Re: [PATCH v4 00/11] PCI VPD access fixes

[Hannes Reinecke <hare@suse.de>]

On 02/24/2016 06:36 AM, Bjorn Helgaas wrote:
> On Wed, Feb 24, 2016 at 05:48:33AM +0800, Hannes Reinecke wrote:
>> On 02/23/2016 10:07 PM, Bjorn Helgaas wrote:
>>> Hi Hannes,
>>>
>>> Thanks for taking a look at the rest of these.
>>>
>>> On Mon, Feb 22, 2016 at 06:46:23PM -0600, Bjorn Helgaas wrote:
>>>> Hi Hannes,
>>>>
>>>> This is a revision of your v3 series:
>>>> http://lkml.kernel.org/r/1455525722-122040-1-git-send-email-hare@suse.de
>>>>
>>>> Here's the description from your v3 posting:
>>>>
>>>>   the current PCI VPD page access assumes that the entire possible VPD
>>>>   data is readable. However, the spec only guarantees a VPD data up to
>>>>   the 'end' marker, with everything beyond that being undefined.
>>>>   This causes a system lockup on certain devices.
>>>>
>>>>   With this patch we always set the VPD sysfs attribute size to '0', and
>>>>   calculate the available VPD size on the first access.
>>>>   If no valid data can be read an I/O error is returned.
>>>
>>> Just to see if I have this right: the VPD file size in sysfs will
>>> always appear as zero, regardless of whether it has been read or
>>> written, right?  I don't think the user-visible size should change.
>>>
>> That is correct.
>> As the actual size is evaluated on the first access, we don't have it
>> available when creating the sysfs attribute itself.
>> And when using the nominal size of 32k some bright program might try to
>> jump to somewhere in the middle of the data, which will make calculating
>> the validity of this horribly complex.
>> Setting it to '0' is an easy way of avoiding this kinda games.
>>
>> So yes, there will be a user-visible change, but it shouldn't affect the
>> programs accessing this attribute.
>> lspci works happily with these changes
> 
> What is the user-visible change?  Here's what I'm thinking.  If we do
> this:
> 
>   ls -l /sys/.../vpd
>   dd if=/sys/.../vpd bs=1 count=1
>   ls -l /sys/.../vpd
> 
> Do we see different sizes from the two "ls" invocations?  My thought
> is that we should see '0' both times, because I don't really think
> that output should change depending on previous actions of this user
> or other users.
> 
Originally we have:

# ls -l 0000:07:00.0/vpd
-rw------- 1 root root 32768 Feb 24 01:29 0000:07:00.0/vpd

and with this patchset we have:

# ls -l 0000:07:00.0/vpd
-rw------- 1 root root 0 Feb 24 01:29 0000:07:00.0/vpd

So only programs doing a 'stat' on the device node will see a difference.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
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