linux-kernel - Re: [PATCH] Make efi-pstore return a unique id

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Date:	Sat, 02 Nov 2013 09:15:35 +0800
From:	Madper Xie <cxie@...hat.com>
To:	Richard Weinberger <richard@....at>
Cc:	Seiji Aguchi <seiji.aguchi@....com>,
	Tony Luck <tony.luck@...il.com>, Madper Xie <cxie@...hat.com>,
	"matt.fleming\@intel.com" <matt.fleming@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linux EFI <linux-efi@...r.kernel.org>,
	谢成骏 <bbboson@...il.com>
Subject: Re: [PATCH] Make efi-pstore return a unique id


richard@....at writes:

> Am 01.11.2013 20:22, schrieb Seiji Aguchi:
>>>>> +{
>>>>> +       char id_str[64];
>>>>> +       u64 id = 0;
>>>>> +
>>>>> +       sprintf(id_str, "%lu%u%d", timestamp, part, count);
>>>>> +       if (kstrtoull(id_str, 10, &id))
>>>>> +               pr_warn("efi-pstore: failed to generate id\n");
>>>>> +       return id;
>>>>> +}
>>>>
>>>> This is just odd. You make a string from three ints and then a parse
>>>> it to a int again.
>>>
>>> Agreed.  I liked your ((timestamp * 100 + part) * 100 + count function much
>>> more than this.
>> 
>> I was worried that the part and count could be more than 100.
>> If it happens, the id may not be unique...
>> 
>> But, currently, size of nvram storage is limited, so it is a corner case.
>> I respect your opinion.
>
Is it really safe? for now I have more than 100 entries:
[root@...p-13-41 rhel6]# ls -l /dev/pstore/ | wc -l
124
The maximum part of my records is 16. But I not sure if overflow will
happen in some special case, like a very long dmesg output. or a server
never reboot, and too many warnings make count++...
So is it necessary to check count < 100 or 100 =< count < 1000 ?
> What about feeding the bytes of all three integers into a non-cryptographic hash function?
> Using this way you get a cheap unique id.
>
> Thanks,
> //richard
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-- 
Best,
Madper Xie.
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