signal: make kill_ok_by_cred() return bool

kill_ok_by_cred() already behaves like a boolean function.  Let's actually
declare it as such too.

Link: http://lkml.kernel.org/r/20180602103653.18181-7-christian@brauner.io
Signed-off-by: Christian Brauner <christian@brauner.io>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: James Morris <james.morris@microsoft.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -717,21 +717,16 @@ static inline bool si_fromuser(const struct siginfo *info)
 /*
  * called with RCU read lock from check_kill_permission()
  */
-static int kill_ok_by_cred(struct task_struct *t)
+static bool kill_ok_by_cred(struct task_struct *t)
 {
 	const struct cred *cred = current_cred();
 	const struct cred *tcred = __task_cred(t);
 
-	if (uid_eq(cred->euid, tcred->suid) ||
-	    uid_eq(cred->euid, tcred->uid)  ||
-	    uid_eq(cred->uid,  tcred->suid) ||
-	    uid_eq(cred->uid,  tcred->uid))
-		return 1;
-
-	if (ns_capable(tcred->user_ns, CAP_KILL))
-		return 1;
-
-	return 0;
+	return uid_eq(cred->euid, tcred->suid) ||
+	       uid_eq(cred->euid, tcred->uid) ||
+	       uid_eq(cred->uid, tcred->suid) ||
+	       uid_eq(cred->uid, tcred->uid) ||
+	       ns_capable(tcred->user_ns, CAP_KILL);
 }
 
 /*