fix: reuse cgroup2 superblock options when mount cgroupfs

src/cgroups/cgfsng.c

@@ -399,27 +399,35 @@ static void trim(char *s)
  */
 static int __cg_mount_direct(struct hierarchy *h, const char *controllerpath)
 {
-	 __do_free char *controllers = NULL;
-	 char *fstype = "cgroup2";
-	 unsigned long flags = 0;
-	 int ret;
-
-	 flags |= MS_NOSUID;
-	 flags |= MS_NOEXEC;
-	 flags |= MS_NODEV;
-	 flags |= MS_RELATIME;
-
-	 if (h->version != CGROUP2_SUPER_MAGIC) {
-		 controllers = lxc_string_join(",", (const char **)h->controllers, false);
-		 if (!controllers)
-			 return -ENOMEM;
-		 fstype = "cgroup";
+	__do_free char *controllers = NULL;
+	__do_free const char *opts = NULL;
+	char *fstype = "cgroup2";
+	unsigned long flags = 0;
+	int ret;
+
+	flags |= MS_NOSUID;
+	flags |= MS_NOEXEC;
+	flags |= MS_NODEV;
+	flags |= MS_RELATIME;
+
+	if (h->version != CGROUP2_SUPER_MAGIC) {
+		controllers = lxc_string_join(",", (const char **)h->controllers, false);
+		if (!controllers)
+			return -ENOMEM;
+		fstype = "cgroup";
+		ret = mount("cgroup", controllerpath, fstype, flags, controllers);
+	} else {
+		opts = get_mount_opts(DEFAULT_CGROUP_MOUNTPOINT, fstype);
+		if (!opts) {
+			return -1;
+		}
+		const char *super_opts = extract_cgroup2_super_opts(opts);
+		ret = mount(fstype, controllerpath, fstype, flags, super_opts);
 	}
 
-	ret = mount("cgroup", controllerpath, fstype, flags, controllers);
 	if (ret < 0)
 		return -1;
-
+	
 	return 0;
 }
 

src/cgroups/cgroup_utils.c

@@ -13,6 +13,7 @@
 #include <sys/types.h>
 #include <sys/vfs.h>
 #include <unistd.h>
+#include <mntent.h>
 
 #include "../macro.h"
 #include "../memory_utils.h"
@@ -810,3 +811,60 @@ int cgroup_walkup_to_root(int cgroup2_root_fd, int hierarchy_fd,
 
 	return log_error_errno(-ELOOP, ELOOP, "To many nested cgroups or invalid mount tree. Terminating walk");
 }
+
+// return first matching cgroup2 super option from opts string
+// eg: "rw,nosuid,nodev,noexec,relatime,memory_localevents,memory_recursiveprot"
+// returns "memory_localevents,memory_recursiveprot"
+const char *extract_cgroup2_super_opts(const char *opts) {
+	static const char *wanted_opts[] = {
+		"nsdelegate",
+		"memory_recursiveprot",
+		"memory_localevents",
+		NULL
+	};
+	if (opts == NULL)
+		return NULL;
+
+	const char *p = opts;
+	while (*p) {
+		const char *start = p;
+		while (*p && *p != ',')
+			p++;
+		size_t len = p - start;
+		for (int i = 0; wanted_opts[i] != NULL; i++) {
+			size_t wanted_len = strlen(wanted_opts[i]);
+			if (len == wanted_len && strncmp(start, wanted_opts[i], len) == 0){
+				return start;
+			}
+		}
+		if (*p == ',')
+			p++;
+	}
+	return NULL;
+}
+
+// return mount options for given target and expected fs type
+// eg: target="/sys/fs/cgroup", expect_type="cgroup2"
+// return NULL if not found, else strdup'ed string of mount options
+// the caller is responsible for freeing the returned string
+char *get_mount_opts(const char *target, const char *expect_type) {
+	if (target == NULL || expect_type == NULL)
+		return NULL;
+
+	char *res = NULL;
+	FILE *fp = setmntent("/proc/self/mounts", "r");
+	if (!fp)
+		return NULL;
+
+	struct mntent *ent;
+	while ((ent = getmntent(fp)) != NULL) {
+		if (strncmp(ent->mnt_dir, target, strlen(target)) == 0 
+			&& strncmp(ent->mnt_type, expect_type, strlen(expect_type)) == 0) {
+			res = strdup(ent->mnt_opts); // allocate, caller frees
+			goto out;
+		}
+	}
+out:
+	endmntent(fp);
+	return res;
+}

src/cgroups/cgroup_utils.h

@@ -98,4 +98,7 @@ static inline bool is_empty_string(const char *s)
 	return !s || strcmp(s, "") == 0;
 }
 
+const char *extract_cgroup2_super_opts(const char *opts);
+char *get_mount_opts(const char *target, const char *expect_type);
+
 #endif /* __LXC_CGROUP_UTILS_H */

tests/meson.build

@@ -152,3 +152,16 @@ test_programs += executable(
     include_directories: config_include,
     install: false,
     build_by_default: want_tests != false)
+
+test_cgroup_utils = executable(
+    'test_cgroup_utils',
+    [
+        'test_cgroup_utils.c',
+        '../src/cgroups/cgroup_utils.c',
+        '../src/utils.c'
+    ],
+    include_directories: config_include,
+    install: false,
+    build_by_default: want_tests != false )
+
+test('test_cgroup_utils', test_cgroup_utils)

tests/test_cgroup_utils.c

@@ -0,0 +1,26 @@
+#include "src/cgroups/cgroup_utils.h"
+
+void test_extract_cgroup2_super_opts(void) {
+    const char *opts = "rw,nosuid,nodev,noexec,relatime,memory_localevents,memory_recursiveprot";
+    const char *result = extract_cgroup2_super_opts(opts);
+    if (result == NULL || strcmp(result, "memory_localevents,memory_recursiveprot") != 0) {
+        fprintf(stderr, "Test failed: expected 'memory_localevents,memory_recursiveprot', got '%s'\n", result ? result : "NULL");
+        exit(1);
+    }
+}
+
+void test_extract_cgroup2_super_opts_not_match(void) {
+    const char *opts = "rw,nosuid,nodev,noexec,relatime";
+    const char *result = extract_cgroup2_super_opts(opts);
+    if (result != NULL) {
+        fprintf(stderr, "Test failed: expected NULL, got '%s'\n", result);
+        exit(1);
+    }
+}
+
+int main(int argc, char *argv[]) {
+    test_extract_cgroup2_super_opts();
+    test_extract_cgroup2_super_opts_not_match();
+	printf("All tests passed\n");
+    return 0;
+}