jump_label: Warn on failed jump_label patching attempt

Currently when the jump label code encounters an address which isn't
recognized by kernel_text_address(), it just silently fails.

This can be dangerous because jump labels are used in a variety of
places, and are generally expected to work.  Convert the silent failure
to a warning.

This won't warn about attempted writes to tracepoints in __init code
after initmem has been freed, as those are already guarded by the
entry->code check.
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/de3a271c93807adb7ed48f4e946b4f9156617680.1519051220.git.jpoimboe@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/jump_label.c

@@ -367,12 +367,15 @@ static void __jump_label_update(struct static_key *key,
 {
 	for (; (entry < stop) && (jump_entry_key(entry) == key); entry++) {
 		/*
-		 * entry->code set to 0 invalidates module init text sections
-		 * kernel_text_address() verifies we are not in core kernel
-		 * init code, see jump_label_invalidate_module_init().
+		 * An entry->code of 0 indicates an entry which has been
+		 * disabled because it was in an init text area.
 		 */
-		if (entry->code && kernel_text_address(entry->code))
+		if (entry->code) {
+			if (kernel_text_address(entry->code))
 				arch_jump_label_transform(entry, jump_label_type(entry));
+			else
+				WARN_ONCE(1, "can't patch jump_label at %pS", (void *)entry->code);
+		}
 	}
 }