Re: [Patch v2 1/3] fs: add filp_clone_open API

[Mateusz Guzik <mguzik-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Thu, Feb 25, 2016 at 11:36:48AM -0800, James Bottomley wrote:
> I need an API that allows me to obtain a clone of the current file
> pointer to pass in to an exec handler.  I've labelled this as an
> internal API because I can't see how it would be useful outside of the
> fs subsystem.  The use case will be a persistent binfmt_misc handler.
> 
> Signed-off-by: James Bottomley <James.Bottomley-d9PhHud1JfjCXq6kfMZ53/egYHeGw8Jk@public.gmane.org>
> ---
>  fs/internal.h |  1 +
>  fs/open.c     | 20 ++++++++++++++++++++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/fs/internal.h b/fs/internal.h
> index b71deee..c8ca0c9 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -108,6 +108,7 @@ extern long do_handle_open(int mountdirfd,
>  			   struct file_handle __user *ufh, int open_flag);
>  extern int open_check_o_direct(struct file *f);
>  extern int vfs_open(const struct path *, struct file *, const struct cred *);
> +extern struct file *filp_clone_open(struct file *);
>  
>  /*
>   * inode.c
> diff --git a/fs/open.c b/fs/open.c
> index 55bdc75..bb7ffd6 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -1004,6 +1004,26 @@ struct file *file_open_root(struct dentry *dentry, struct vfsmount *mnt,
>  }
>  EXPORT_SYMBOL(file_open_root);
>  
> +struct file *filp_clone_open(struct file *oldfile)
> +{
> +	struct file *file;
> +	int retval;
> +
> +	file = get_empty_filp();
> +	if (IS_ERR(file))
> +		return file;
> +
> +	file->f_flags = oldfile->f_flags;
> +	retval = vfs_open(&oldfile->f_path, file, oldfile->f_cred);
> +	if (retval) {
> +		fput(file);

You likely want put_file instead, although if I read it right this is
only cosmetics for consistency with dentry_open.

Also vast majority of the file uses 'error' instead of 'retval'. Any
reason to deviate here?

> +		return ERR_PTR(retval);
> +	}
> +
> +	return file;
> +}
> +EXPORT_SYMBOL(filp_clone_open);
> +
>  long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
>  {
>  	struct open_flags op;
> -- 
> 2.6.2
> 

-- 
Mateusz Guzik