Re: [PATCH] powerpc/pseries: fix of_read_drc_info_cell() to point at next record

[Michael Ellerman <mpe@ellerman.id.au>]

Tyrel Datwyler <tyreld@linux.ibm.com> writes:
> On 3/11/20 10:43 PM, Michael Ellerman wrote:
>> Tyrel Datwyler <tyreld@linux.ibm.com> writes:
>>> On 3/10/20 10:25 AM, Nathan Lynch wrote:
>>>> Tyrel Datwyler <tyreld@linux.ibm.com> writes:
>>>>> The expectation is that when calling of_read_drc_info_cell()
>>>>> repeatedly to parse multiple drc-info records that the in/out curval
>>>>> parameter points at the start of the next record on return. However,
>>>>> the current behavior has curval still pointing at the final value of
>>>>> the record just parsed. The result of which is that if the
>>>>> ibm,drc-info property contains multiple properties the parsed value
>>>>> of the drc_type for any record after the first has the power_domain
>>>>> value of the previous record appended to the type string.
>>>>>
>>>>> Ex: observed the following 0xffffffff prepended to PHB
>>>>>
>>>>> [   69.485037] drc-info: type: \xff\xff\xff\xffPHB, prefix: PHB , index_start: 0x20000001
>>>>> [   69.485038] drc-info: suffix_start: 1, sequential_elems: 3072, sequential_inc: 1
>>>>> [   69.485038] drc-info: power-domain: 0xffffffff, last_index: 0x20000c00
>>>>>
>>>>> Fix by incrementing curval past the power_domain value to point at
>>>>> drc_type string of next record.
>>>>>
>>>>> Fixes: a29396653b8bf ("pseries/drc-info: Search DRC properties for CPU indexes")
>>>>
>>>> I have a different commit hash for that:
>>>> e83636ac3334 pseries/drc-info: Search DRC properties for CPU indexes
>>>
>>> Oof, looks like I grabbed the commit hash from the SLES 15 SP1 branch. You have
>>> the correct upstream commit.
>>>
>>> Fixes: e83636ac3334 ("pseries/drc-info: Search DRC properties for CPU indexes")
>>>
>>> Michael, let me know if you want me to resubmit, or if you will fixup the Fixes
>>> tag on your end?
>> 
>> I can update the Fixes tag.
>> 
>> What's the practical effect of this bug? It seems like it should break
>> all the hotplug stuff, but presumably it doesn't in practice for some
>> reason?
>> 
>> It would also be *really* nice if we had some unit tests for this
>> parsing code, it's demonstrably very bug prone.
>
> In practice PHBs are the only type of connector in the ibm,drc-info property
> that has multiple records. So, it breaks PHB hotplug, but by chance not pci,
> cpu, slot, or mem because they happen to only ever be a single record.

Thanks. I folded that into the change log.

cheers