powerpc/stackprotector: work around stack-guard init from atomic

BugLink: https://bugs.launchpad.net/bugs/2060704

This is invoked from the secondary CPU in atomic context. On x86 we use
tsc instead. On Power we XOR it against mftb() so lets use stack address
as the initial value.

Cc: stable-rt@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Kevin Becker <kevin.becker@canonical.com>

arch/powerpc/include/asm/stackprotector.h

@@ -19,8 +19,13 @@
  */
 static __always_inline void boot_init_stack_canary(void)
 {
-	unsigned long canary = get_random_canary();
+	unsigned long canary;
 
+#ifndef CONFIG_PREEMPT_RT
+	canary = get_random_canary();
+#else
+	canary = ((unsigned long)&canary) & CANARY_MASK;
+#endif
 	current->stack_canary = canary;
 #ifdef CONFIG_PPC64
 	get_paca()->canary = canary;