cgroup2: switch to option-by-option parsing

[again, carved out of patch by dhowells]
[NB: we probably want to handle "source" in parse_param here]
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/cgroup/cgroup.c

@@ -54,6 +54,7 @@
 #include <linux/proc_ns.h>
 #include <linux/nsproxy.h>
 #include <linux/file.h>
+#include <linux/fs_parser.h>
 #include <linux/sched/cputime.h>
 #include <linux/psi.h>
 #include <net/sock.h>
@@ -1772,26 +1773,37 @@ int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
 	return len;
 }
 
-static int parse_cgroup_root_flags(char *data, unsigned int *root_flags)
-{
-	char *token;
+enum cgroup2_param {
+	Opt_nsdelegate,
+	nr__cgroup2_params
+};
 
-	*root_flags = 0;
+static const struct fs_parameter_spec cgroup2_param_specs[] = {
+	fsparam_flag  ("nsdelegate",		Opt_nsdelegate),
+	{}
+};
 
-	if (!data || *data == '\0')
-		return 0;
+static const struct fs_parameter_description cgroup2_fs_parameters = {
+	.name		= "cgroup2",
+	.specs		= cgroup2_param_specs,
+};
 
-	while ((token = strsep(&data, ",")) != NULL) {
-		if (!strcmp(token, "nsdelegate")) {
-			*root_flags |= CGRP_ROOT_NS_DELEGATE;
-			continue;
-		}
+static int cgroup2_parse_param(struct fs_context *fc, struct fs_parameter *param)
+{
+	struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
+	struct fs_parse_result result;
+	int opt;
 
-		pr_err("cgroup2: unknown option \"%s\"\n", token);
-		return -EINVAL;
-	}
+	opt = fs_parse(fc, &cgroup2_fs_parameters, param, &result);
+	if (opt < 0)
+		return opt;
 
-	return 0;
+	switch (opt) {
+	case Opt_nsdelegate:
+		ctx->flags |= CGRP_ROOT_NS_DELEGATE;
+		return 0;
+	}
+	return -EINVAL;
 }
 
 static void apply_cgroup_root_flags(unsigned int root_flags)
@@ -2074,15 +2086,6 @@ static void cgroup_fs_context_free(struct fs_context *fc)
 	kfree(ctx);
 }
 
-static int cgroup_parse_monolithic(struct fs_context *fc, void *data)
-{
-	struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
-
-	if (data)
-		security_sb_eat_lsm_opts(data, &fc->security);
-	return parse_cgroup_root_flags(data, &ctx->flags);
-}
-
 static int cgroup_get_tree(struct fs_context *fc)
 {
 	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
@@ -2108,7 +2111,7 @@ static int cgroup_get_tree(struct fs_context *fc)
 
 static const struct fs_context_operations cgroup_fs_context_ops = {
 	.free		= cgroup_fs_context_free,
-	.parse_monolithic = cgroup_parse_monolithic,
+	.parse_param	= cgroup2_parse_param,
 	.get_tree	= cgroup_get_tree,
 	.reconfigure	= cgroup_reconfigure,
 };
@@ -2174,10 +2177,11 @@ struct file_system_type cgroup_fs_type = {
 };
 
 static struct file_system_type cgroup2_fs_type = {
-	.name = "cgroup2",
-	.init_fs_context = cgroup_init_fs_context,
-	.kill_sb = cgroup_kill_sb,
-	.fs_flags = FS_USERNS_MOUNT,
+	.name			= "cgroup2",
+	.init_fs_context	= cgroup_init_fs_context,
+	.parameters		= &cgroup2_fs_parameters,
+	.kill_sb		= cgroup_kill_sb,
+	.fs_flags		= FS_USERNS_MOUNT,
 };
 
 int cgroup_path_ns_locked(struct cgroup *cgrp, char *buf, size_t buflen,