overlayfs: Make f_path always point to the overlay and f_inode to the underlay

Make file->f_path always point to the overlay dentry so that the path in
/proc/pid/fd is correct and to ensure that label-based LSMs have access to the
overlay as well as the underlay (path-based LSMs probably don't need it).

Using my union testsuite to set things up, before the patch I see:

	[root@andromeda union-testsuite]# bash 5</mnt/a/foo107
	[root@andromeda union-testsuite]# ls -l /proc/$$/fd/
	...
	lr-x------. 1 root root 64 Jun  5 14:38 5 -> /a/foo107
	[root@andromeda union-testsuite]# stat /mnt/a/foo107
	...
	Device: 23h/35d Inode: 13381       Links: 1
	...
	[root@andromeda union-testsuite]# stat -L /proc/$$/fd/5
	...
	Device: 23h/35d Inode: 13381       Links: 1
	...

After the patch:

	[root@andromeda union-testsuite]# bash 5</mnt/a/foo107
	[root@andromeda union-testsuite]# ls -l /proc/$$/fd/
	...
	lr-x------. 1 root root 64 Jun  5 14:22 5 -> /mnt/a/foo107
	[root@andromeda union-testsuite]# stat /mnt/a/foo107
	...
	Device: 23h/35d Inode: 40346       Links: 1
	...
	[root@andromeda union-testsuite]# stat -L /proc/$$/fd/5
	...
	Device: 23h/35d Inode: 40346       Links: 1
	...

Note the change in where /proc/$$/fd/5 points to in the ls command.  It was
pointing to /a/foo107 (which doesn't exist) and now points to /mnt/a/foo107
(which is correct).

The inode accessed, however, is the lower layer.  The union layer is on device
25h/37d and the upper layer on 24h/36d.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 26 insertions, 23 deletions

diff --git a/fs/open.c b/fs/open.c
index e0250bdcc440..b1c5823b7f11 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -678,18 +678,18 @@ int open_check_o_direct(struct file *f)
 }
 
 static int do_dentry_open(struct file *f,
+			  struct inode *inode,
 			  int (*open)(struct inode *, struct file *),
 			  const struct cred *cred)
 {
 	static const struct file_operations empty_fops = {};
-	struct inode *inode;
 	int error;
 
 	f->f_mode = OPEN_FMODE(f->f_flags) | FMODE_LSEEK |
 				FMODE_PREAD | FMODE_PWRITE;
 
 	path_get(&f->f_path);
-	inode = f->f_inode = f->f_path.dentry->d_inode;
+	f->f_inode = inode;
 	f->f_mapping = inode->i_mapping;
 
 	if (unlikely(f->f_flags & O_PATH)) {
@@ -793,7 +793,8 @@ int finish_open(struct file *file, struct dentry *dentry,
 	BUG_ON(*opened & FILE_OPENED); /* once it's opened, it's opened */
 
 	file->f_path.dentry = dentry;
-	error = do_dentry_open(file, open, current_cred());
+	error = do_dentry_open(file, d_backing_inode(dentry), open,
+			       current_cred());
 	if (!error)
 		*opened |= FILE_OPENED;
 
@@ -822,6 +823,28 @@ int finish_no_open(struct file *file, struct dentry *dentry)
 }
 EXPORT_SYMBOL(finish_no_open);
 
+/**
+ * vfs_open - open the file at the given path
+ * @path: path to open
+ * @file: newly allocated file with f_flag initialized
+ * @cred: credentials to use
+ */
+int vfs_open(const struct path *path, struct file *file,
+	     const struct cred *cred)
+{
+	struct dentry *dentry = path->dentry;
+	struct inode *inode = dentry->d_inode;
+
+	file->f_path = *path;
+	if (dentry->d_flags & DCACHE_OP_SELECT_INODE) {
+		inode = dentry->d_op->d_select_inode(dentry, file->f_flags);
+		if (IS_ERR(inode))
+			return PTR_ERR(inode);
+	}
+
+	return do_dentry_open(file, inode, NULL, cred);
+}
+
 struct file *dentry_open(const struct path *path, int flags,
 			 const struct cred *cred)
 {
@@ -853,26 +876,6 @@ struct file *dentry_open(const struct path *path, int flags,
 }
 EXPORT_SYMBOL(dentry_open);
 
-/**
- * vfs_open - open the file at the given path
- * @path: path to open
- * @filp: newly allocated file with f_flag initialized
- * @cred: credentials to use
- */
-int vfs_open(const struct path *path, struct file *filp,
-	     const struct cred *cred)
-{
-	struct inode *inode = path->dentry->d_inode;
-
-	if (inode->i_op->dentry_open)
-		return inode->i_op->dentry_open(path->dentry, filp, cred);
-	else {
-		filp->f_path = *path;
-		return do_dentry_open(filp, NULL, cred);
-	}
-}
-EXPORT_SYMBOL(vfs_open);
-
 static inline int build_open_flags(int flags, umode_t mode, struct open_flags *op)
 {
 	int lookup_flags = 0;