x86: cleanup stack protector

Impact: cleanup

Make the following cleanups.

* remove duplicate comment from boot_init_stack_canary() which fits
  better in the other place - cpu_idle().

* move stack_canary offset check from __switch_to() to
  boot_init_stack_canary().

Signed-off-by: Tejun Heo <tj@kernel.org>

arch/x86/include/asm/pda.h

@@ -40,6 +40,4 @@ extern void pda_init(int);
 
 #endif
 
-#define refresh_stack_canary() write_pda(stack_canary, current->stack_canary)
-
 #endif /* _ASM_X86_PDA_H */

arch/x86/include/asm/stackprotector.h

@@ -16,13 +16,12 @@ static __always_inline void boot_init_stack_canary(void)
 	u64 tsc;
 
 	/*
-	 * If we're the non-boot CPU, nothing set the PDA stack
-	 * canary up for us - and if we are the boot CPU we have
-	 * a 0 stack canary. This is a good place for updating
-	 * it, as we wont ever return from this function (so the
-	 * invalid canaries already on the stack wont ever
-	 * trigger).
-	 *
+	 * Build time only check to make sure the stack_canary is at
+	 * offset 40 in the pda; this is a gcc ABI requirement
+	 */
+	BUILD_BUG_ON(offsetof(struct x8664_pda, stack_canary) != 40);
+
+	/*
 	 * We both use the random pool and the current TSC as a source
 	 * of randomness. The TSC only matters for very early init,
 	 * there it already has some randomness on most systems. Later

arch/x86/kernel/process_64.c

@@ -638,13 +638,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	percpu_write(kernel_stack,
 		  (unsigned long)task_stack_page(next_p) +
 		  THREAD_SIZE - KERNEL_STACK_OFFSET);
-#ifdef CONFIG_CC_STACKPROTECTOR
-	/*
-	 * Build time only check to make sure the stack_canary is at
-	 * offset 40 in the pda; this is a gcc ABI requirement
-	 */
-	BUILD_BUG_ON(offsetof(struct x8664_pda, stack_canary) != 40);
-#endif
 
 	/*
 	 * Now maybe reload the debug registers and handle I/O bitmaps