cgroup: let a symlink too be created with a cftype file

This commit enables a cftype to have a symlink (of any name) that
points to the file associated with the cftype.
Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/cgroup-defs.h

@@ -106,6 +106,8 @@ enum {
 	CFTYPE_WORLD_WRITABLE	= (1 << 4),	/* (DON'T USE FOR NEW FILES) S_IWUGO */
 	CFTYPE_DEBUG		= (1 << 5),	/* create when cgroup_debug */
 
+	CFTYPE_SYMLINKED	= (1 << 6),	/* pointed to by symlink too */
+
 	/* internal flags, do not use outside cgroup core proper */
 	__CFTYPE_ONLY_ON_DFL	= (1 << 16),	/* only on default hierarchy */
 	__CFTYPE_NOT_ON_DFL	= (1 << 17),	/* not on default hierarchy */
@@ -543,6 +545,7 @@ struct cftype {
 	 * end of cftype array.
 	 */
 	char name[MAX_CFTYPE_NAME];
+	char link_name[MAX_CFTYPE_NAME];
 	unsigned long private;
 
 	/*

kernel/cgroup/cgroup.c

@@ -1460,8 +1460,8 @@ struct cgroup *task_cgroup_from_root(struct task_struct *task,
 
 static struct kernfs_syscall_ops cgroup_kf_syscall_ops;
 
-static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,
-			      char *buf)
+static char *cgroup_fill_name(struct cgroup *cgrp, const struct cftype *cft,
+			      char *buf, bool write_link_name)
 {
 	struct cgroup_subsys *ss = cft->ss;
 
@@ -1471,13 +1471,26 @@ static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,
 
 		snprintf(buf, CGROUP_FILE_NAME_MAX, "%s%s.%s",
 			 dbg, cgroup_on_dfl(cgrp) ? ss->name : ss->legacy_name,
-			 cft->name);
+			 write_link_name ? cft->link_name : cft->name);
 	} else {
-		strscpy(buf, cft->name, CGROUP_FILE_NAME_MAX);
+		strscpy(buf, write_link_name ? cft->link_name : cft->name,
+			CGROUP_FILE_NAME_MAX);
 	}
 	return buf;
 }
 
+static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,
+			      char *buf)
+{
+	return cgroup_fill_name(cgrp, cft, buf, false);
+}
+
+static char *cgroup_link_name(struct cgroup *cgrp, const struct cftype *cft,
+			      char *buf)
+{
+	return cgroup_fill_name(cgrp, cft, buf, true);
+}
+
 /**
  * cgroup_file_mode - deduce file mode of a control file
  * @cft: the control file in question
@@ -1636,6 +1649,9 @@ static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
 	}
 
 	kernfs_remove_by_name(cgrp->kn, cgroup_file_name(cgrp, cft, name));
+	if (cft->flags & CFTYPE_SYMLINKED)
+		kernfs_remove_by_name(cgrp->kn,
+				      cgroup_link_name(cgrp, cft, name));
 }
 
 /**
@@ -3821,6 +3837,7 @@ static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
 {
 	char name[CGROUP_FILE_NAME_MAX];
 	struct kernfs_node *kn;
+	struct kernfs_node *kn_link;
 	struct lock_class_key *key = NULL;
 	int ret;
 
@@ -3851,6 +3868,14 @@ static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
 		spin_unlock_irq(&cgroup_file_kn_lock);
 	}
 
+	if (cft->flags & CFTYPE_SYMLINKED) {
+		kn_link = kernfs_create_link(cgrp->kn,
+					     cgroup_link_name(cgrp, cft, name),
+					     kn);
+		if (IS_ERR(kn_link))
+			return PTR_ERR(kn_link);
+	}
+
 	return 0;
 }