ARM: move signal handlers into a vdso-like page

Move the signal handlers into a VDSO page rather than keeping them in
the vectors page.  This allows us to place them randomly within this
page, and also map the page at a random location within userspace
further protecting these code fragments from ROP attacks.  The new
VDSO page is also poisoned in the same way as the vector page.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/traps.c

@@ -35,8 +35,6 @@
 #include <asm/tls.h>
 #include <asm/system_misc.h>
 
-#include "signal.h"
-
 static const char *handler[]= { "prefetch abort", "data abort", "address exception", "interrupt" };
 
 void *vectors_page;
@@ -850,13 +848,6 @@ void __init early_trap_init(void *vectors_base)
 
 	kuser_init(vectors_base);
 
-	/*
-	 * Copy signal return handlers into the vector page, and
-	 * set sigreturn to be a pointer to these.
-	 */
-	memcpy((void *)(vectors + KERN_SIGRETURN_CODE - CONFIG_VECTORS_BASE),
-	       sigreturn_codes, sizeof(sigreturn_codes));
-
 	flush_icache_range(vectors, vectors + PAGE_SIZE * 2);
 	modify_domain(DOMAIN_USER, DOMAIN_CLIENT);
 #else /* ifndef CONFIG_CPU_V7M */