Re: [RFC][PATCH] fix short copy handling in copy_mc_pipe_to_iter()

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Jun 13, 2022 at 11:28:34PM +0100, Al Viro wrote:
> On Mon, Jun 13, 2022 at 10:54:36AM -0700, Linus Torvalds wrote:
> > On Sun, Jun 12, 2022 at 5:10 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> > >
> > > Unlike other copying operations on ITER_PIPE, copy_mc_to_iter() can
> > > result in a short copy.  In that case we need to trim the unused
> > > buffers, as well as the length of partially filled one - it's not
> > > enough to set ->head, ->iov_offset and ->count to reflect how
> > > much had we copied.  Not hard to fix, fortunately...
> > >
> > > I'd put a helper (pipe_discard_from(pipe, head)) into pipe_fs_i.h,
> > > rather than iov_iter.c -
> > 
> > Actually, since this "copy_mc_xyz()" stuff is going to be entirely
> > impossible to debug and replicate for any normal situation, I would
> > suggest we take the approach that we (long ago) used to take with
> > copy_from_user(): zero out the destination buffer, so that developers
> > that can't test the faulting behavior don't have to worry about it.
> > 
> > And then the existing code is fine: it will break out of the loop, but
> > it won't do the odd revert games and the "randomnoise.len -= rem"
> > thing that I can't wrap my head around.
> > 
> > Hmm?
> 
> Not really - we would need to zero the rest of those pages somehow.
> They are already allocated and linked into pipe; leaving them
> there (and subsequent ones hadn't seen any stores whatsoever - they
> are fresh out of alloc_page(GFP_USER)) is a non-starter.
> 
> We could do allocation as we go, but that's a much more intrusive
> change...

FWIW, I've got quite a bit of cleanups in the local tree; reordering and
cleaning that queue up at the moment, will post tonight or tomorrow.

I've looked into doing allocations page-by-page (instead of single
push_pipe(), followed by copying into those).  Doable, but it ends
up being much messier.

IMO this "truncate on failure" approach is saner.