kernfs: attach uuid for every kernfs and report it in fsid

JIRA: https://issues.redhat.com/browse/RHEL-8810

The following two commits added the same thing for tmpfs:

* commit 2b4db79618 ("tmpfs: generate random sb->s_uuid")
* commit 59cda49ecf ("shmem: allow reporting fanotify events with file handles on tmpfs")

Having fsid allows using fanotify, which is especially handy for cgroups,
where one might be interested in knowing when they are created or removed.

Signed-off-by: Ivan Babrou <ivan@cloudflare.com>
Acked-by: Jan Kara <jack@suse.cz>
Acked-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230731184731.64568-1-ivan@cloudflare.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 0ce7c12e88cfa3e4cb0800e8172e5befea5c199a)
Signed-off-by: Ryan Sullivan <rysulliv@redhat.com>

fs/kernfs/mount.c

@@ -16,6 +16,8 @@
 #include <linux/namei.h>
 #include <linux/seq_file.h>
 #include <linux/exportfs.h>
+#include <linux/uuid.h>
+#include <linux/statfs.h>
 
 #include "kernfs-internal.h"
 
@@ -45,8 +47,15 @@ static int kernfs_sop_show_path(struct seq_file *sf, struct dentry *dentry)
 	return 0;
 }
 
+static int kernfs_statfs(struct dentry *dentry, struct kstatfs *buf)
+{
+	simple_statfs(dentry, buf);
+	buf->f_fsid = uuid_to_fsid(dentry->d_sb->s_uuid.b);
+	return 0;
+}
+
 const struct super_operations kernfs_sops = {
-	.statfs		= simple_statfs,
+	.statfs		= kernfs_statfs,
 	.drop_inode	= generic_delete_inode,
 	.evict_inode	= kernfs_evict_inode,
 
@@ -351,6 +360,8 @@ int kernfs_get_tree(struct fs_context *fc)
 		}
 		sb->s_flags |= SB_ACTIVE;
 
+		uuid_gen(&sb->s_uuid);
+
 		down_write(&root->kernfs_supers_rwsem);
 		list_add(&info->node, &info->root->supers);
 		up_write(&root->kernfs_supers_rwsem);