x86/mm/numa: Remove uninitialized_var() usage

Using uninitialized_var() is dangerous as it papers over real bugs[1]
(or can in the future), and suppresses unrelated compiler warnings (e.g.
"unused variable"). If the compiler thinks it is uninitialized, either
simply initialize the variable or make compiler changes. As a precursor
to removing[2] this[3] macro[4], refactor code to avoid its need.

The original reason for its use here was to work around the #ifdef
being the only place the variable was used. This is better expressed
using IS_ENABLED() and a new code block where the variable can be used
unconditionally.

[1] https://lore.kernel.org/lkml/20200603174714.192027-1-glider@google.com/
[2] https://lore.kernel.org/lkml/CA+55aFw+Vbj0i=1TGqCR5vQkCzWJ0QxK6CernOU6eedsudAixw@mail.gmail.com/
[3] https://lore.kernel.org/lkml/CA+55aFwgbgqhbp1fkxvRKEpzyR5J8n1vKT1VZdz9knmPuXhOeg@mail.gmail.com/
[4] https://lore.kernel.org/lkml/CA+55aFz2500WfbKXAx8s67wrm9=yVJu65TpLgN_ybYNv0VEOKA@mail.gmail.com/

Fixes: 1e01979c ("x86, numa: Implement pfn -> nid mapping granularity check")
Signed-off-by: Kees Cook <keescook@chromium.org>

arch/x86/mm/numa.c

@@ -543,7 +543,6 @@ static void __init numa_clear_kernel_node_hotplug(void)
 
 static int __init numa_register_memblks(struct numa_meminfo *mi)
 {
-	unsigned long uninitialized_var(pfn_align);
 	int i, nid;
 
 	/* Account for nodes with cpus and no memory */
@@ -571,15 +570,16 @@ static int __init numa_register_memblks(struct numa_meminfo *mi)
 	 * If sections array is gonna be used for pfn -> nid mapping, check
 	 * whether its granularity is fine enough.
 	 */
-#ifdef NODE_NOT_IN_PAGE_FLAGS
-	pfn_align = node_map_pfn_alignment();
-	if (pfn_align && pfn_align < PAGES_PER_SECTION) {
-		printk(KERN_WARNING "Node alignment %LuMB < min %LuMB, rejecting NUMA config\n",
-		       PFN_PHYS(pfn_align) >> 20,
-		       PFN_PHYS(PAGES_PER_SECTION) >> 20);
-		return -EINVAL;
+	if (IS_ENABLED(NODE_NOT_IN_PAGE_FLAGS)) {
+		unsigned long pfn_align = node_map_pfn_alignment();
+
+		if (pfn_align && pfn_align < PAGES_PER_SECTION) {
+			pr_warn("Node alignment %LuMB < min %LuMB, rejecting NUMA config\n",
+				PFN_PHYS(pfn_align) >> 20,
+				PFN_PHYS(PAGES_PER_SECTION) >> 20);
+			return -EINVAL;
+		}
 	}
-#endif
 	if (!numa_meminfo_cover_memory(mi))
 		return -EINVAL;
 

include/linux/page-flags-layout.h

@@ -98,9 +98,11 @@
 /*
  * We are going to use the flags for the page to node mapping if its in
  * there.  This includes the case where there is no node, so it is implicit.
+ * Note that this #define MUST have a value so that it can be tested with
+ * the IS_ENABLED() macro.
  */
 #if !(NODES_WIDTH > 0 || NODES_SHIFT == 0)
-#define NODE_NOT_IN_PAGE_FLAGS
+#define NODE_NOT_IN_PAGE_FLAGS 1
 #endif
 
 #if defined(CONFIG_NUMA_BALANCING) && LAST_CPUPID_WIDTH == 0