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.

[[email protected]: fix build with CONFIG_SLUB_DEBUG=n]

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Stephen Boyd <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Petr Mladek <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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);

lib/vsprintf.c

@@ -2186,7 +2186,7 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
 bool no_hash_pointers __ro_after_init;
 EXPORT_SYMBOL_GPL(no_hash_pointers);
 
-static int __init no_hash_pointers_enable(char *str)
+int __init no_hash_pointers_enable(char *str)
 {
 	if (no_hash_pointers)
 		return 0;

mm/slub.c

@@ -118,12 +118,26 @@
  */
 
 #ifdef CONFIG_SLUB_DEBUG
+
 #ifdef CONFIG_SLUB_DEBUG_ON
 DEFINE_STATIC_KEY_TRUE(slub_debug_enabled);
 #else
 DEFINE_STATIC_KEY_FALSE(slub_debug_enabled);
 #endif
-#endif
+
+static inline bool __slub_debug_enabled(void)
+{
+	return static_branch_unlikely(&slub_debug_enabled);
+}
+
+#else		/* CONFIG_SLUB_DEBUG */
+
+static inline bool __slub_debug_enabled(void)
+{
+	return false;
+}
+
+#endif		/* CONFIG_SLUB_DEBUG */
 
 static inline bool kmem_cache_debug(struct kmem_cache *s)
 {
@@ -4487,6 +4501,10 @@ void __init kmem_cache_init(void)
 	if (debug_guardpage_minorder())
 		slub_max_order = 0;
 
+	/* Print slub debugging pointers without hashing */
+	if (__slub_debug_enabled())
+		no_hash_pointers_enable(NULL);
+
 	kmem_cache_node = &boot_kmem_cache_node;
 	kmem_cache = &boot_kmem_cache;