Merge branch 'vsock-add-write-once-semantics-to-child_ns_mode'

Bobby Eshleman says:

====================
vsock: add write-once semantics to child_ns_mode

Two administrator processes may race when setting child_ns_mode: one
sets it to "local" and creates a namespace, but another changes it to
"global" in between. The first process ends up with a namespace in the
wrong mode. Make child_ns_mode write-once so that a namespace manager
can set it once, check the value, and be guaranteed it won't change
before creating its namespaces. Writing a different value after the
first write returns -EBUSY.

One patch for the implementation, one for docs, and one for tests.

v2: https://lore.kernel.org/r/20260218-vsock-ns-write-once-v2-0-19e4c50d509a@meta.com
v1: https://lore.kernel.org/r/20260217-vsock-ns-write-once-v1-1-a1fb30f289a9@meta.com
====================

Link: https://patch.msgid.link/20260223-vsock-ns-write-once-v3-0-c0cde6959923@meta.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 21 insertions, 20 deletions

diff --git a/tools/testing/selftests/vsock/vmtest.sh b/tools/testing/selftests/vsock/vmtest.sh
index dc8dbe74a6d0..86e338886b33 100755
--- a/tools/testing/selftests/vsock/vmtest.sh
+++ b/tools/testing/selftests/vsock/vmtest.sh
@@ -210,16 +210,21 @@ check_result() {
 }
 
 add_namespaces() {
-	local orig_mode
-	orig_mode=$(cat /proc/sys/net/vsock/child_ns_mode)
-
-	for mode in "${NS_MODES[@]}"; do
-		echo "${mode}" > /proc/sys/net/vsock/child_ns_mode
-		ip netns add "${mode}0" 2>/dev/null
-		ip netns add "${mode}1" 2>/dev/null
-	done
-
-	echo "${orig_mode}" > /proc/sys/net/vsock/child_ns_mode
+	ip netns add "global-parent" 2>/dev/null
+	echo "global" | ip netns exec "global-parent" \
+		tee /proc/sys/net/vsock/child_ns_mode &>/dev/null
+	ip netns add "local-parent" 2>/dev/null
+	echo "local" | ip netns exec "local-parent" \
+		tee /proc/sys/net/vsock/child_ns_mode &>/dev/null
+
+	nsenter --net=/var/run/netns/global-parent \
+		ip netns add "global0" 2>/dev/null
+	nsenter --net=/var/run/netns/global-parent \
+		ip netns add "global1" 2>/dev/null
+	nsenter --net=/var/run/netns/local-parent \
+		ip netns add "local0" 2>/dev/null
+	nsenter --net=/var/run/netns/local-parent \
+		ip netns add "local1" 2>/dev/null
 }
 
 init_namespaces() {
@@ -237,6 +242,8 @@ del_namespaces() {
 		log_host "removed ns ${mode}0"
 		log_host "removed ns ${mode}1"
 	done
+	ip netns del "global-parent" &>/dev/null
+	ip netns del "local-parent" &>/dev/null
 }
 
 vm_ssh() {
@@ -287,7 +294,7 @@ check_args() {
 }
 
 check_deps() {
-	for dep in vng ${QEMU} busybox pkill ssh ss socat; do
+	for dep in vng ${QEMU} busybox pkill ssh ss socat nsenter; do
 		if [[ ! -x $(command -v "${dep}") ]]; then
 			echo -e "skip:    dependency ${dep} not found!\n"
 			exit "${KSFT_SKIP}"
@@ -1231,12 +1238,8 @@ test_ns_local_same_cid_ok() {
 }
 
 test_ns_host_vsock_child_ns_mode_ok() {
-	local orig_mode
-	local rc
-
-	orig_mode=$(cat /proc/sys/net/vsock/child_ns_mode)
+	local rc="${KSFT_PASS}"
 
-	rc="${KSFT_PASS}"
 	for mode in "${NS_MODES[@]}"; do
 		local ns="${mode}0"
 
@@ -1246,15 +1249,13 @@ test_ns_host_vsock_child_ns_mode_ok() {
 			continue
 		fi
 
-		if ! echo "${mode}" > /proc/sys/net/vsock/child_ns_mode; then
-			log_host "child_ns_mode should be writable to ${mode}"
+		if ! echo "${mode}" | ip netns exec "${ns}" \
+			tee /proc/sys/net/vsock/child_ns_mode &>/dev/null; then
 			rc="${KSFT_FAIL}"
 			continue
 		fi
 	done
 
-	echo "${orig_mode}" > /proc/sys/net/vsock/child_ns_mode
-
 	return "${rc}"
 }