Re: [PATCH RESEND x3 v9 1/9] iov_iter: add copy_struct_from_iter()

[Omar Sandoval <osandov@osandov.com>]

On Fri, Jun 18, 2021 at 07:49:53PM +0000, Al Viro wrote:
> On Fri, Jun 18, 2021 at 07:42:41PM +0000, Al Viro wrote:
> 
> > Pipe ones are strictly destinations - they can't be sources.  So if you
> > see it called for one of those, you've a bug.
> > 
> > Xarray ones are *not* - they can be sources, and that's missing here.
> > 
> > Much more unpleasant, though, is that this thing has hard dependency on
> > nr_seg == 1 *AND* openly suggests the use of iov_iter_single_seg_count(),
> > which is completely wrong.  That sucker has some weird users left (as
> > of #work.iov_iter), but all of them are actually due to API deficiencies
> > and I very much hope to kill that thing off.
> > 
> > Why not simply add iov_iter_check_zeroes(), that would be called after
> > copy_from_iter() and verified that all that's left in the iterator
> > consists of zeroes?  Then this copy_struct_from_...() would be
> > trivial to express through those two.  And check_zeroes would also
> > be trivial, especially on top of #work.iov_iter.  With no calls of
> > iov_iter_advance() at all, while we are at it...
> > 
> > IDGI... Omar, what semantics do you really want from that primitive?
> 
> And for pity sake, let's not do that EXPORT_SYMBOL_GPL() posturing there.
> If it's a sane general-purpose API, it doesn't matter who uses it;
> if it's not, it shouldn't be exported in the first place.
> 
> It can be implemented via the already exported primitives, so it's
> not as if we prevented anyone from doing an equivalent...

Fair enough, I'll fix that.