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

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Message-ID: <51BF0AD7.1070101@schinagl.nl>
Date:	Mon, 17 Jun 2013 15:10:47 +0200
From:	Oliver Schinagl <oliver+list@...inagl.nl>
To:	Tomasz Figa <t.figa@...sung.com>
CC:	linux-arm-kernel@...ts.infradead.org,
	Tomasz Figa <tomasz.figa@...il.com>, linux@....linux.org.uk,
	arnd@...db.de, Oliver Schinagl <oliver@...inagl.nl>,
	gregkh@...uxfoundation.org, linus.walleij@...aro.org,
	linux-kernel@...r.kernel.org, andy.shevchenko@...il.com,
	maxime.ripard@...e-electrons.com, linux-sunxi@...glegroups.com
Subject: Re: [PATCH 1/2] Initial support for Allwinner's Security ID fuses

On 17-06-13 14:51, Tomasz Figa wrote:
> On Monday 17 of June 2013 12:36:47 Oliver Schinagl wrote:
>> On 15-06-13 12:28, Tomasz Figa wrote:
>>> Hi,
>>>
>>> Some comments inline.
>>
>> Thank you
>
> You're welcome. :)
>
>>> On Saturday 15 of June 2013 01:16:20 Oliver Schinagl wrote:
>>>> From: Oliver Schinagl <oliver@...inagl.nl>
>>>>
>>>> Allwinner has electric fuses (efuse) on their line of chips. This driver
>>>> reads those fuses, seeds the kernel entropy and exports them as a sysfs
>>>> node.
<snip>
>> I will change the comment, 'and 4 byte sized keys per SID' is probably
>> better
>> The array is 128 bits split into 32 bit words. Each 32 bit word consists
>> of 8 bits (1 byte).
>> So 4 * 4 = 16 bytes (SID_SIZE), is 128 bits.
>>
>
> What about:
>
> 	/* There are 4 keys. */
> 	#define SID_KEYS 4
> 	/* Each key is 4 byte long (32-bit). */
> 	#define SID_SIZE (SID_KEYS * 4)
>
I'll ommit the 'long (32-bit)' part but yeah that's probably enough.

<snip>
>>>> +
>>>> +	if (offset >= SID_SIZE)
>>>> +		goto exit;
>>>>
>>> 		return 0; ...
>>
>> I did say in the changelog I opted for goto over return. But since
>> everybody keeps preferring returns (I personally like 'one single exit
>> point much more' I have already changed it all over to many returns, who
>> am I to argue :)
>
> Well, single exit points makes sense (and is much nicer) when you have
> something to do before exiting, take error paths as an example. But jumping
> just to return makes no sense, because when reading the code you must scroll
> down to the label to check what actually happens.
But functions shouldn't be so large :p But that is the first reasonable 
reason I can live with :)

<snip>
>>
>>>> +
>>>> +	for (i = 0; i < SID_SIZE; i++)
>>>> +		entropy[i] = sunxi_sid_read_byte(sid_reg_base, i);
>>>
>>> You seem to read bytes into an array of ints. Your entropy data will
>>> always have most significant 24-bits cleared. Is this behavior correct?
>>
>> Yes, though I changed it so that entropy is an array of u8's, since
>> that's what sunxi_sid_read_byte returns.
>>
>>>> +	add_device_randomness(entropy, SID_SIZE);
>>>
>>> Now I'm pretty sure that above is not the correct behavior. You are adding
>>> here first 16 bytes (=SID_SIZE) of entropy[], while it is an array of 16
>>> ints (=4*SID_SIZE)...
>>
>> Well technically, doesn't to compiler see that entropy is never larger
>> then 8 bits and thus uses only 8 bits? uint8_atleast or something. But
>> yeah, it's better to use the specified size to not waste 24 empty bits.
>
> I mean, the loop fills the array with SID_SIZE ints, each with 3 zero bytes and
> 1 byte of actual data, so you get:
>
> S0 0x00 0x00 0x00 S1 0x00 0x00 0x00 ... S15 0x00 0x00 0x00
>
Ok, I get that
> but by calling add_device_randomness() with SID_SIZE as size argument, you add
> only 16 first bytes of data from the array:
>
> S0 0x00 0x00 0x00 S1 0x00 0x00 0x00 ... S3 0x00 0x00 0x00
That bit I'm not quite sure I understand:

We have an array of ints, { 0x00000000, 0x00000000, 0x00000000 .... }
We read 1 byte and copy it to the array (x16) (say sid = 0xdeadbeef...)
{ 0x000000de, 0x000000ad, 0x000000be, ... }

Now we pass this array to add_randomness(array, 16). So add_randomness 
sees 16 ints in an array. So while there will be a lot of extra zero's, 
there still be 16 elements copied/processed, no?

Otherwise, how does add_randomness() know it's dealing with bytes or 
ints? it just see's the pointer to an int array that is 16 long? Or what 
am I overlooking?

I did already change the array to be u8 big so it is only to help me 
understand.
>
> (little endianness assumed)
>
> Best regards,
> Tomasz
>
> (for rest of comments I think it's enough said in Russell's and Maxime's
> replies)
Yes it has :) thanks to all of you

Oliver
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