ovl: set i_ino to the value of st_ino for NFS export

Eddie Horng reported that readdir of an overlayfs directory that
was exported via NFSv3 returns entries with d_type set to DT_UNKNOWN.
The reason is that while preparing the response for readdirplus, nfsd
checks inside encode_entryplus_baggage() that a child dentry's inode
number matches the value of d_ino returns by overlayfs readdir iterator.

Because the overlayfs inodes use arbitrary inode numbers that are not
correlated with the values of st_ino/d_ino, NFSv3 falls back to not
encoding d_type. Although this is an allowed behavior, we can fix it for
the case of all overlayfs layers on the same underlying filesystem.

When NFS export is enabled and d_ino is consistent with st_ino
(samefs), set the same value also to i_ino in ovl_fill_inode() for all
overlayfs inodes, nfsd readdirplus sanity checks will pass.
ovl_fill_inode() may be called from ovl_new_inode(), before real inode
was created with ino arg 0. In that case, i_ino will be updated to real
upper inode i_ino on ovl_inode_init() or ovl_inode_update().
Reported-by: Eddie Horng <eddiehorng.tw@gmail.com>
Tested-by: Eddie Horng <eddiehorng.tw@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Fixes: 8383f174 ("ovl: wire up NFS export operations")
Cc: <stable@vger.kernel.org> #v4.16
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/inode.c

@@ -459,8 +459,19 @@ static inline void ovl_lockdep_annotate_inode_mutex_key(struct inode *inode)
 #endif
 }
 
-static void ovl_fill_inode(struct inode *inode, umode_t mode, dev_t rdev)
+static void ovl_fill_inode(struct inode *inode, umode_t mode, dev_t rdev,
+			   unsigned long ino)
 {
+	/*
+	 * When NFS export is enabled and d_ino is consistent with st_ino
+	 * (samefs), set the same value to i_ino, because nfsd readdirplus
+	 * compares d_ino values to i_ino values of child entries. When called
+	 * from ovl_new_inode(), ino arg is 0, so i_ino will be updated to real
+	 * upper inode i_ino on ovl_inode_init() or ovl_inode_update().
+	 */
+	if (inode->i_sb->s_export_op && ovl_same_sb(inode->i_sb))
+		inode->i_ino = ino;
+	else
 		inode->i_ino = get_next_ino();
 	inode->i_mode = mode;
 	inode->i_flags |= S_NOCMTIME;
@@ -597,7 +608,7 @@ struct inode *ovl_new_inode(struct super_block *sb, umode_t mode, dev_t rdev)
 
 	inode = new_inode(sb);
 	if (inode)
-		ovl_fill_inode(inode, mode, rdev);
+		ovl_fill_inode(inode, mode, rdev, 0);
 
 	return inode;
 }
@@ -710,6 +721,7 @@ struct inode *ovl_get_inode(struct super_block *sb, struct dentry *upperdentry,
 	struct inode *inode;
 	bool bylower = ovl_hash_bylower(sb, upperdentry, lowerdentry, index);
 	bool is_dir;
+	unsigned long ino = 0;
 
 	if (!realinode)
 		realinode = d_inode(lowerdentry);
@@ -748,13 +760,14 @@ struct inode *ovl_get_inode(struct super_block *sb, struct dentry *upperdentry,
 		if (!is_dir)
 			nlink = ovl_get_nlink(lowerdentry, upperdentry, nlink);
 		set_nlink(inode, nlink);
+		ino = key->i_ino;
 	} else {
 		/* Lower hardlink that will be broken on copy up */
 		inode = new_inode(sb);
 		if (!inode)
 			goto out_nomem;
 	}
-	ovl_fill_inode(inode, realinode->i_mode, realinode->i_rdev);
+	ovl_fill_inode(inode, realinode->i_mode, realinode->i_rdev, ino);
 	ovl_inode_init(inode, upperdentry, lowerdentry);
 
 	if (upperdentry && ovl_is_impuredir(upperdentry))

fs/overlayfs/util.c

@@ -279,12 +279,16 @@ void ovl_dentry_set_redirect(struct dentry *dentry, const char *redirect)
 void ovl_inode_init(struct inode *inode, struct dentry *upperdentry,
 		    struct dentry *lowerdentry)
 {
+	struct inode *realinode = d_inode(upperdentry ?: lowerdentry);
+
 	if (upperdentry)
 		OVL_I(inode)->__upperdentry = upperdentry;
 	if (lowerdentry)
 		OVL_I(inode)->lower = igrab(d_inode(lowerdentry));
 
-	ovl_copyattr(d_inode(upperdentry ?: lowerdentry), inode);
+	ovl_copyattr(realinode, inode);
+	if (!inode->i_ino)
+		inode->i_ino = realinode->i_ino;
 }
 
 void ovl_inode_update(struct inode *inode, struct dentry *upperdentry)
@@ -299,6 +303,8 @@ void ovl_inode_update(struct inode *inode, struct dentry *upperdentry)
 	smp_wmb();
 	OVL_I(inode)->__upperdentry = upperdentry;
 	if (inode_unhashed(inode)) {
+		if (!inode->i_ino)
+			inode->i_ino = upperinode->i_ino;
 		inode->i_private = upperinode;
 		__insert_inode_hash(inode, (unsigned long) upperinode);
 	}