Re: [PATCH] nfsd: Fix error return values for nfsd4_clone_file_range()

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Jan 25, 2019 at 12:50:09AM -0500, Trond Myklebust wrote:
> On Thu, 2019-01-24 at 19:46 -0500, J. Bruce Fields wrote:
> > On Mon, Jan 21, 2019 at 03:58:38PM -0500, Trond Myklebust wrote:
> > > If the parameter 'count' is non-zero, nfsd4_clone_file_range() will
> > > currently clobber all errors returned by vfs_clone_file_range() and
> > > replace them with EINVAL.
> > 
> > Oops, thanks for the fix.  I'm still a little confused, though:
...
> > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> > > index 9824e32b2f23..7dc98e14655d 100644
> > > --- a/fs/nfsd/vfs.c
> > > +++ b/fs/nfsd/vfs.c
> > > @@ -557,9 +557,11 @@ __be32 nfsd4_clone_file_range(struct file
> > > *src, u64 src_pos, struct file *dst,
> > >  	loff_t cloned;
> > >  
> > >  	cloned = vfs_clone_file_range(src, src_pos, dst, dst_pos,
> > > count, 0);
> > > +	if (cloned < 0)
> > > +		return nfserrno(cloned);
> > >  	if (count && cloned != count)
> > > -		cloned = -EINVAL;
> > > -	return nfserrno(cloned < 0 ? cloned : 0);
> > > +		return nfserrno(-EINVAL);
> > > +	return 0;
> > 
> > I still don't understand the cloned != count case.  I thought clone
> > was
> > supposed to be all-or-nothing and atomic, can it really return a
> > short
> > copy?  And how is that inval, shouldn't that be serverfault?
> 
> That, quite frankly, seems like more of a question for Darrick, not me.
> I haven't changed that part of the code.
> 
> The main thing I care about is being able to correctly report
> EOPNOTSUPP errors for the vast majority of filesystems that don't
> support clone() or dedup().

Makes sense, and I'm happy just to apply this and then sort out the rest in a
subsequent patch, but I'd really like to understand; Darrick?:

ioctl_file_clone also converts short copies to EINVAL:

	if (cloned < 0)
                ret = cloned;
        else if (olen && cloned != olen)
                ret = -EINVAL;
        else
                ret = 0;

Maybe that happens iff we hit EOF in the short file?

Does that mean we can successfully copy up to EOF and then return -EINVAL?
That sounds wrong.

There's a man page (IOCTL-FICLONERANGE(2)) but it doesn't cover this case.

--b.