Re: [RFC PATCH 2/2] ovl: enhance write permission check for writable open

[Miklos Szeredi <miklos@szeredi.hu>]

On Sat, 24 Apr 2021 at 16:04, Chengguang Xu <cgxu519@mykernel.net> wrote:
>
> Check upper file's write permission when open on writable mode.

This should already be done in ovl_open() -> ovl_open_realfile() ->
open_with_fake_path() -> do_dentry_open().

Do you have a test case indicating that the writecount test is not working?

Thanks,
Miklos

>
> NOTE: lower files may be shared between differnt overlayfs instances,
> so we skip the check of lower file to avoid introducing interferes.
>
> Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
> ---
>  fs/overlayfs/file.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index 6e454a294046..1c3c24d07d01 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -144,12 +144,18 @@ static int ovl_real_fdget(const struct file *file, struct fd *real)
>  static int ovl_open(struct inode *inode, struct file *file)
>  {
>         struct file *realfile;
> +       struct inode *upperinode;
>         int err;
>
>         err = ovl_maybe_copy_up(file_dentry(file), file->f_flags);
>         if (err)
>                 return err;
>
> +       upperinode = ovl_inode_upper(inode);
> +       if (((file->f_mode & FMODE_WRITE) || file->f_flags & O_TRUNC) &&
> +           (upperinode && atomic_read(&upperinode->i_writecount) < 0))
> +               return -ETXTBSY;
> +
>         /* No longer need these flags, so don't pass them on to underlying fs */
>         file->f_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);
>
> --
> 2.27.0
>
>