[PATCH] prctl: add PR_[GS]ET_KILLABLE

["Jürg Billeter" <j@bitron.ch>]

PR_SET_KILLABLE clears the SIGNAL_UNKILLABLE flag. This allows
CLONE_NEWPID tasks to restore normal signal behavior, opting out of the
special signal protection for init processes.

This is required for job control in a shell that uses CLONE_NEWPID for
child processes.

This prctl does not allow setting the SIGNAL_UNKILLABLE flag, only
clearing.

Signed-off-by: Jürg Billeter <j@bitron.ch>
---
 include/uapi/linux/prctl.h |  4 ++++
 kernel/sys.c               | 11 +++++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index c0d7ea0bf5b6..92afb63da727 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -219,4 +219,8 @@ struct prctl_mm_map {
 # define PR_SPEC_DISABLE		(1UL << 2)
 # define PR_SPEC_FORCE_DISABLE		(1UL << 3)
 
+/* Control SIGNAL_UNKILLABLE */
+#define PR_GET_KILLABLE			54
+#define PR_SET_KILLABLE			55
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index 38509dc1f77b..264de630d548 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2484,6 +2484,17 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			return -EINVAL;
 		error = arch_prctl_spec_ctrl_set(me, arg2, arg3);
 		break;
+	case PR_GET_KILLABLE:
+		if (arg3 || arg4 || arg5)
+			return -EINVAL;
+		error = put_user(!(me->signal->flags & SIGNAL_UNKILLABLE),
+				 (int __user *)arg2);
+		break;
+	case PR_SET_KILLABLE:
+		if (arg2 != 1 || arg3 || arg4 || arg5)
+			return -EINVAL;
+		me->signal->flags &= ~SIGNAL_UNKILLABLE;
+		break;
 	default:
 		error = -EINVAL;
 		break;
-- 
2.18.0