locks: fix file locking on overlayfs

This patch allows flock, posix locks, ofd locks and leases to work
correctly on overlayfs.

Instead of using the underlying inode for storing lock context use the
overlay inode.  This allows locks to be persistent across copy-up.

This is done by introducing locks_inode() helper and using it instead of
file_inode() to get the inode in locking code.  For non-overlayfs the two
are equivalent, except for an extra pointer dereference in locks_inode().

Since lock operations are in "struct file_operations" we must also make
sure not to call underlying filesystem's lock operations.  Introcude a
super block flag MS_NOREMOTELOCK to this effect.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Acked-by: Jeff Layton <jlayton@poochiereds.net>
Cc: "J. Bruce Fields" <bfields@fieldses.org>

1 files changed, 14 insertions, 2 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7db097d673a8..8ee0f011547f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1065,6 +1065,18 @@ struct file_lock_context {
 
 extern void send_sigio(struct fown_struct *fown, int fd, int band);
 
+/*
+ * Return the inode to use for locking
+ *
+ * For overlayfs this should be the overlay inode, not the real inode returned
+ * by file_inode().  For any other fs file_inode(filp) and locks_inode(filp) are
+ * equal.
+ */
+static inline struct inode *locks_inode(const struct file *f)
+{
+        return f->f_path.dentry->d_inode;
+}
+
 #ifdef CONFIG_FILE_LOCKING
 extern int fcntl_getlk(struct file *, unsigned int, struct flock __user *);
 extern int fcntl_setlk(unsigned int, struct file *, unsigned int,
@@ -1252,7 +1264,7 @@ static inline struct dentry *file_dentry(const struct file *file)
 
 static inline int locks_lock_file_wait(struct file *filp, struct file_lock *fl)
 {
-        return locks_lock_inode_wait(file_inode(filp), fl);
+        return locks_lock_inode_wait(locks_inode(filp), fl);
 }
 
 struct fasync_struct {
@@ -2155,7 +2167,7 @@ static inline int mandatory_lock(struct inode *ino)
 
 static inline int locks_verify_locked(struct file *file)
 {
-        if (mandatory_lock(file_inode(file)))
+        if (mandatory_lock(locks_inode(file)))
                 return locks_mandatory_locked(file);
         return 0;
 }