slub/memcg: cure the brainless abuse of sysfs attributes

memcg_propagate_slab_attrs() abuses the sysfs attribute file functions
to propagate settings from the root kmem_cache to a newly created
kmem_cache.  It does that with:

     attr->show(root, buf);
     attr->store(new, buf, strlen(bug);

Aside of being a lazy and absurd hackery this is broken because it does
not check the return value of the show() function.

Some of the show() functions return 0 w/o touching the buffer.  That
means in such a case the store function is called with the stale content
of the previous show().  That causes nonsense like invoking
kmem_cache_shrink() on a newly created kmem_cache.  In the worst case it
would cause handing in an uninitialized buffer.

This should be rewritten proper by adding a propagate() callback to
those slub_attributes which must be propagated and avoid that insane
conversion to and from ASCII, but that's too large for a hot fix.

Check at least the return value of the show() function, so calling
store() with stale content is prevented.

Steven said:
 "It can cause a deadlock with get_online_cpus() that has been uncovered
  by recent cpu hotplug and lockdep changes that Thomas and Peter have
  been doing.

     Possible unsafe locking scenario:

           CPU0                    CPU1
           ----                    ----
      lock(cpu_hotplug.lock);
                                   lock(slab_mutex);
                                   lock(cpu_hotplug.lock);
      lock(slab_mutex);

     *** DEADLOCK ***"

Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1705201244540.2255@nanos
Signed-off-by: Thomas Gleixner <[email protected]>
Reported-by: Steven Rostedt <[email protected]>
Acked-by: David Rientjes <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/slub.c

@@ -5512,6 +5512,7 @@ static void memcg_propagate_slab_attrs(struct kmem_cache *s)
 		char mbuf[64];
 		char *buf;
 		struct slab_attribute *attr = to_slab_attr(slab_attrs[i]);
+		ssize_t len;
 
 		if (!attr || !attr->store || !attr->show)
 			continue;
@@ -5536,8 +5537,9 @@ static void memcg_propagate_slab_attrs(struct kmem_cache *s)
 			buf = buffer;
 		}
 
-		attr->show(root_cache, buf);
-		attr->store(s, buf, strlen(buf));
+		len = attr->show(root_cache, buf);
+		if (len > 0)
+			attr->store(s, buf, len);
 	}
 
 	if (buffer)