vfs: add i_op->dentry_open()

Add a new inode operation i_op->dentry_open().  This is for stacked filesystems
that want to return a struct file from a different filesystem.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

5 files changed, 40 insertions, 5 deletions

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 94d93b1f8b53..b30753cbf431 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -67,6 +67,7 @@ prototypes:
                                 struct file *, unsigned open_flag,
                                 umode_t create_mode, int *opened);
         int (*tmpfile) (struct inode *, struct dentry *, umode_t);
+        int (*dentry_open)(struct dentry *, struct file *, const struct cred *);
 
 locking rules:
         all may block
@@ -96,6 +97,7 @@ fiemap:		no
 update_time:    no
 atomic_open:    yes
 tmpfile:        no
+dentry_open:    no
 
         Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on
 victim.
diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index fceff7c00a3c..20bf204426ca 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -364,6 +364,7 @@ struct inode_operations {
         int (*atomic_open)(struct inode *, struct dentry *, struct file *,
                         unsigned open_flag, umode_t create_mode, int *opened);
         int (*tmpfile) (struct inode *, struct dentry *, umode_t);
+        int (*dentry_open)(struct dentry *, struct file *, const struct cred *);
 };
 
 Again, all methods are called without any locks being held, unless
@@ -696,6 +697,12 @@ struct address_space_operations {
         but instead uses bmap to find out where the blocks in the file
         are and uses those addresses directly.
 
+  dentry_open: *WARNING: probably going away soon, do not use!* This is an
+        alternative to f_op->open(), the difference is that this method may open
+        a file not necessarily originating from the same filesystem as the one
+        i_op->open() was called on.  It may be useful for stacking filesystems
+        which want to allow native I/O directly on underlying files.
+
 
   invalidatepage: If a page has PagePrivate set, then invalidatepage
         will be called when part or all of the page is to be removed
diff --git a/fs/namei.c b/fs/namei.c
index 43927d14db67..75306b3c9526 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3064,9 +3064,12 @@ finish_open_created:
         error = may_open(&nd->path, acc_mode, open_flag);
         if (error)
                 goto out;
-        file->f_path.mnt = nd->path.mnt;
-        error = finish_open(file, nd->path.dentry, NULL, opened);
-        if (error) {
+
+        BUG_ON(*opened & FILE_OPENED); /* once it's opened, it's opened */
+        error = vfs_open(&nd->path, file, current_cred());
+        if (!error) {
+                *opened |= FILE_OPENED;
+        } else {
                 if (error == -EOPENSTALE)
                         goto stale_open;
                 goto out;
diff --git a/fs/open.c b/fs/open.c
index d6fd3acde134..de92c13b58be 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -823,8 +823,7 @@ struct file *dentry_open(const struct path *path, int flags,
         f = get_empty_filp();
         if (!IS_ERR(f)) {
                 f->f_flags = flags;
-                f->f_path = *path;
-                error = do_dentry_open(f, NULL, cred);
+                error = vfs_open(path, f, cred);
                 if (!error) {
                         /* from now on we need fput() to dispose of f */
                         error = open_check_o_direct(f);
@@ -841,6 +840,26 @@ 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;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index a957d4366c24..5cf7f6759679 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1528,6 +1528,9 @@ struct inode_operations {
                            umode_t create_mode, int *opened);
         int (*tmpfile) (struct inode *, struct dentry *, umode_t);
         int (*set_acl)(struct inode *, struct posix_acl *, int);
+
+        /* WARNING: probably going away soon, do not use! */
+        int (*dentry_open)(struct dentry *, struct file *, const struct cred *);
 } ____cacheline_aligned;
 
 ssize_t rw_copy_check_uvector(int type, const struct iovec __user * uvector,
@@ -2040,6 +2043,7 @@ extern struct file *file_open_name(struct filename *, int, umode_t);
 extern struct file *filp_open(const char *, int, umode_t);
 extern struct file *file_open_root(struct dentry *, struct vfsmount *,
                                    const char *, int);
+extern int vfs_open(const struct path *, struct file *, const struct cred *);
 extern struct file * dentry_open(const struct path *, int, const struct cred *);
 extern int filp_close(struct file *, fl_owner_t id);