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

[Linus Torvalds]

On Sun, Jun 12, 2022 at 5:10 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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?

                Linus