[PATCH] overlayfs: implement show_options

[PATCH] overlayfs: implement show_options

[Erez Zadok]

Overlayfs: implement show_options

This is useful because of the stacking nature of overlayfs.  Users like to
find out (via /proc/mounts) which lower/upper directory were used at mount
time.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index be3c4a9..ff55a00 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -15,12 +15,19 @@
 #include <linux/slab.h>
 #include <linux/parser.h>
 #include <linux/module.h>
+#include <linux/seq_file.h>
 #include "overlayfs.h"
 
 MODULE_AUTHOR("Miklos Szeredi <miklos@szeredi.hu>");
 MODULE_DESCRIPTION("Overlay filesystem");
 MODULE_LICENSE("GPL");
 
+struct ovl_config {
+	char *lowerdir;
+	char *upperdir;
+};
+
+/* private information held for overlayfs's superblock */
 struct ovl_fs {
 	/*
 	 * Keep "double reference" on upper dentries, so that
@@ -28,8 +35,11 @@ struct ovl_fs {
 	 */
 	struct vfsmount *upper_mnt;
 	struct vfsmount *lower_mnt;
+	/* pathnames of lower and upper dirs, for show_options */
+	struct ovl_config config;
 };
 
+/* private information held for every overlayfs dentry */
 struct ovl_entry {
 	struct dentry *__upperdentry;
 	struct dentry *lowerdentry;
@@ -363,6 +373,8 @@ static void ovl_put_super(struct super_block *sb)
 	mntput(ufs->upper_mnt);
 	mntput(ufs->lower_mnt);
 
+	kfree(ufs->config.lowerdir);
+	kfree(ufs->config.upperdir);
 	kfree(ufs);
 }
 
@@ -404,15 +416,27 @@ static int ovl_statfs(struct dentry *dentry, struct kstatfs *buf)
 	return path.dentry->d_sb->s_op->statfs(path.dentry, buf);
 }
 
+/**
+ * ovl_show_options
+ *
+ * Prints the mount options for a given superblock.
+ * Returns zero; does not fail.
+ */
+static int ovl_show_options(struct seq_file *m, struct vfsmount *mnt)
+{
+	struct super_block *sb = mnt->mnt_sb;
+	struct ovl_fs *ufs = sb->s_fs_info;
+
+	seq_printf(m, ",lowerdir=%s", ufs->config.lowerdir);
+	seq_printf(m, ",upperdir=%s", ufs->config.upperdir);
+	return 0;
+}
+
 static const struct super_operations ovl_super_operations = {
 	.put_super	= ovl_put_super,
 	.remount_fs	= ovl_remount_fs,
 	.statfs		= ovl_statfs,
-};
-
-struct ovl_config {
-	char *lowerdir;
-	char *upperdir;
+	.show_options	= ovl_show_options,
 };
 
 enum {
@@ -472,37 +496,36 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	struct dentry *root_dentry;
 	struct ovl_entry *oe;
 	struct ovl_fs *ufs;
-	struct ovl_config config;
 	int err;
 
-	err = ovl_parse_opt((char *) data, &config);
-	if (err)
+	err = -ENOMEM;
+	ufs = kmalloc(sizeof(struct ovl_fs), GFP_KERNEL);
+	if (!ufs)
 		goto out;
 
+	err = ovl_parse_opt((char *) data, &ufs->config);
+	if (err)
+		goto out_free_ufs;
+
 	err = -EINVAL;
-	if (!config.upperdir || !config.lowerdir) {
+	if (!ufs->config.upperdir || !ufs->config.lowerdir) {
 		printk(KERN_ERR "overlayfs: missing upperdir or lowerdir\n");
 		goto out_free_config;
 	}
 
-	err = -ENOMEM;
-	ufs = kmalloc(sizeof(struct ovl_fs), GFP_KERNEL);
-	if (!ufs)
-		goto out_free_config;
-
 	oe = ovl_alloc_entry();
 	if (oe == NULL)
-		goto out_free_ufs;
+		goto out_free_config;
 
 	root_inode = ovl_new_inode(sb, S_IFDIR, oe);
 	if (!root_inode)
 		goto out_free_oe;
 
-	err = kern_path(config.upperdir, LOOKUP_FOLLOW, &upperpath);
+	err = kern_path(ufs->config.upperdir, LOOKUP_FOLLOW, &upperpath);
 	if (err)
 		goto out_put_root;
 
-	err = kern_path(config.lowerdir, LOOKUP_FOLLOW, &lowerpath);
+	err = kern_path(ufs->config.lowerdir, LOOKUP_FOLLOW, &lowerpath);
 	if (err)
 		goto out_put_upperpath;
 
@@ -566,11 +589,11 @@ out_put_root:
 	iput(root_inode);
 out_free_oe:
 	kfree(oe);
+out_free_config:
+	kfree(ufs->config.lowerdir);
+	kfree(ufs->config.upperdir);
 out_free_ufs:
 	kfree(ufs);
-out_free_config:
-	kfree(config.lowerdir);
-	kfree(config.upperdir);
 out:
 	return err;
 }

Re: [PATCH] overlayfs: implement show_options

[Miklos Szeredi]

Erez Zadok <ezk@fsl.cs.sunysb.edu> writes:

> Overlayfs: implement show_options
>
> This is useful because of the stacking nature of overlayfs.  Users like to
> find out (via /proc/mounts) which lower/upper directory were used at mount
> time.

This is indeed useful.  Thanks, applied.

Miklos