landlock: detect support at runtime

And ignore landlock-related commands if Landlock is unsupported at
runtime.

src/firejail/firejail.h

@@ -968,6 +968,7 @@ void oom_set(const char *oom_string);
 // landlock.c
 #ifdef HAVE_LANDLOCK
 int ll_get_fd(void);
+int ll_is_supported(void);
 int ll_read(const char *allowed_path);
 int ll_write(const char *allowed_path);
 int ll_special(const char *allowed_path);

src/firejail/landlock.c

@@ -28,6 +28,7 @@
 #include <fcntl.h>
 
 static int ll_ruleset_fd = -1;
+static int ll_abi = -1;
 
 int ll_get_fd(void) {
 	return ll_ruleset_fd;
@@ -59,7 +60,30 @@ landlock_restrict_self(const int ruleset_fd, const __u32 flags) {
 }
 #endif
 
+int ll_is_supported(void) {
+	if (ll_abi != -1)
+		goto out;
+
+	int ll_abi = landlock_create_ruleset(NULL, 0,
+	                                     LANDLOCK_CREATE_RULESET_VERSION);
+	if (ll_abi < 1) {
+		ll_abi = 0;
+		fprintf(stderr, "Warning: Landlock is disabled or not supported: %s, "
+		                "ignoring landlock commands\n",
+			strerror(errno));
+		goto out;
+	}
+	if (arg_debug) {
+		printf("Detected Landlock ABI version %d\n", ll_abi);
+	}
+out:
+	return ll_abi;
+}
+
 static int ll_create_full_ruleset() {
+	if (!ll_is_supported())
+		return -1;
+
 	int error;
 	struct landlock_ruleset_attr attr;
 	attr.handled_access_fs =
@@ -86,6 +110,9 @@ static int ll_create_full_ruleset() {
 }
 
 int ll_read(const char *allowed_path) {
+	if (!ll_is_supported())
+		return 0;
+
 	if (ll_ruleset_fd == -1)
 		ll_ruleset_fd = ll_create_full_ruleset();
 
@@ -115,6 +142,9 @@ int ll_read(const char *allowed_path) {
 }
 
 int ll_write(const char *allowed_path) {
+	if (!ll_is_supported())
+		return 0;
+
 	if (ll_ruleset_fd == -1)
 		ll_ruleset_fd = ll_create_full_ruleset();
 
@@ -148,6 +178,9 @@ int ll_write(const char *allowed_path) {
 }
 
 int ll_special(const char *allowed_path) {
+	if (!ll_is_supported())
+		return 0;
+
 	if (ll_ruleset_fd == -1)
 		ll_ruleset_fd = ll_create_full_ruleset();
 
@@ -179,6 +212,9 @@ int ll_special(const char *allowed_path) {
 }
 
 int ll_exec(const char *allowed_path) {
+	if (!ll_is_supported())
+		return 0;
+
 	if (ll_ruleset_fd == -1)
 		ll_ruleset_fd = ll_create_full_ruleset();
 
@@ -209,6 +245,9 @@ int ll_exec(const char *allowed_path) {
 int ll_basic_system(void) {
 	assert(cfg.homedir);
 
+	if (!ll_is_supported())
+		return 0;
+
 	if (ll_ruleset_fd == -1)
 		ll_ruleset_fd = ll_create_full_ruleset();
 
@@ -256,6 +295,9 @@ int ll_basic_system(void) {
 }
 
 int ll_restrict(__u32 flags) {
+	if (!ll_is_supported())
+		return 0;
+
 	int (*fnc[])(const char *) = {
 		ll_read,
 		ll_write,
@@ -298,6 +340,9 @@ void ll_add_profile(int type, const char *data) {
 	assert(type < LL_MAX);
 	assert(data);
 
+	if (!ll_is_supported())
+		return;
+
 	const char *str = data;
 	while (*str == ' ' || *str == '\t')
 		str++;