Re: [PATCH 01/11] vfs: copy_file_range source range over EOF should fail

Olga Kornievskaia
2018-12-04T21:47:18Z

On Tue, Dec 4, 2018 at 4:35 PM Dave Chinner <david@fromorbit.com> wrote:
>
> On Tue, Dec 04, 2018 at 07:13:32AM -0800, Christoph Hellwig wrote:
> > On Mon, Dec 03, 2018 at 02:46:20PM +0200, Amir Goldstein wrote:
> > > > From: Dave Chinner <dchinner@redhat.com>
> > > >
> > > > The man page says:
> > > >
> > > > EINVAL Requested range extends beyond the end of the source file
> > > >
> > > > But the current behaviour is that copy_file_range does a short
> > > > copy up to the source file EOF. Fix the kernel behaviour to match
> > > > the behaviour described in the man page.
> >
> > I think the behavior implemented is a lot more useful than the one
> > documented..
>
> The current behaviour is really nasty. Because copy_file_range() can
> return short copies, the caller has to implement a loop to ensure
> the range hey want get copied. When the source range you are
> trying to copy overlaps source EOF, this loop:
>
> while (len > 0) {
> ret = copy_file_range(... len ...)
> ...
> off_in += ret;
> off_out += ret;
> len -= ret;
> }
>
> Currently the fallback code copies up to the end of the source file
> on the first copy and then fails the second copy with EINVAL because
> the source range is now completely beyond EOF.
>
> So, from an application perspective, did the copy succeed or did it
> fail?
>
> Existing tools that exercise copy_file_range (like xfs_io) consider
> this a failure, because the second copy_file_range() call returns
> EINVAL and not some "there is no more to copy" marker like read()
> returning 0 bytes when attempting to read beyond EOF.
>
> IOWs, we cannot tell the difference between a real error and a short
> copy because the input range spans EOF and it was silently
> shortened. That's the API problem we need to fix here - the existing
> behaviour is really crappy for applications. Erroring out
> immmediately is one solution, and it's what the man page says should
> happen so that is what I implemented.
>
> Realistically, though, I think an attempt to read beyond EOF for the
> copy should result in behaviour like read() (i.e. return 0 bytes),
> not EINVAL. The existing behaviour needs to change, though.

There are two checks to consider
1. pos_in >= EOF should return EINVAL
2. however what's perhaps should be relaxed is pos_in+len >= EOF
should return a short copy.

Having check#1 enforced allows to us to differentiate between a real
error and a short copy.

>
> > > i_size_read()...
> > >
> > > Otherwise
> > > Reviewed-by: Amir Goldstein <amir73il@gmail.com>
> >
> > Looks like this doesn't even compile?
>
> It's fixed in a later patch that consolidates the checks into a
> generic check function, but I'm not sure why my "compile every
> patch" script didn't catch this.
>
> Cheers,
>
> -Dave.
> --
> Dave Chinner
> david@fromorbit.com