Re: [PATCH V2 2/2] mm/highmem: Lift memcpy_[to|from]_page to core

[Matthew Wilcox]

On Mon, Dec 07, 2020 at 03:34:44PM -0800, Dan Williams wrote:
> On Mon, Dec 7, 2020 at 3:27 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Mon, Dec 07, 2020 at 02:57:03PM -0800, ira.weiny@xxxxxxxxx wrote:
> > > +static inline void memcpy_page(struct page *dst_page, size_t dst_off,
> > > +                            struct page *src_page, size_t src_off,
> > > +                            size_t len)
> > > +{
> > > +     char *dst = kmap_local_page(dst_page);
> > > +     char *src = kmap_local_page(src_page);
> >
> > I appreciate you've only moved these, but please add:
> >
> >         BUG_ON(dst_off + len > PAGE_SIZE || src_off + len > PAGE_SIZE);
> 
> I imagine it's not outside the realm of possibility that some driver
> on CONFIG_HIGHMEM=n is violating this assumption and getting away with
> it because kmap_atomic() of contiguous pages "just works (TM)".
> Shouldn't this WARN rather than BUG so that the user can report the
> buggy driver and not have a dead system?

As opposed to (on a HIGHMEM=y system) silently corrupting data that
is on the next page of memory?  I suppose ideally ...

	if (WARN_ON(dst_off + len > PAGE_SIZE))
		len = PAGE_SIZE - dst_off;
	if (WARN_ON(src_off + len > PAGE_SIZE))
		len = PAGE_SIZE - src_off;

and then we just truncate the data of the offending caller instead of
corrupting innocent data that happens to be adjacent.  Although that's
not ideal either ... I dunno, what's the least bad poison to drink here?