[PATCH v2 1/3] signal: change force_sig_info_to_task() to call __send_signal_locked()

[Oleg Nesterov]

force_sig_info_to_task() calls send_signal_locked() which does two
things on top of __send_signal_locked():

1. The namespace translation of si_pid/si_uid. However, forced signals
   carry fault info (si_addr, si_call_addr, si_syscall), not pid/uid.
   The force_sig*() API should never be used to send signals with
   meaningful si_pid/si_uid, the forced signals are always "from kernel".

   There are few users of force_sig(SIGKILL), and in this case
   send_signal_locked() -> has_si_pid_and_uid() returns true.
   However, __send_signal_locked() simply ignores kernel_siginfo if
   sig == SIGKILL.

   (and in fact force_sig(SIGKILL) makes little sense, they should
    use send_sig(SIGKILL, p, 1) instead)

2. The "force" computation. However, for the forced signals, the
   unconditional force == true works just fine.

   If the target is ptraced, the "force" arg has no effect unless
   sig == SIGKILL.

   Otherwise, this check in sig_task_ignored()

	if (unlikely(t->signal->flags & SIGNAL_UNKILLABLE) &&
	    handler == SIG_DFL && !(force && sig_kernel_only(sig)))
		return true;

   has no effect, force_sig_info_to_task() clears SIGNAL_UNKILLABLE
   if handler == SIG_DFL.

   The only behavioral difference is another check in sig_task_ignored:

	if (unlikely((t->flags & PF_KTHREAD) &&
		     (handler == SIG_KTHREAD_KERNEL) && !force))

   So with this patch a kthread that called allow_kernel_signal()
   for a fault signal would now receive the forced signal instead
   of silently ignoring it.

   And this is arguably more correct, even if I don't think that
   the force_sig*() API should be used in this case.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 kernel/signal.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 9c2b32c4d755..68af503ed43c 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1315,7 +1315,7 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t,
 	if (action->sa.sa_handler == SIG_DFL &&
 	    (!t->ptrace || (handler == HANDLER_EXIT)))
 		t->signal->flags &= ~SIGNAL_UNKILLABLE;
-	ret = send_signal_locked(sig, info, t, PIDTYPE_PID);
+	ret = __send_signal_locked(sig, info, t, PIDTYPE_PID, true);
 	/* This can happen if the signal was already pending and blocked */
 	if (!task_sigpending(t))
 		signal_wake_up(t, 0);
-- 
2.52.0