linux: Provide dummy seccomp(2)

Don't emit messages; this isn't any different from a Linux kernel
built without OPTIONS_SECCOMP, so the userspace already needs to know
how to deal with it.  This is also similar with how we handle seccomp
in linux_prctl().

Sponsored By:	EPSRC
Differential Revision:	https://reviews.freebsd.org/D33808

sys/compat/linux/linux_dummy.c

@@ -112,8 +112,6 @@ DUMMY(kcmp);
 DUMMY(finit_module);
 DUMMY(sched_setattr);
 DUMMY(sched_getattr);
-/* Linux 3.17: */
-DUMMY(seccomp);
 /* Linux 3.18: */
 DUMMY(bpf);
 /* Linux 3.19: */

sys/compat/linux/linux_misc.c

@@ -2880,3 +2880,19 @@ linux_poll(struct thread *td, struct linux_poll_args *args)
 	    tsp, NULL, 0));
 }
 #endif /* __i386__ || __amd64__ */
+
+int
+linux_seccomp(struct thread *td, struct linux_seccomp_args *args)
+{
+
+	switch (args->op) {
+	case LINUX_SECCOMP_GET_ACTION_AVAIL:
+		return (EOPNOTSUPP);
+	default:
+		/*
+		 * Ignore unknown operations, just like Linux kernel built
+		 * without CONFIG_SECCOMP.
+		 */
+		return (EINVAL);
+	}
+}

sys/compat/linux/linux_misc.h

@@ -153,6 +153,9 @@ extern int stclohz;
 /* Linux syslog flags */
 #define	LINUX_SYSLOG_ACTION_READ_ALL	3
 
+/* Linux seccomp flags */
+#define	LINUX_SECCOMP_GET_ACTION_AVAIL	2
+
 #if defined(__aarch64__) || (defined(__amd64__) && !defined(COMPAT_LINUX32))
 int linux_ptrace_status(struct thread *td, int pid, int status);
 #endif