netdev - Re: [PATCH v3 17/33] nds32: VDSO support

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Message-ID: <20171208102149.iqiieszktwzorkuw@lakrids.cambridge.arm.com>
Date:   Fri, 8 Dec 2017 10:21:49 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Greentime Hu <green.hu@...il.com>
Cc:     greentime@...estech.com, linux-kernel@...r.kernel.org,
        arnd@...db.de, linux-arch@...r.kernel.org, tglx@...utronix.de,
        jason@...edaemon.net, marc.zyngier@....com, robh+dt@...nel.org,
        netdev@...r.kernel.org, deanbo422@...il.com,
        devicetree@...r.kernel.org, viro@...iv.linux.org.uk,
        dhowells@...hat.com, will.deacon@....com,
        daniel.lezcano@...aro.org, linux-serial@...r.kernel.org,
        geert.uytterhoeven@...il.com, linus.walleij@...aro.org,
        greg@...ah.com, Vincent Chen <vincentc@...estech.com>
Subject: Re: [PATCH v3 17/33] nds32: VDSO support

On Fri, Dec 08, 2017 at 05:12:00PM +0800, Greentime Hu wrote:
> From: Greentime Hu <greentime@...estech.com>
> 
> This patch adds VDSO support. The VDSO code is currently used for
> sys_rt_sigreturn() and optimised gettimeofday() (using the SoC timer counter).

[...]

> +static int grab_timer_node_info(void)
> +{
> +	struct device_node *timer_node;
> +
> +	timer_node = of_find_node_by_name(NULL, "timer");

Please use a compatible string, rather than matching the timer by name.

It's plausible that you have multiple nodes called "timer" in the DT,
under different parent nodes, and this might not be the device you
think it is. I see your dt in patch 24 has two timer nodes.

It would be best if your clocksource driver exposed some stuct that you
looked at here, so that you're guaranteed to user the same device.

> +	of_property_read_u32(timer_node, "cycle-count-offset",
> +			     &vdso_data->cycle_count_offset);
> +	vdso_data->cycle_count_down =
> +	    of_property_read_bool(timer_node, "cycle-count-down");

... and then you'd only need to parse these in one place, too.

IIUC these are proeprties for the atcpit device, which has no
documentation or driver in this series.

So I'm rather confused as to what's going on here.

> +	return of_address_to_resource(timer_node, 0, &timer_res);
> +}

> +int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
> +{

> +	/*Map timer to user space */
> +	vdso_base += PAGE_SIZE;
> +	prot = __pgprot(_PAGE_V | _PAGE_M_UR_KR | _PAGE_D |
> +			_PAGE_G | _PAGE_C_DEV);
> +	ret = io_remap_pfn_range(vma, vdso_base, timer_res.start >> PAGE_SHIFT,
> +				 PAGE_SIZE, prot);
> +	if (ret)
> +		goto up_fail;

Maybe this is fine, but it looks a bit suspicious.

Is it safe to map IO memory to a userspace process like this?

In general that isn't safe, since userspace could access other registers
(if those exist), perform accesses that change the state of hardware, or
make unsupported access types (e.g. unaligned, atomic) that result in
errors the kernel can't handle.

Does none of that apply here?

Thanks,
Mark.