mm/slub: fix incorrect interpretation of s->offset

mm/slub: fix incorrect interpretation of s->offset

In a couple of places in the slub memory allocator, the code uses
"s->offset" as a check to see if the free pointer is put right after the
object.  That check is no longer true with commit 36cdf4a3eca7 ("slub:
relocate freelist pointer to middle of object").

As a result, echoing "1" into the validate sysfs file, e.g.  of dentry,
may cause a bunch of "Freepointer corrupt" error reports like the
following to appear with the system in panic afterwards.

  =============================================================================
  BUG dentry(666:pmcd.service) (Tainted: G    B): Freepointer corrupt
  -----------------------------------------------------------------------------

To fix it, use the check "s->offset == s->inuse" in the new helper
function freeptr_outside_object() instead.  Also add another helper
function get_info_end() to return the end of info block (inuse + free
pointer if not overlapping with object).

Fixes: 36cdf4a3eca7 ("slub: relocate freelist pointer to middle of object")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Rafael Aquini <aquini@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Vitaly Nikolenko <vnik@duasynt.com>
Cc: Silvio Cesare <silvio.cesare@gmail.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Markus Elfring <Markus.Elfring@web.de>
Cc: Changbin Du <changbin.du@gmail.com>
Link: http://lkml.kernel.org/r/20200429135328.26976-1-longman@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>