linux-kernel - Re: [PATCH 1/2] Initial support for Allwinner's Security ID fuses

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Date:	Mon, 24 Jun 2013 19:11:35 +0200
From:	Oliver Schinagl <oliver+list@...inagl.nl>
To:	Greg KH <gregkh@...uxfoundation.org>
CC:	Maxime Ripard <maxime.ripard@...e-electrons.com>, arnd@...db.de,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	andy.shevchenko@...il.com, linux@....linux.org.uk,
	linus.walleij@...aro.org, linux-sunxi@...glegroups.com,
	Oliver Schinagl <oliver@...inagl.nl>
Subject: Re: [PATCH 1/2] Initial support for Allwinner's Security ID fuses

Hey Greg,
On 06/24/13 18:04, Greg KH wrote:
> On Mon, Jun 24, 2013 at 11:29:42AM +0200, Maxime Ripard wrote:
>> Hi Greg,
>>
>> On Mon, Jun 17, 2013 at 03:58:47PM -0700, Greg KH wrote:
>>> On Mon, Jun 17, 2013 at 10:59:37PM +0200, Oliver Schinagl wrote:
>>
>> [..]
>>
>>>> +static int __init sunxi_sid_probe(struct platform_device *pdev)
>>>> +{
>>>> +	u8 entropy[SID_SIZE];
>>>> +	unsigned int i;
>>>> +	struct resource *res;
>>>> +	void __iomem *sid_reg_base;
>>>> +	int ret;
>>>> +
>>>> +	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
>>>> +	sid_reg_base = devm_ioremap_resource(&pdev->dev, res);
>>>> +	if (IS_ERR(sid_reg_base))
>>>> +		return PTR_ERR(sid_reg_base);
>>>> +	platform_set_drvdata(pdev, sid_reg_base);
>>>> +
>>>> +	ret = device_create_bin_file(&pdev->dev, &sid_bin_attr);
>>>> +	if (ret)
>>>> +		return ret;
>>>
>>> You just raced with userspace, having the file show up after the device
>>> was announced to users that it was there.  Please use the proper device
>>> file api to add default attributes to prevent this from happening.
>>
>> Sorry if the question looks dumb, but what kind of race can we generate
>> here?
>
> Userspace gets told about the device from the driver core, udev runs and
> reads all of the attributes, then your probe function comes along and
> adds a new attribute.  Userspace will then not know about it at all.
>
>> The device_create_bin_file is the last call that we make (if we except
>> the entropy stuff, but it doesn't really matter here), so after we
>> created the file, we have everything properly initialised so that our
>> functions can be called, right?
>>
>> And another dumb question for you, what is the "proper device file API"
>> you are referring to ? :)
>
> Please read Documentation/driver_model/device.txt and see the section on
> Attributes for what to do.  If you have specific questions after reading
> that, please let me know.
Since Maxime kinda asked for me, I hope you don't mind me following up.

That doc doesn't mention the binary interface at all. Initially I had 
both devices up, the 'read' device as a textual representation and added 
the binary one later. Maxime and I decided the binary one made more 
sense, as the only textual user would be a human and they don't poke 
that entry that often.

So what default way exists for binary files or how would that be solved?

Oliver
>
> thanks,
>
> greg k-h
>

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