Subject: Re: [PATCH 02/10] fs: don't allow splice read/write without explicit ops

From: Christoph Hellwig <[email-masked]>
Date: Tue, 27 Oct 2020 10:54:55 +0100

On Tue, Oct 27, 2020 at 09:51:34AM +0000, David Howells wrote:
> David Howells <[email-masked]> wrote:
> 
> > > default_file_splice_write is the last piece of generic code that uses
> > > set_fs to make the uaccess routines operate on kernel pointers.  It
> > > implements a "fallback loop" for splicing from files that do not actually
> > > provide a proper splice_read method.  The usual file systems and other
> > > high bandwith instances all provide a ->splice_read, so this just removes
> > > support for various device drivers and procfs/debugfs files.  If splice
> > > support for any of those turns out to be important it can be added back
> > > by switching them to the iter ops and using generic_file_splice_read.
> > 
> > Hmmm...  this causes the copy_file_range() syscall to fail with EINVAL in some
> > places where before it used to work.
> > 
> > For my part, it causes the generic/112 xfstest to fail with afs, but there may
> > be other places.
> > 
> > Is this a regression we need to fix in the VFS core?  Or is it something we
> > need to fix in xfstests and assume userspace will fallback to doing it itself?
> 
> That said, for afs at least, the fix seems to be just this:

And that is the correct fix, I was about to send it to you.

We can't have a "generic" splice using ->read/->write without set_fs,
in addition to the iter_file_splice_write based version being a lot
more efficient than what you had before.