+RET=0
+ip netns exec "$failed_ns" sh -c \
+	'printf x > /proc/sys/net/ipv4/conf/lo/arp_ignore' 2>/dev/null
+check_fail $? "invalid arp_ignore write was accepted"
+ip netns exec "$failed_ns" sysctl -qw \
+	net.ipv4.conf.default.arp_ignore="$failed_next"
+check_err $? "failed to update default arp_ignore"
+value=$(ip netns exec "$failed_ns" sysctl -n net.ipv4.conf.lo.arp_ignore)
+check_err $? "failed to read lo arp_ignore"
+check_err "$((value != failed_next))" \
+	"failed write prevented lo from inheriting arp_ignore"
+log_test "Failed devinet write preserves inheritance"
+
+RET=0
+ip netns exec "$explicit_ns" sysctl -qw \
+	net.ipv4.conf.lo.arp_ignore="$explicit_value"
+check_err $? "failed to explicitly set lo arp_ignore"
+ip netns exec "$explicit_ns" sysctl -qw \
+	net.ipv4.conf.default.arp_ignore="$explicit_next"
+check_err $? "failed to update default arp_ignore"
+value=$(ip netns exec "$explicit_ns" sysctl -n net.ipv4.conf.lo.arp_ignore)
+check_err $? "failed to read lo arp_ignore"
+check_err "$((value != explicit_value))" \
+	"explicit lo arp_ignore unexpectedly inherited the default"
+log_test "Valid same-value devinet write prevents inheritance"
+
+exit "$EXIT_STATUS"
-- 
2.43.0