x86/unwind: Fix guess-unwinder regression

My attempt at fixing some KASAN false positive warnings was rather brain
dead, and it broke the guess unwinder.  With frame pointers disabled,
/proc/<pid>/stack is broken:

  # cat /proc/1/stack
  [<ffffffffffffffff>] 0xffffffffffffffff

Restore the code flow to more closely resemble its previous state, while
still using READ_ONCE_NOCHECK() macros to silence KASAN false positives.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: c2d75e03d6 ("x86/unwind: Prevent KASAN false positive warnings in guess unwinder")
Link: http://lkml.kernel.org/r/b824f92c2c22eca5ec95ac56bd2a7c84cf0b9df9.1480309971.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/unwind_guess.c

@@ -7,11 +7,13 @@
 
 unsigned long unwind_get_return_address(struct unwind_state *state)
 {
-	unsigned long addr = READ_ONCE_NOCHECK(*state->sp);
+	unsigned long addr;
 
 	if (unwind_done(state))
 		return 0;
 
+	addr = READ_ONCE_NOCHECK(*state->sp);
+
 	return ftrace_graph_ret_addr(state->task, &state->graph_idx,
 				     addr, state->sp);
 }
@@ -25,11 +27,12 @@ bool unwind_next_frame(struct unwind_state *state)
 		return false;
 
 	do {
-		unsigned long addr = READ_ONCE_NOCHECK(*state->sp);
+		for (state->sp++; state->sp < info->end; state->sp++) {
+			unsigned long addr = READ_ONCE_NOCHECK(*state->sp);
 
-		for (state->sp++; state->sp < info->end; state->sp++)
 			if (__kernel_text_address(addr))
 				return true;
+		}
 
 		state->sp = info->next_sp;