[PATCH 1/1] backing-file: covert to using fops->splice_write

[Amir Goldstein]

On Fri, Jul 5, 2024 at 11:17 AM <ed.tsai at mediatek.com> wrote:
>
> From: Ed Tsai <ed.tsai at mediatek.com>
>
> Filesystems may define their own splice write. Therefore, use file
> fops instead of invoking iter_file_splice_write() directly.

This looks sane, but can you share the scenario where you ran into this?
or did you find this via code audit?

I can think of these cases:
1. overlayfs with fuse (virtiofs) upper
2. fuse passthrough over fuse
3. fuse passthrough over overlayfs
4. fuse passthrough over gfs2 or some out of tree fs

The first two will not cause any harm,
In case #3, according to the comment above ovl_splice_write()
the current code could even deadlock.

So do you have a reproduction?

Thanks,
Amir.

>
> Signed-off-by: Ed Tsai <ed.tsai at mediatek.com>
> ---
>  fs/backing-file.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/fs/backing-file.c b/fs/backing-file.c
> index 740185198db3..687a7fae7d25 100644
> --- a/fs/backing-file.c
> +++ b/fs/backing-file.c
> @@ -280,13 +280,16 @@ ssize_t backing_file_splice_write(struct pipe_inode_info *pipe,
>         if (WARN_ON_ONCE(!(out->f_mode & FMODE_BACKING)))
>                 return -EIO;
>
> +       if (out->f_op->splice_write)
> +               return -EINVAL;
> +
>         ret = file_remove_privs(ctx->user_file);
>         if (ret)
>                 return ret;
>
>         old_cred = override_creds(ctx->cred);
>         file_start_write(out);
> -       ret = iter_file_splice_write(pipe, out, ppos, len, flags);
> +       ret = out->f_op->splice_write(pipe, out, ppos, len, flags);
>         file_end_write(out);
>         revert_creds(old_cred);
>
> --
> 2.18.0
>