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:

quoted

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.