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Date:   Fri, 11 Sep 2020 11:44:31 +0200
From:   peterz@infradead.org
To:     Gabriel Krisman Bertazi <krisman@collabora.com>
Cc:     luto@kernel.org, tglx@linutronix.de, keescook@chromium.org,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org, willy@infradead.org,
        linux-kselftest@vger.kernel.org, shuah@kernel.org,
        kernel@collabora.com, Paul Gofman <gofmanp@gmail.com>
Subject: Re: [PATCH v6 5/9] kernel: Implement selective syscall userspace
 redirection
Message-ID: <20200911094431.GF1362448@hirez.programming.kicks-ass.net>
References: <20200904203147.2908430-1-krisman@collabora.com>
 <20200904203147.2908430-6-krisman@collabora.com>
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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.