ARC: handle gcc generated __builtin_trap()

gcc toggle -fisolate-erroneous-paths-dereference (default at -O2
onwards) isolates faulty code paths such as null pointer access, divide
by zero etc by emitting __builtin_trap()

Newer ARC gcc generates TRAP_S 5 instruction which needs to be handled
and treated like any other unexpected exception
  - user mode  : task terminated with a SEGV
  - kernel mode: die() called after register and stack dump
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/traps.c

@@ -83,6 +83,7 @@ DO_ERROR_INFO(SIGILL, "Illegal Insn (or Seq)", insterror_is_error, ILL_ILLOPC)
 DO_ERROR_INFO(SIGBUS, "Invalid Mem Access", __weak do_memory_error, BUS_ADRERR)
 DO_ERROR_INFO(SIGTRAP, "Breakpoint Set", trap_is_brkpt, TRAP_BRKPT)
 DO_ERROR_INFO(SIGBUS, "Misaligned Access", do_misaligned_error, BUS_ADRALN)
+DO_ERROR_INFO(SIGSEGV, "gcc generated __builtin_trap", do_trap5_error, 0)
 
 /*
  * Entry Point for Misaligned Data access Exception, for emulating in software
@@ -115,6 +116,8 @@ void do_machine_check_fault(unsigned long address, struct pt_regs *regs)
  * Thus TRAP_S <n> can be used for specific purpose
  *  -1 used for software breakpointing (gdb)
  *  -2 used by kprobes
+ *  -5 __builtin_trap() generated by gcc (2018.03 onwards) for toggle such as
+ *     -fno-isolate-erroneous-paths-dereference
  */
 void do_non_swi_trap(unsigned long address, struct pt_regs *regs)
 {
@@ -134,6 +137,9 @@ void do_non_swi_trap(unsigned long address, struct pt_regs *regs)
 		kgdb_trap(regs);
 		break;
 
+	case 5:
+		do_trap5_error(address, regs);
+		break;
 	default:
 		break;
 	}

arch/arc/kernel/troubleshoot.c

@@ -163,6 +163,9 @@ static void show_ecr_verbose(struct pt_regs *regs)
 		else
 			pr_cont("Bus Error, check PRM\n");
 #endif
+	} else if (vec == ECR_V_TRAP) {
+		if (regs->ecr_param == 5)
+			pr_cont("gcc generated __builtin_trap\n");
 	} else {
 		pr_cont("Check Programmer's Manual\n");
 	}