arm64: Treat handle_arch_irq as a function pointer

handle_arch_irq isn't actually text, it's just a function pointer.
It doesn't need to be stored in the text section and doing so
causes problesm if we ever want to make the kernel text read only.
Declare handle_arch_irq as a proper function pointer stored in
the data section.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/irq.h

@@ -3,7 +3,6 @@
 
 #include <asm-generic/irq.h>
 
-extern void (*handle_arch_irq)(struct pt_regs *);
 extern void migrate_irqs(void);
 extern void set_handle_irq(void (*handle_irq)(struct pt_regs *));
 

arch/arm64/kernel/entry.S

@@ -167,7 +167,8 @@ tsk	.req	x28		// current thread_info
  * Interrupt handling.
  */
 	.macro	irq_handler
-	ldr	x1, handle_arch_irq
+	adrp	x1, handle_arch_irq
+	ldr	x1, [x1, #:lo12:handle_arch_irq]
 	mov	x0, sp
 	blr	x1
 	.endm
@@ -699,6 +700,3 @@ ENTRY(sys_rt_sigreturn_wrapper)
 	mov	x0, sp
 	b	sys_rt_sigreturn
 ENDPROC(sys_rt_sigreturn_wrapper)
-
-ENTRY(handle_arch_irq)
-	.quad	0

arch/arm64/kernel/irq.c

@@ -40,6 +40,8 @@ int arch_show_interrupts(struct seq_file *p, int prec)
 	return 0;
 }
 
+void (*handle_arch_irq)(struct pt_regs *) = NULL;
+
 void __init set_handle_irq(void (*handle_irq)(struct pt_regs *))
 {
 	if (handle_arch_irq)