signal: make kill_as_cred_perm() return bool

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

Link: http://lkml.kernel.org/r/20180602103653.18181-3-christian@brauner.ioSigned-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

@@ -1339,14 +1339,15 @@ static int kill_proc_info(int sig, struct siginfo *info, pid_t pid)
 	return error;
 }
 
-static int kill_as_cred_perm(const struct cred *cred,
+static inline bool kill_as_cred_perm(const struct cred *cred,
 				     struct task_struct *target)
 {
 	const struct cred *pcred = __task_cred(target);
-	if (!uid_eq(cred->euid, pcred->suid) && !uid_eq(cred->euid, pcred->uid) &&
-	    !uid_eq(cred->uid,  pcred->suid) && !uid_eq(cred->uid,  pcred->uid))
-		return 0;
-	return 1;
+
+	return uid_eq(cred->euid, pcred->suid) ||
+	       uid_eq(cred->euid, pcred->uid) ||
+	       uid_eq(cred->uid, pcred->suid) ||
+	       uid_eq(cred->uid, pcred->uid);
 }
 
 /* like kill_pid_info(), but doesn't use uid/euid of "current" */