On Fri, Sep 04, 2020 at 04:31:43PM -0400, Gabriel Krisman Bertazi wrote:

+int set_syscall_user_dispatch(int mode, unsigned long dispatcher_start,
+			      unsigned long dispatcher_end, char __user *selector)
+{
+	switch (mode) {
+	case PR_SYS_DISPATCH_OFF:

...

+	case PR_SYS_DISPATCH_ON:

...

+	default:
+		return -EINVAL;

...

+	case PR_SET_SYSCALL_USER_DISPATCH:
+		error = set_syscall_user_dispatch((int) arg2, arg3, arg4,
+						  (char __user *) arg5);

This makes aliases of DISPATCH_OFF and DISPATCH_ON every 4GB throughout
the 64-bit space of arg2.  I don't think that was intentional (nor
desirable).  I'd suggest just making 'mode' a long and dropping the cast.