HWPOISON: Clean up PR_MCE_KILL interface

While writing the manpage I noticed some shortcomings in the
current interface.

- Define symbolic names for all the different values
- Boundary check the kill mode values
- For symmetry add a get interface too. This allows library
code to get/set the current state.
- For consistency define a PR_MCE_KILL_DEFAULT value
Signed-off-by: Andi Kleen <ak@linux.intel.com>

include/linux/prctl.h

@@ -88,6 +88,18 @@
 #define PR_TASK_PERF_EVENTS_DISABLE		31
 #define PR_TASK_PERF_EVENTS_ENABLE		32
 
+/*
+ * Set early/late kill mode for hwpoison memory corruption.
+ * This influences when the process gets killed on a memory corruption.
+ */
 #define PR_MCE_KILL	33
+# define PR_MCE_KILL_CLEAR   0
+# define PR_MCE_KILL_SET     1
+
+# define PR_MCE_KILL_LATE    0
+# define PR_MCE_KILL_EARLY   1
+# define PR_MCE_KILL_DEFAULT 2
+
+#define PR_MCE_KILL_GET 34
 
 #endif /* _LINUX_PRCTL_H */

kernel/sys.c

@@ -1546,24 +1546,37 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			if (arg4 | arg5)
 				return -EINVAL;
 			switch (arg2) {
-			case 0:
+			case PR_MCE_KILL_CLEAR:
 				if (arg3 != 0)
 					return -EINVAL;
 				current->flags &= ~PF_MCE_PROCESS;
 				break;
-			case 1:
+			case PR_MCE_KILL_SET:
 				current->flags |= PF_MCE_PROCESS;
-				if (arg3 != 0)
+				if (arg3 == PR_MCE_KILL_EARLY)
 					current->flags |= PF_MCE_EARLY;
-				else
+				else if (arg3 == PR_MCE_KILL_LATE)
 					current->flags &= ~PF_MCE_EARLY;
+				else if (arg3 == PR_MCE_KILL_DEFAULT)
+					current->flags &=
+						~(PF_MCE_EARLY|PF_MCE_PROCESS);
+				else
+					return -EINVAL;
 				break;
 			default:
 				return -EINVAL;
 			}
 			error = 0;
 			break;
-
+		case PR_MCE_KILL_GET:
+			if (arg2 | arg3 | arg4 | arg5)
+				return -EINVAL;
+			if (current->flags & PF_MCE_PROCESS)
+				error = (current->flags & PF_MCE_EARLY) ?
+					PR_MCE_KILL_EARLY : PR_MCE_KILL_LATE;
+			else
+				error = PR_MCE_KILL_DEFAULT;
+			break;
 		default:
 			error = -EINVAL;
 			break;