[PATCH 1/6] samples/landlock: Implement best-effort fallback for network rules.

["Günther Noack" <gnoack3000@gmail.com>]

The sandboxer sample tool added network rules unconditionally, even on
systems that only support lower Landlock ABI versions, resulting in
errors.

This change implements the correct best-effort fallback: As on older
Landlock ABI versions, the given operation is not restrictable, is
also does not need to be allow-listed.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 samples/landlock/sandboxer.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/samples/landlock/sandboxer.c b/samples/landlock/sandboxer.c
index 030583273f3f..1c514efecafb 100644
--- a/samples/landlock/sandboxer.c
+++ b/samples/landlock/sandboxer.c
@@ -198,6 +198,10 @@ static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
 		.allowed_access = allowed_access,
 	};
 
+	/* A rule without access rights and flags is a no-op. */
+	if (!allowed_access && !flags)
+		return 0;
+
 	env_port_name = getenv(env_var);
 	if (!env_port_name)
 		return 0;
@@ -657,19 +661,27 @@ int main(const int argc, char *const argv[], char *const *const envp)
 	}
 
 	if (populate_ruleset_net(ENV_TCP_BIND_NAME, ruleset_fd,
-				 LANDLOCK_ACCESS_NET_BIND_TCP, 0)) {
+				 ruleset_attr.handled_access_net &
+					 LANDLOCK_ACCESS_NET_BIND_TCP,
+				 0)) {
 		goto err_close_ruleset;
 	}
 	if (populate_ruleset_net(ENV_TCP_CONNECT_NAME, ruleset_fd,
-				 LANDLOCK_ACCESS_NET_CONNECT_TCP, 0)) {
+				 ruleset_attr.handled_access_net &
+					 LANDLOCK_ACCESS_NET_CONNECT_TCP,
+				 0)) {
 		goto err_close_ruleset;
 	}
 	if (populate_ruleset_net(ENV_UDP_BIND_NAME, ruleset_fd,
-				 LANDLOCK_ACCESS_NET_BIND_UDP, 0)) {
+				 ruleset_attr.handled_access_net &
+					 LANDLOCK_ACCESS_NET_BIND_UDP,
+				 0)) {
 		goto err_close_ruleset;
 	}
 	if (populate_ruleset_net(ENV_UDP_CONNECT_SEND_NAME, ruleset_fd,
-				 LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP, 0)) {
+				 ruleset_attr.handled_access_net &
+					 LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP,
+				 0)) {
 		goto err_close_ruleset;
 	}
 
-- 
2.55.0