Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable

On Fri, Aug 27, 2021 at 09:48:55PM +0000, Al Viro wrote:

	[btrfs]search_ioctl()
Broken with memory poisoning, for either variant of semantics.  Same for
arm64 sub-page permission differences, I think.

So we have 3 callers where we want all-or-nothing semantics - two in
arch/x86/kernel/fpu/signal.c and one in btrfs.  HWPOISON will be a problem
for all 3, AFAICS...

IOW, it looks like we have two different things mixed here - one that wants
to try and fault stuff in, with callers caring only about having _something_
faulted in (most of the users) and one that wants to make sure we *can* do
stores or loads on each byte in the affected area.

Just accessing a byte in each page really won't suffice for the second kind.
Neither will g-u-p use, unless we teach it about HWPOISON and other fun
beasts...  Looks like we want that thing to be a separate primitive; for
btrfs I'd probably replace fault_in_pages_writeable() with clear_user()
as a quick fix for now...

Comments?

Wait a sec...  Wasn't HWPOISON a per-page thing?  arm64 definitely does have
smaller-than-page areas with different permissions, so btrfs search_ioctl()
has a problem there, but arch/x86/kernel/fpu/signal.c doesn't have to deal
with that...

Sigh...  I really need more coffee...