Re: [PATCH v6 5/9] kernel: Implement selective syscall userspace redirection

[peterz]

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

> +struct syscall_user_dispatch {
> +     char __user *selector;
> +     unsigned long dispatcher_start;
> +     unsigned long dispatcher_end;
> +};

> +int do_syscall_user_dispatch(struct pt_regs *regs)
> +{
> +     struct syscall_user_dispatch *sd = &current->syscall_dispatch;
> +     unsigned long ip = instruction_pointer(regs);
> +     char state;
> +
> +     if (likely(ip >= sd->dispatcher_start && ip <= sd->dispatcher_end))
> +             return 0;

If you use {offset,size}, instead of {start,end}, you can write the
above like:

        if (ip - sd->dispatcher_offset < sd->dispatcher_size)
                return 0;

which is just a single branch.