mm, slub: introduce kmem_cache_debug_flags()

There are few places that call kmem_cache_debug(s) (which tests if any of
debug flags are enabled for a cache) immediately followed by a test for a
specific flag.  The compiler can probably eliminate the extra check, but
we can make the code nicer by introducing kmem_cache_debug_flags() that
works like kmem_cache_debug() (including the static key check) but tests
for specific flag(s).  The next patches will add more users.

[vbabka@suse.cz: change return from int to bool, per Kees.  Add VM_WARN_ON_ONCE() for invalid flags, per Roman]
  Link: http://lkml.kernel.org/r/949b90ed-e0f0-07d7-4d21-e30ec0958a7c@suse.cz

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Christoph Lameter <cl@linux.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Jann Horn <jannh@google.com>
Cc: Vijayanand Jitta <vjitta@codeaurora.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/20200610163135.17364-8-vbabka@suse.cz
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -122,18 +122,29 @@ DEFINE_STATIC_KEY_FALSE(slub_debug_enabled);
 #endif
 #endif
 
-static inline int kmem_cache_debug(struct kmem_cache *s)
+/*
+ * Returns true if any of the specified slub_debug flags is enabled for the
+ * cache. Use only for flags parsed by setup_slub_debug() as it also enables
+ * the static key.
+ */
+static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t flags)
 {
+	VM_WARN_ON_ONCE(!(flags & SLAB_DEBUG_FLAGS));
 #ifdef CONFIG_SLUB_DEBUG
 	if (static_branch_unlikely(&slub_debug_enabled))
-		return s->flags & SLAB_DEBUG_FLAGS;
+		return s->flags & flags;
 #endif
-	return 0;
+	return false;
+}
+
+static inline bool kmem_cache_debug(struct kmem_cache *s)
+{
+	return kmem_cache_debug_flags(s, SLAB_DEBUG_FLAGS);
 }
 
 void *fixup_red_left(struct kmem_cache *s, void *p)
 {
-	if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE)
+	if (kmem_cache_debug_flags(s, SLAB_RED_ZONE))
 		p += s->red_left_pad;
 
 	return p;
@@ -4060,7 +4071,7 @@ void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
 	offset = (ptr - page_address(page)) % s->size;
 
 	/* Adjust for redzone and reject if within the redzone. */
-	if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) {
+	if (kmem_cache_debug_flags(s, SLAB_RED_ZONE)) {
 		if (offset < s->red_left_pad)
 			usercopy_abort("SLUB object in left red zone",
 				       s->name, to_user, offset, n);