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Date:   Thu, 22 Jun 2023 10:26:35 -0700
From:   Isaac Manjarres <isaacmanjarres@google.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     John Stultz <jstultz@google.com>, Tony Luck <tony.luck@intel.com>,
        "Guilherme G. Piccoli" <gpiccoli@igalia.com>,
        "Isaac J. Manjarres" <isaacm@codeaurora.org>,
        Mukesh Ojha <mojha@codeaurora.org>,
        Prasad Sodagudi <psodagud@codeaurora.org>,
        kernel-team@android.com, linux-hardening@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] pstore/ram: Add support for dynamically allocated
 ramoops memory regions
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 <202306212212.5E53607@keescook>
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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?

> > 
> > > Thus invoke of_reserved_mem_lookup() in case the call to
> > > platform_get_resource() fails in order to support dynamically
> > > allocated ramoops memory regions.
> > >
> > > Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
> > > Signed-off-by: Mukesh Ojha <mojha@codeaurora.org>
> > > Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>
> > > Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>
> 
> I think this should have "Co-developed-by:"s for each person, since this
> isn't explicitly a S-o-B chain...

Noted. I'll fix this up for v2 of the patch.

> > > @@ -643,6 +644,7 @@ static int ramoops_parse_dt(struct platform_device *pdev,
> > >  {
> > >         struct device_node *of_node = pdev->dev.of_node;
> > >         struct device_node *parent_node;
> > > +       struct reserved_mem *rmem;
> > >         struct resource *res;
> > >         u32 value;
> > >         int ret;
> > > @@ -651,13 +653,20 @@ static int ramoops_parse_dt(struct platform_device *pdev,
> > >
> > >         res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> > >         if (!res) {
> > > -               dev_err(&pdev->dev,
> > > -                       "failed to locate DT /reserved-memory resource\n");
> > > -               return -EINVAL;
> > > +               rmem = of_reserved_mem_lookup(of_node);
> > 
> > Nit: you could keep rmem scoped locally here.
> > 
> > Otherwise the code looks sane, I just suspect the commit message could
> > be more clear in explaining the need/utility of the dts entry using
> > alloc-ranges.
> 
> I haven't looked closely at the API here, but does this need a "put"
> like the "get" stuff? (I assume not, given the "lookup" is on a node...)

No, it doesn't need a put, since of_reserved_mem_lookup() doesn't
acquire a reference to anything.

Thanks,
Isaac