[PATCH] [GLIBC RFC] clone3: add CLONE3_RESET_SIGHAND

[Christian Brauner <christian.brauner@ubuntu.com>]

Reset all signal handlers of the child not set to SIG_IGN to SIG_DFL.
Mutually exclusive with CLONE_SIGHAND to not disturb other threads
signal handler.

Questions for glibc before going to send this for review on lkml:
- Is it sufficient for glibc to get EINVAL when CLONE3_RESET_SIGNALS is
  passed to determine kernel support for it?
- Do you really want to have only those signals set to SIG_DFL which are
  not SIG_IGN or do you want to enforce _all_ signals are set to
  SIG_DFL even if they are SIG_IGN?

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 include/uapi/linux/sched.h | 3 +++
 kernel/fork.c              | 8 +++++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 99335e1f4a27..e3bc43efbbba 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -33,6 +33,9 @@
 #define CLONE_NEWNET		0x40000000	/* New network namespace */
 #define CLONE_IO		0x80000000	/* Clone io context */
 
+/* Flags for the clone3() syscall */
+#define CLONE3_RESET_SIGHAND 0x100000000ULL /* Reset any signal handler which is not SIG_IGN or SIG_DFL to SIG_DFL. */
+
 #ifndef __ASSEMBLY__
 /**
  * struct clone_args - arguments for the clone3 syscall
diff --git a/kernel/fork.c b/kernel/fork.c
index 1f6c45f6a734..3bced4a2931e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1516,6 +1516,9 @@ static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
 	refcount_set(&sig->count, 1);
 	spin_lock_irq(&current->sighand->siglock);
 	memcpy(sig->action, current->sighand->action, sizeof(sig->action));
+	/* Reset all signal handler not set to SIG_IGN to SIG_DFL. */
+	if (clone_flags & CLONE3_RESET_SIGHAND)
+		flush_signal_handlers(tsk, 0);
 	spin_unlock_irq(&current->sighand->siglock);
 	return 0;
 }
@@ -2567,7 +2570,7 @@ static bool clone3_args_valid(const struct kernel_clone_args *kargs)
 	 * All lower bits of the flag word are taken.
 	 * Verify that no other unknown flags are passed along.
 	 */
-	if (kargs->flags & ~CLONE_LEGACY_FLAGS)
+	if (kargs->flags & ~(CLONE_LEGACY_FLAGS | CLONE3_RESET_SIGNALS))
 		return false;
 
 	/*
@@ -2577,6 +2580,9 @@ static bool clone3_args_valid(const struct kernel_clone_args *kargs)
 	if (kargs->flags & (CLONE_DETACHED | CSIGNAL))
 		return false;
 
+	if (kargs->flags & (CLONE_SIGHAND | CLONE3_RESET_SIGHAND))
+		return false;
+
 	if ((kargs->flags & (CLONE_THREAD | CLONE_PARENT)) &&
 	    kargs->exit_signal)
 		return false;
-- 
2.23.0