mm, slub: restore initial kmem_cache flags

The routine that applies debug flags to the kmem_cache slabs
inadvertantly prevents non-debug flags from being applied to those
same objects.  That is, if slub_debug=<flag>,<slab> is specified,
non-debugged slabs will end up having flags of zero, and the slabs
may be unusable.

Fix this by including the input flags for non-matching slabs with the
contents of slub_debug, so that the caches are created as expected
alongside any debugging options that may be requested.  With this, we
can remove the check for a NULL slub_debug_string, since it's covered
by the loop itself.

Fixes: e17f1df ("mm, slub: extend slub_debug syntax for multiple blocks")
Signed-off-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Kees Cook <keescook@chromium.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Link: https://lkml.kernel.org/r/20200930161931.28575-1-farman@linux.ibm.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -1413,10 +1413,6 @@ slab_flags_t kmem_cache_flags(unsigned int object_size,
 	char *next_block;
 	slab_flags_t block_flags;
 
-	/* If slub_debug = 0, it folds into the if conditional. */
-	if (!slub_debug_string)
-		return flags | slub_debug;
-
 	len = strlen(name);
 	next_block = slub_debug_string;
 	/* Go through all blocks of debug options, see if any matches our slab's name */
@@ -1450,7 +1446,7 @@ slab_flags_t kmem_cache_flags(unsigned int object_size,
 		}
 	}
 
-	return slub_debug;
+	return flags | slub_debug;
 }
 #else /* !CONFIG_SLUB_DEBUG */
 static inline void setup_object_debug(struct kmem_cache *s,