No changes in compiled code. The patch adds the new helper, si_fromuser()

and changes check_kill_permission() to use this helper.

The real effect of this patch is that from now we "officially" consider
SEND_SIG_NOINFO signal as "from user-space" signals. This is already true
if we look at the code which uses SEND_SIG_NOINFO, except __send_signal()
has another opinion - see the next patch.

The naming of these special SEND_SIG_XXX siginfo's is really bad
imho.  From __send_signal()'s pov they mean

	SEND_SIG_NOINFO		from user
	SEND_SIG_PRIV		from kernel
	SEND_SIG_FORCED		no info
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Reviewed-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/sched.h

@@ -2093,11 +2093,6 @@ static inline int kill_cad_pid(int sig, int priv)
 #define SEND_SIG_PRIV	((struct siginfo *) 1)
 #define SEND_SIG_FORCED	((struct siginfo *) 2)
 
-static inline int is_si_special(const struct siginfo *info)
-{
-	return info <= SEND_SIG_FORCED;
-}
-
 /*
  * True if we are on the alternate signal stack.
  */

kernel/signal.c

@@ -606,6 +606,17 @@ static int rm_from_queue(unsigned long mask, struct sigpending *s)
 	return 1;
 }
 
+static inline int is_si_special(const struct siginfo *info)
+{
+	return info <= SEND_SIG_FORCED;
+}
+
+static inline bool si_fromuser(const struct siginfo *info)
+{
+	return info == SEND_SIG_NOINFO ||
+		(!is_si_special(info) && SI_FROMUSER(info));
+}
+
 /*
  * Bad permissions for sending the signal
  * - the caller must hold at least the RCU read lock
@@ -620,7 +631,7 @@ static int check_kill_permission(int sig, struct siginfo *info,
 	if (!valid_signal(sig))
 		return -EINVAL;
 
-	if (info != SEND_SIG_NOINFO && (is_si_special(info) || SI_FROMKERNEL(info)))
+	if (!si_fromuser(info))
 		return 0;
 
 	error = audit_signal_info(sig, t); /* Let audit system see the signal */
@@ -1176,8 +1187,7 @@ int kill_pid_info_as_uid(int sig, struct siginfo *info, struct pid *pid,
 		goto out_unlock;
 	}
 	pcred = __task_cred(p);
-	if ((info == SEND_SIG_NOINFO ||
-	     (!is_si_special(info) && SI_FROMUSER(info))) &&
+	if (si_fromuser(info) &&
 	    euid != pcred->suid && euid != pcred->uid &&
 	    uid  != pcred->suid && uid  != pcred->uid) {
 		ret = -EPERM;