linux-kernel - Re: [RFC PATCH 19/35] ovl: readd reflink/copyfile/dedup support

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Date:   Tue, 17 Apr 2018 23:31:44 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     overlayfs <linux-unionfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 19/35] ovl: readd reflink/copyfile/dedup support

On Thu, Apr 12, 2018 at 6:08 PM, Miklos Szeredi <mszeredi@...hat.com> wrote:
> Since set of arguments are so similar, handle in a common helper.
>
> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
> ---
>  fs/overlayfs/file.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 79 insertions(+)
>
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index 9670e160967e..39b1b73334ad 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -352,6 +352,81 @@ long ovl_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
>         return ret;
>  }
>
> +enum ovl_copyop {
> +       OVL_COPY,
> +       OVL_CLONE,
> +       OVL_DEDUPE,
> +};
> +
> +static ssize_t ovl_copyfile(struct file *file_in, loff_t pos_in,
> +                           struct file *file_out, loff_t pos_out,
> +                           u64 len, unsigned int flags, enum ovl_copyop op)
> +{
> +       struct inode *inode_out = file_inode(file_out);
> +       struct fd real_in, real_out;
> +       const struct cred *old_cred;
> +       int ret;
> +
> +       ret = ovl_real_file(file_out, &real_out);
> +       if (ret)
> +               return ret;
> +
> +       ret = ovl_real_file(file_in, &real_in);
> +       if (ret) {
> +               fdput(real_out);
> +               return ret;
> +       }
> +
> +       old_cred = ovl_override_creds(file_inode(file_out)->i_sb);
> +       switch (op) {
> +       case OVL_COPY:
> +               ret = vfs_copy_file_range(real_in.file, pos_in,
> +                                         real_out.file, pos_out, len, flags);

Problem:
vfs_copy_file_range(ovl_lower_file, ovl_upper_file) on non samefs
will get -EXDEV from  ovl_copy_file_range(), so will not fall back
to do_splice_direct().
We may be better off checking in_sb != out_sb and returning
-EOPNOTSUPP? not sure.


> +               break;
> +
> +       case OVL_CLONE:
> +               ret = vfs_clone_file_range(real_in.file, pos_in,
> +                                          real_out.file, pos_out, len);
> +               break;
> +
> +       case OVL_DEDUPE:
> +               ret = vfs_dedupe_file_range_one(real_in.file, pos_in, len,
> +                                               real_out.file, pos_out);

Problem:
real_out can be a readonly fd (for is_admin), so we will be deduping
the lower file.
I guess this problem is mitigated in current code by may_write_real().

How can we deal with that sort of "write leak" without patching
 mnt_want_write_file()?

Thanks,
Amir.