linux-kernel - Re: [PATCH] pstore/ram: Add support for dynamically allocated ramoops memory regions

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Message-ID: <696269e1-8b97-66ed-c9b0-ce1b8d637d24@quicinc.com>
Date:   Mon, 26 Jun 2023 23:04:11 +0530
From:   Mukesh Ojha <quic_mojha@...cinc.com>
To:     Elliot Berman <quic_eberman@...cinc.com>,
        Kees Cook <kees@...nel.org>,
        Isaac Manjarres <isaacmanjarres@...gle.com>,
        Kees Cook <keescook@...omium.org>
CC:     John Stultz <jstultz@...gle.com>, Tony Luck <tony.luck@...el.com>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        <kernel-team@...roid.com>, <linux-hardening@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        "Trilok Soni" <quic_tsoni@...cinc.com>,
        Satya Durga Srinivasu Prabhala <quic_satyap@...cinc.com>
Subject: Re: [PATCH] pstore/ram: Add support for dynamically allocated ramoops
 memory regions



On 6/23/2023 1:21 AM, Elliot Berman wrote:
> 
> 
> On 6/22/2023 10:58 AM, Kees Cook wrote:
>> On June 22, 2023 10:26:35 AM PDT, Isaac Manjarres 
>> <isaacmanjarres@...gle.com> wrote:
>>> On Wed, Jun 21, 2023 at 10:15:45PM -0700, Kees Cook wrote:
>>>> On Wed, Jun 21, 2023 at 09:47:26PM -0700, John Stultz wrote:
>>>>>> The reserved memory region for ramoops is assumed to be at a fixed
>>>>>> and known location when read from the devicetree. This is not 
>>>>>> desirable
>>>>>> in environments where it is preferred for the region to be 
>>>>>> dynamically
>>>>>> allocated early during boot (i.e. the memory region is defined with
>>>>>> the "alloc-ranges" property instead of the "reg" property).
>>>>>>
>>>>>
>>>>> Thanks for sending this out, Isaac!
>>>>>
>>>>> Apologies, I've forgotten much of the details around dt bindings here,
>>>>> so forgive my questions:
>>>>> If the memory is dynamically allocated from a specific range, is it
>>>>> guaranteed to be consistently the same address boot to boot?
>>>>>
>>>>>> Since ramoops regions are part of the reserved-memory devicetree
>>>>>> node, they exist in the reserved_mem array. This means that the
>>>>>> of_reserved_mem_lookup() function can be used to retrieve the
>>>>>> reserved_mem structure for the ramoops region, and that structure
>>>>>> contains the base and size of the region, even if it has been
>>>>>> dynamically allocated.
>>>>>
>>>>> I think this is answering my question above, but it's a little opaque,
>>>>> so I'm not sure.
>>>>
>>>> Yeah, I had exactly the same question: will this be the same
>>>> boot-to-boot?
>>>
>>> Hi Kees,
>>>
>>> Thank you for taking a look at this patch and for your review! When the
>>> alloc-ranges property is used to describe a memory region, the memory
>>> region will always be allocated within that range, but it's not
>>> guaranteed to be allocated at the same base address across reboots.
>>>
>>> I had proposed re-wording the end of the commit message in my response
>>> to John as follows:
>>>
>>> "...and that structure contains the address of the base of the region
>>> that was allocated at boot anywhere within the range specified by the
>>> "alloc-ranges" devicetree property."
>>>
>>> Does that clarify things better?
>>
>> I am probably misunderstanding something still, but it it varies from 
>> boot to boot, what utility is there for pstore if it changes? I.e. the 
>> things written during the last boot would then no longer accessible at 
>> the next boot? E.g.:
>>
>> Boot 1.
>> Get address Foo.
>> Crash, write to Foo.
>> Boot 2.
>> Get address Bar, different from Foo.
>> Nothing found at Bar, so nothing populated in pstorefs; crash report 
>> from Boot 1 unavailable.
>>
>> I feel like there is something I don't understand about the Foo/Bar 
>> addresses in my example.
>>
> 
> I believe this is being added to support the QCOM SoC minidump feature. 
> Mukesh has posted it on the mailing lists here:
> 
> https://lore.kernel.org/all/1683133352-10046-1-git-send-email-quic_mojha@quicinc.com/
> 
> https://lore.kernel.org/all/1683133352-10046-10-git-send-email-quic_mojha@quicinc.com/
> 
> Mukesh, could you comment whether this patch is wanted for us in the 
> version you have posted? It looks like maybe not based on the commit 
> text in patch #9.

No, this is no needed after patch #9 .

I have tried multiple attempt already to get this patch in

https://lore.kernel.org/lkml/1675330081-15029-2-git-send-email-quic_mojha@quicinc.com/

later tried the approach of patch #9 along with minidump series..


- Mukesh

> 
>   - Elliot