x86/traps: Use a new on_thread_stack() helper to clean up an assertion

Let's keep the stack-related logic together rather than open-coding
a comparison in an assertion in the traps code.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/856b15bee1f55017b8f79d3758b0d51c48a08cf8.1509609304.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/processor.h

@@ -541,6 +541,12 @@ static inline unsigned long current_top_of_stack(void)
 #endif
 }
 
+static inline bool on_thread_stack(void)
+{
+	return (unsigned long)(current_top_of_stack() -
+			       current_stack_pointer) < THREAD_SIZE;
+}
+
 #ifdef CONFIG_PARAVIRT
 #include <asm/paravirt.h>
 #else

arch/x86/kernel/traps.c

@@ -141,8 +141,7 @@ void ist_begin_non_atomic(struct pt_regs *regs)
 	 * will catch asm bugs and any attempt to use ist_preempt_enable
 	 * from double_fault.
 	 */
-	BUG_ON((unsigned long)(current_top_of_stack() -
-			       current_stack_pointer) >= THREAD_SIZE);
+	BUG_ON(!on_thread_stack());
 
 	preempt_enable_no_resched();
 }