ovl: cleanup orphan index entries

index entry should live only as long as there are upper or lower
hardlinks.

Cleanup orphan index entries on mount and when dropping the last
overlay inode nlink.

When about to cleanup or link up to orphan index and the index inode
nlink > 1, admit that something went wrong and adjust overlay nlink
to index inode nlink - 1 to prevent it from dropping below zero.
This could happen when adding lower hardlinks underneath a mounted
overlay and then trying to unlink them.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/inode.c

@@ -510,7 +510,7 @@ int ovl_set_nlink_lower(struct dentry *dentry)
 	return ovl_set_nlink_common(dentry, ovl_dentry_lower(dentry), "L%+i");
 }
 
-static unsigned int ovl_get_nlink(struct dentry *lowerdentry,
+unsigned int ovl_get_nlink(struct dentry *lowerdentry,
 			   struct dentry *upperdentry,
 			   unsigned int fallback)
 {

fs/overlayfs/namei.c

@@ -425,6 +425,11 @@ int ovl_verify_index(struct dentry *index, struct path *lowerstack,
 	if (err)
 		goto fail;
 
+	/* Check if index is orphan and don't warn before cleaning it */
+	if (d_inode(index)->i_nlink == 1 &&
+	    ovl_get_nlink(index, origin.dentry, 0) == 0)
+		err = -ENOENT;
+
 	dput(origin.dentry);
 out:
 	kfree(fh);

fs/overlayfs/overlayfs.h

@@ -263,6 +263,9 @@ int ovl_indexdir_cleanup(struct dentry *dentry, struct vfsmount *mnt,
 /* inode.c */
 int ovl_set_nlink_upper(struct dentry *dentry);
 int ovl_set_nlink_lower(struct dentry *dentry);
+unsigned int ovl_get_nlink(struct dentry *lowerdentry,
+			   struct dentry *upperdentry,
+			   unsigned int fallback);
 int ovl_setattr(struct dentry *dentry, struct iattr *attr);
 int ovl_getattr(const struct path *path, struct kstat *stat,
 		u32 request_mask, unsigned int flags);

fs/overlayfs/super.c

@@ -1069,7 +1069,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 			if (err)
 				pr_err("overlayfs: failed to verify index dir origin\n");
 
-			/* Cleanup bad/stale index entries */
+			/* Cleanup bad/stale/orphan index entries */
 			if (!err)
 				err = ovl_indexdir_cleanup(ufs->indexdir,
 							   ufs->upper_mnt,

fs/overlayfs/util.c

@@ -14,6 +14,8 @@
 #include <linux/xattr.h>
 #include <linux/exportfs.h>
 #include <linux/uuid.h>
+#include <linux/namei.h>
+#include <linux/ratelimit.h>
 #include "overlayfs.h"
 #include "ovl_entry.h"
 
@@ -411,6 +413,58 @@ void ovl_inuse_unlock(struct dentry *dentry)
 	}
 }
 
+/* Called must hold OVL_I(inode)->oi_lock */
+static void ovl_cleanup_index(struct dentry *dentry)
+{
+	struct inode *dir = ovl_indexdir(dentry->d_sb)->d_inode;
+	struct dentry *lowerdentry = ovl_dentry_lower(dentry);
+	struct dentry *upperdentry = ovl_dentry_upper(dentry);
+	struct dentry *index = NULL;
+	struct inode *inode;
+	struct qstr name;
+	int err;
+
+	err = ovl_get_index_name(lowerdentry, &name);
+	if (err)
+		goto fail;
+
+	inode = d_inode(upperdentry);
+	if (inode->i_nlink != 1) {
+		pr_warn_ratelimited("overlayfs: cleanup linked index (%pd2, ino=%lu, nlink=%u)\n",
+				    upperdentry, inode->i_ino, inode->i_nlink);
+		/*
+		 * We either have a bug with persistent union nlink or a lower
+		 * hardlink was added while overlay is mounted. Adding a lower
+		 * hardlink and then unlinking all overlay hardlinks would drop
+		 * overlay nlink to zero before all upper inodes are unlinked.
+		 * As a safety measure, when that situation is detected, set
+		 * the overlay nlink to the index inode nlink minus one for the
+		 * index entry itself.
+		 */
+		set_nlink(d_inode(dentry), inode->i_nlink - 1);
+		ovl_set_nlink_upper(dentry);
+		goto out;
+	}
+
+	inode_lock_nested(dir, I_MUTEX_PARENT);
+	/* TODO: whiteout instead of cleanup to block future open by handle */
+	index = lookup_one_len(name.name, ovl_indexdir(dentry->d_sb), name.len);
+	err = PTR_ERR(index);
+	if (!IS_ERR(index))
+		err = ovl_cleanup(dir, index);
+	inode_unlock(dir);
+	if (err)
+		goto fail;
+
+out:
+	dput(index);
+	return;
+
+fail:
+	pr_err("overlayfs: cleanup index of '%pd2' failed (%i)\n", dentry, err);
+	goto out;
+}
+
 /*
  * Operations that change overlay inode and upper inode nlink need to be
  * synchronized with copy up for persistent nlink accounting.
@@ -473,6 +527,16 @@ int ovl_nlink_start(struct dentry *dentry, bool *locked)
 
 void ovl_nlink_end(struct dentry *dentry, bool locked)
 {
-	if (locked)
+	if (locked) {
+		if (ovl_test_flag(OVL_INDEX, d_inode(dentry)) &&
+		    d_inode(dentry)->i_nlink == 0) {
+			const struct cred *old_cred;
+
+			old_cred = ovl_override_creds(dentry->d_sb);
+			ovl_cleanup_index(dentry);
+			revert_creds(old_cred);
+		}
+
 		mutex_unlock(&OVL_I(d_inode(dentry))->lock);
+	}
 }