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From:   Kees Cook <keescook@chromium.org>
Date:   Mon, 6 May 2019 14:09:55 -0700
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Subject: Re: [PATCH] pstore/ram: Improve backward compatibility with older Chromebooks
To:     Douglas Anderson <dianders@chromium.org>,
        Rob Herring <robh@kernel.org>
Cc:     Anton Vorontsov <anton@enomsg.org>,
        linux-rockchip@lists.infradead.org, jwerner@chromium.org,
        Guenter Roeck <groeck@chromium.org>,
        Matthias Kaehlcke <mka@chromium.org>,
        Brian Norris <briannorris@chromium.org>,
        Colin Cross <ccross@android.com>,
        Tony Luck <tony.luck@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
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From: Douglas Anderson <dianders@chromium.org>
Date: Fri, May 3, 2019 at 10:48 AM
To: Kees Cook, Anton Vorontsov
Cc: <linux-rockchip@lists.infradead.org>, <jwerner@chromium.org>,
<groeck@chromium.org>, <mka@chromium.org>, <briannorris@chromium.org>,
Douglas Anderson, Colin Cross, Tony Luck,
<linux-kernel@vger.kernel.org>

> When you try to run an upstream kernel on an old ARM-based Chromebook
> you'll find that console-ramoops doesn't work.
>
> Old ARM-based Chromebooks, before <https://crrev.com/c/439792>
> ("ramoops: support upstream {console,pmsg,ftrace}-size properties")
> used to create a "ramoops" node at the top level that looked like:
>
> / {
>   ramoops {
>     compatible = "ramoops";
>     reg = <...>;
>     record-size = <...>;
>     dump-oops;
>   };
> };
>
> ...and these Chromebooks assumed that the downstream kernel would make
> console_size / pmsg_size match the record size.  The above ramoops
> node was added by the firmware so it's not easy to make any changes.
>
> Let's match the expected behavior, but only for those using the old
> backward-compatible way of working where ramoops is right under the
> root node.
>
> NOTE: if there are some out-of-tree devices that had ramoops at the
> top level, left everything but the record size as 0, and somehow
> doesn't want this behavior, we can try to add more conditions here.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>

I like this; thanks! Rob is this okay by you? I just want to
double-check since it's part of the DT parsing logic.

I'll pick it up and add a Cc: stable.

-Kees

> ---
>
>  fs/pstore/ram.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
> index c5c685589e36..8df3bfa2837f 100644
> --- a/fs/pstore/ram.c
> +++ b/fs/pstore/ram.c
> @@ -669,6 +669,7 @@ static int ramoops_parse_dt(struct platform_device *pdev,
>                             struct ramoops_platform_data *pdata)
>  {
>         struct device_node *of_node = pdev->dev.of_node;
> +       struct device_node *parent_node;
>         struct resource *res;
>         u32 value;
>         int ret;
> @@ -703,6 +704,23 @@ static int ramoops_parse_dt(struct platform_device *pdev,
>
>  #undef parse_size
>
> +       /*
> +        * Some old Chromebooks relied on the kernel setting the console_size
> +        * and pmsg_size to the record size since that's what the downstream
> +        * kernel did.  These same Chromebooks had "ramoops" straight under
> +        * the root node which isn't according to the upstream bindings.  Let's
> +        * make those old Chromebooks work by detecting this and mimicing the
> +        * expected behavior.
> +        */
> +       parent_node = of_get_parent(of_node);
> +       if (of_node_is_root(parent_node) &&
> +           !pdata->console_size && !pdata->ftrace_size &&
> +           !pdata->pmsg_size && !pdata->ecc_info.ecc_size) {
> +               pdata->console_size = pdata->record_size;
> +               pdata->pmsg_size = pdata->record_size;
> +       }
> +       of_node_put(parent_node);
> +
>         return 0;
>  }
>
> --
> 2.21.0.1020.gf2820cf01a-goog
>


-- 
Kees Cook