new helper: open_with_fake_path()

open a file by given inode, faking ->f_path.  Use with shitloads
of caution - at the very least you'd damn better make sure that
some dentry alias of that inode is pinned down by the path in
question.  Again, this is no general-purpose interface and I hope
it will eventually go away.  Right now overlayfs wants something
like that, but nothing else should.

Any out-of-tree code with bright idea of using this one *will*
eventually get hurt, with zero notice and great delight on my part.
I refuse to use EXPORT_SYMBOL_GPL(), especially in situations when
it's really EXPORT_SYMBOL_DONT_USE_IT(), but don't take that export
as "you are welcome to use it".

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/open.c b/fs/open.c
index ee89324..dd15711 100644 (file)
--- a/fs/open.c
+++ b/fs/open.c
@@ -925,6 +925,24 @@ struct file *dentry_open(const struct path *path, int flags,
 }
 EXPORT_SYMBOL(dentry_open);
 
+struct file *open_with_fake_path(const struct path *path, int flags,
+                               struct inode *inode, const struct cred *cred)
+{
+       struct file *f = alloc_empty_file(flags, cred);
+       if (!IS_ERR(f)) {
+               int error;
+
+               f->f_path = *path;
+               error = do_dentry_open(f, inode, NULL);
+               if (error) {
+                       fput(f);
+                       f = ERR_PTR(error);
+               }
+       }
+       return f;
+}
+EXPORT_SYMBOL(open_with_fake_path);
+
 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 05f3472..4ff7b70 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2424,6 +2424,8 @@ extern struct file *filp_open(const char *, int, umode_t);
 extern struct file *file_open_root(struct dentry *, struct vfsmount *,
                                   const char *, int, umode_t);
 extern struct file * dentry_open(const struct path *, int, const struct cred *);
+extern struct file * open_with_fake_path(const struct path *, int,
+                                        struct inode*, const struct cred *);
 static inline struct file *file_clone_open(struct file *file)
 {
        return dentry_open(&file->f_path, file->f_flags, file->f_cred);