ARM: 8807/1: mm: Facilitate debugging CONFIG_KUSER_HELPERS disabled

Some software such as perf makes unconditional use of the special
[vectors] page which is only provided when CONFIG_KUSER_HELPERS is
enabled in the kernel.

Facilitate the debugging of such situations by printing a debug message
to the kernel log showing the task name and the faulting address.
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Suggested-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/mm/fault.c

@@ -173,6 +173,12 @@ __do_user_fault(struct task_struct *tsk, unsigned long addr,
 		show_regs(regs);
 	}
 #endif
+#ifndef CONFIG_KUSER_HELPERS
+	if ((sig == SIGSEGV) && ((addr & PAGE_MASK) == 0xffff0000))
+		printk_ratelimited(KERN_DEBUG
+				   "%s: CONFIG_KUSER_HELPERS disabled at 0x%08lx\n",
+				   tsk->comm, addr);
+#endif
 
 	tsk->thread.address = addr;
 	tsk->thread.error_code = fsr;