kbuild: introduce __init_refok/__initdata_refok to supress section mismatch warnings

Throughout the kernel there are a few legitimite references
to init or exit sections. Most of these are covered by the
patterns included in modpost but a few nees special attention.
To avoid hardcoding a lot of function names in modpost introduce
a marker so relevant function/data can be marked.
When modpost see a reference to a init/exit function from
a function/data marked no warning will be issued.

Idea from: Andrew Morton <[e-mail]>

Signed-off-by: Sam Ravnborg <[e-mail]>
Cc: Andrew Morton <[e-mail]>

include/asm-generic/vmlinux.lds.h

···
 
 /* .data section */
 #define DATA_DATA							\
-	*(.data)
+	*(.data)							\
+	*(.data.init.refok)
 
 #define RODATA								\
 	. = ALIGN(4096);						\
···
  * during second ld run in second ld pass when generating System.map */
 #define TEXT_TEXT							\
 		ALIGN_FUNCTION();					\
-		*(.text)
+		*(.text)						\
+		*(.text.init.refok)
 
 /* sched.text is aling to function alignment to secure we have same
  * address even at second ld pass when generating System.map */

include/linux/init.h

···
 #define __exitdata	__attribute__ ((__section__(".exit.data")))
 #define __exit_call	__attribute_used__ __attribute__ ((__section__ (".exitcall.exit")))
 
+/* modpost check for section mismatches during the kernel build.
+ * A section mismatch happens when there are references from a
+ * code or data section to an init section (both code or data).
+ * The init sections are (for most archs) discarded by the kernel
+ * when early init has completed so all such references are potential bugs.
+ * For exit sections the same issue exists.
+ * The following markers are used for the cases where the reference to
+ * the init/exit section (code or data) is valid and will teach modpost
+ * not to issue a warning.
+ * The markers follow same syntax rules as __init / __initdata. */
+#define __init_refok     noinline __attribute__ ((__section__ (".text.init.refok")))
+#define __initdata_refok          __attribute__ ((__section__ (".data.init.refok")))
+
 #ifdef MODULE
 #define __exit		__attribute__ ((__section__(".exit.text")))
 #else

scripts/mod/modpost.c

···
 
 /**
  * Whitelist to allow certain references to pass with no warning.
+ *
+ * Pattern 0:
+ *   Do not warn if funtion/data are marked with __init_refok/__initdata_refok.
+ *   The pattern is identified by:
+ *   fromsec = .text.init.refok | .data.init.refok
+ *
  * Pattern 1:
  *   If a module parameter is declared __initdata and permissions=0
  *   then this is legal despite the warning generated.
···
 		"zone_wait_table_init",
 		NULL
 	};
+
+	/* Check for pattern 0 */
+	if ((strcmp(fromsec, ".text.init.refok") == 0) ||
+	    (strcmp(fromsec, ".data.init.refok") == 0))
+		return 1;
 
 	/* Check for pattern 1 */
 	if (strcmp(tosec, ".init.data") != 0)