Re: [PATCH v2] vfs: prevent copy_file_range to copy across devices

[Steve French]

On Mon, Feb 15, 2021 at 10:11 AM Trond Myklebust
<trondmy@xxxxxxxxxxxxxxx> wrote:
>
> On Mon, 2021-02-15 at 15:43 +0000, Luis Henriques wrote:
> > Nicolas Boichat reported an issue when trying to use the
> > copy_file_range
> > syscall on a tracefs file.  It failed silently because the file
> > content is
> > generated on-the-fly (reporting a size of zero) and copy_file_range
> > needs
> > to know in advance how much data is present.
>
> That explanation makes no sense whatsoever. copy_file_range is a non-
> atomic operation and so the file can change while being copied. Any
> determination of 'how much data is present' that is made in advance
> would therefore be a flaw in the copy process being used (i.e.
> do_splice_direct()). Does sendfile() also 'issue' in the same way?

I agree that the explanation of the tracefs problem motivating this
patch doesn't make sense.


-- 
Thanks,

Steve