x86/debug: Handle warnings before the notifier chain, to fix KGDB crash

Commit:

9a93848fe787 ("x86/debug: Implement __WARN() using UD0")

turned warnings into UD0, but the fixup code only runs after the
notify_die() chain. This is a problem, in particular, with kgdb,
which kicks in as if it was a BUG().

Fix this by running the fixup code before the notifier chain in
the invalid op handler path.

Signed-off-by: Alexander Shishkin <[e-mail]>
Tested-by: Ilya Dryomov <[e-mail]>
Acked-by: Daniel Thompson <[e-mail]>
Acked-by: Thomas Gleixner <[e-mail]>
Cc: Jason Wessel <[e-mail]>
Cc: Arjan van de Ven <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Richard Weinberger <[e-mail]>
Cc: <[e-mail]> # v4.12+
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/traps.c

···
 		if (fixup_exception(regs, trapnr))
 			return 0;
 
-		if (fixup_bug(regs, trapnr))
-			return 0;
-
 		tsk->thread.error_code = error_code;
 		tsk->thread.trap_nr = trapnr;
 		die(str, regs, error_code);
···
 	siginfo_t info;
 
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
+
+	/*
+	 * WARN*()s end up here; fix them up before we call the
+	 * notifier chain.
+	 */
+	if (!user_mode(regs) && fixup_bug(regs, trapnr))
+		return;
 
 	if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) !=
 			NOTIFY_STOP) {