Re: [PATCH 02/11] vfs: introduce generic_copy_file_range()

[Amir Goldstein <amir73il@gmail.com>]

On Mon, Dec 3, 2018 at 10:34 AM Dave Chinner <david@fromorbit.com> wrote:
>
> From: Dave Chinner <dchinner@redhat.com>
>
> Right now if vfs_copy_file_range() does not use any offload
> mechanism, it falls back to calling do_splice_direct(). This fails
> to do basic sanity checks on the files being copied. Before we
> start adding this necessarily functionality to the fallback path,
> separate it out into generic_copy_file_range().
>
> generic_copy_file_range() has the same prototype as
> ->copy_file_range() so that filesystems can use it in their custom
> ->copy_file_range() method if they so choose.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

Looks good.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>

Question:
2 years ago you suggested that I covert the overlayfs copy up
code that does a do_direct_splice() with a loop of vfs_copy_file_range():
https://marc.info/?l=linux-fsdevel&m=147369468521525&w=2
We ended up with a slightly different solution, but with your recent
changes, I can get back to your original proposal.

Back then, I wondered whether it makes sense to push the killable
loop of shorter do_direct_splice() calls into the vfs helper.
What do you think about adding this to generic_copy_file_range()
now? (I can do that after your changes are merged).

The fact that userspace *can* enter a very long unkillable loop
with current copy_file_range() syscall doesn't mean that we
*should* persist this situation. After all, fixing the brokenness
of the existing interface is what you set out to do.

With that change in place, overlayfs could call only
vfs_copy_file_range() as you suggested and not as a fallback to
do_clone_file_range().

Thanks,
Amir.

>  fs/read_write.c    | 35 ++++++++++++++++++++++++++++++++---
>  include/linux/fs.h |  3 +++
>  2 files changed, 35 insertions(+), 3 deletions(-)
>
> diff --git a/fs/read_write.c b/fs/read_write.c
> index 09d1816cf3cf..50114694c98b 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1540,6 +1540,36 @@ COMPAT_SYSCALL_DEFINE4(sendfile64, int, out_fd, int, in_fd,
>  }
>  #endif
>
> +/**
> + * generic_copy_file_range - copy data between two files
> + * @file_in:   file structure to read from
> + * @pos_in:    file offset to read from
> + * @file_out:  file structure to write data to
> + * @pos_out:   file offset to write data to
> + * @len:       amount of data to copy
> + * @flags:     copy flags
> + *
> + * This is a generic filesystem helper to copy data from one file to another.
> + * It has no constraints on the source or destination file owners - the files
> + * can belong to different superblocks and different filesystem types. Short
> + * copies are allowed.
> + *
> + * This should be called from the @file_out filesystem, as per the
> + * ->copy_file_range() method.
> + *
> + * Returns the number of bytes copied or a negative error indicating the
> + * failure.
> + */
> +
> +ssize_t generic_copy_file_range(struct file *file_in, loff_t pos_in,
> +                           struct file *file_out, loff_t pos_out,
> +                           size_t len, unsigned int flags)
> +{
> +       return do_splice_direct(file_in, &pos_in, file_out, &pos_out,
> +                       len > MAX_RW_COUNT ? MAX_RW_COUNT : len, 0);
> +}
> +EXPORT_SYMBOL(generic_copy_file_range);
> +
>  /*
>   * copy_file_range() differs from regular file read and write in that it
>   * specifically allows return partial success.  When it does so is up to
> @@ -1611,9 +1641,8 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
>                         goto done;
>         }
>
> -       ret = do_splice_direct(file_in, &pos_in, file_out, &pos_out,
> -                       len > MAX_RW_COUNT ? MAX_RW_COUNT : len, 0);
> -
> +       ret = generic_copy_file_range(file_in, &pos_in, file_out, &pos_out,
> +                                       len, flags);
>  done:
>         if (ret > 0) {
>                 fsnotify_access(file_in);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index c95c0807471f..a4478764cf63 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1874,6 +1874,9 @@ extern ssize_t vfs_readv(struct file *, const struct iovec __user *,
>                 unsigned long, loff_t *, rwf_t);
>  extern ssize_t vfs_copy_file_range(struct file *, loff_t , struct file *,
>                                    loff_t, size_t, unsigned int);
> +extern ssize_t generic_copy_file_range(struct file *file_in, loff_t pos_in,
> +                               struct file *file_out, loff_t pos_out,
> +                               size_t len, unsigned int flags);
>  extern int generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
>                                          struct file *file_out, loff_t pos_out,
>                                          loff_t *count,
> --
> 2.19.1
>