slub: force on no_hash_pointers when slub_debug is enabled

Obscuring the pointers that slub shows when debugging makes for some
confusing slub debug messages:

 Padding overwritten. 0x0000000079f0674a-0x000000000d4dce17

Those addresses are hashed for kernel security reasons.  If we're trying
to be secure with slub_debug on the commandline we have some big problems
given that we dump whole chunks of kernel memory to the kernel logs.
Let's force on the no_hash_pointers commandline flag when slub_debug is on
the commandline.  This makes slub debug messages more meaningful and if by
chance a kernel address is in some slub debug object dump we will have a
better chance of figuring out what went wrong.

Note that we don't use %px in the slub code because we want to reduce the
number of places that %px is used in the kernel.  This also nicely prints
a big fat warning at kernel boot if slub_debug is on the commandline so
that we know that this kernel shouldn't be used on production systems.

[akpm@linux-foundation.org: fix build with CONFIG_SLUB_DEBUG=n]

Link: https://lkml.kernel.org/r/20210601182202.3011020-5-swboyd@chromium.org
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Petr Mladek <pmladek@suse.com>
Cc: Joe Perches <joe@perches.com>
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>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kernel.h

@@ -357,6 +357,8 @@ int sscanf(const char *, const char *, ...);
 extern __scanf(2, 0)
 int vsscanf(const char *, const char *, va_list);
 
+extern int no_hash_pointers_enable(char *str);
+
 extern int get_option(char **str, int *pint);
 extern char *get_options(const char *str, int nints, int *ints);
 extern unsigned long long memparse(const char *ptr, char **retptr);