[PATCH 2/2] fs/overlay: use same inodes for hardlinks

[Alexander Morozov <alexandr.morozov@docker.com>]

Fix problem with accessing hardlink to unix-socket.  unix_find_other
function from net/unix/af_unix.c uses d_backing_inode helper which in
current implementation doesn't return any "backing" inode, but just
dentry->d_inode, so, there from kern_path we have path with dentry from
overlayfs and not underlying file system. Further in code in
unix_find_socket_byinode we looking for unix-socket by inode, but due
overlayfs implementation that inode for hardlink will be different from
original inode of unix-socket.
Now ovl_link doesn't create new inode, but uses old->d_inode for
instantiating new dentry. Not use inc_nlink, because nlink seems to be
correctly propagated from underlying fs where it need.

Signed-off-by: Alexander Morozov <lk4d4@docker.com>
---
 fs/overlayfs/dir.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 487a157..a55c89b 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -164,8 +164,14 @@ static void ovl_dentry_update_instantiate(struct dentry *dentry,
 {
 	ovl_dentry_version_inc(dentry->d_parent);
 	ovl_dentry_update(dentry, newdentry);
-	ovl_copyattr(newdentry->d_inode, inode);
-	d_instantiate(dentry, inode);
+	/*
+	 * inode == NULL only in case of hardlink
+	 * for hardlink dentry will be instantiated in ovl_link
+	 */
+	if (inode) {
+		ovl_copyattr(newdentry->d_inode, inode);
+		d_instantiate(dentry, inode);
+	}
 }
 
 static int ovl_create_upper(struct dentry *dentry, struct inode *inode,
@@ -390,16 +396,19 @@ static int ovl_create_or_link(struct dentry *dentry, int mode, dev_t rdev,
 			      const char *link, struct dentry *hardlink)
 {
 	int err;
-	struct inode *inode;
+	struct inode *inode = NULL;
 	struct kstat stat = {
 		.mode = mode,
 		.rdev = rdev,
 	};
 
 	err = -ENOMEM;
-	inode = ovl_new_inode(dentry->d_sb, mode, dentry->d_fsdata);
-	if (!inode)
-		goto out;
+	/* We don't need inode for hardlink */
+	if (!hardlink) {
+		inode = ovl_new_inode(dentry->d_sb, mode, dentry->d_fsdata);
+		if (!inode)
+			goto out;
+	}
 
 	err = ovl_copy_up(dentry->d_parent);
 	if (err)
@@ -498,6 +507,10 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
 
 	upper = ovl_dentry_upper(old);
 	err = ovl_create_or_link(new, upper->d_inode->i_mode, 0, NULL, upper);
+	if (!err) {
+		ihold(old->d_inode);
+		d_instantiate(new, old->d_inode);
+	}
 
 out_drop_write:
 	ovl_drop_write(old);
-- 
2.6.1