[PATCH] slub/memcg: Cure the brainless abuse of sysfs attributes

[Thomas Gleixner]

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.

Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx

---
 mm/slub.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5512,6 +5512,7 @@ static void memcg_propagate_slab_attrs(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(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)