linux-kernel - Re: [PATCH 4.14 07/15] driver core: Extend device_is

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Date:   Fri, 5 Feb 2021 09:52:56 -0800
From:   Saravana Kannan <saravanak@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Stephan Gerhold <stephan@...hold.net>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>
Subject: Re: [PATCH 4.14 07/15] driver core: Extend device_is_dependent()

On Fri, Feb 5, 2021 at 6:14 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
>
> commit 3d1cf435e201d1fd63e4346b141881aed086effd upstream
>
> If the device passed as the target (second argument) to
> device_is_dependent() is not completely registered (that is, it has
> been initialized, but not added yet), but the parent pointer of it
> is set, it may be missing from the list of the parent's children
> and device_for_each_child() called by device_is_dependent() cannot
> be relied on to catch that dependency.
>
> For this reason, modify device_is_dependent() to check the ancestors
> of the target device by following its parent pointer in addition to
> the device_for_each_child() walk.
>
> Fixes: 9ed9895370ae ("driver core: Functional dependencies tracking support")
> Reported-by: Stephan Gerhold <stephan@...hold.net>
> Tested-by: Stephan Gerhold <stephan@...hold.net>
> Reviewed-by: Saravana Kannan <saravanak@...gle.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Link: https://lore.kernel.org/r/17705994.d592GUb2YH@kreacher
> Cc: stable <stable@...r.kernel.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@...il.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  drivers/base/core.c |   17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -96,6 +96,16 @@ void device_links_read_unlock(int not_us
>  }
>  #endif /* !CONFIG_SRCU */
>
> +static bool device_is_ancestor(struct device *dev, struct device *target)
> +{
> +       while (target->parent) {
> +               target = target->parent;
> +               if (dev == target)
> +                       return true;
> +       }
> +       return false;
> +}
> +
>  /**
>   * device_is_dependent - Check if one device depends on another one
>   * @dev: Device to check dependencies for.
> @@ -109,7 +119,12 @@ static int device_is_dependent(struct de
>         struct device_link *link;
>         int ret;
>
> -       if (dev == target)
> +       /*
> +        * The "ancestors" check is needed to catch the case when the target
> +        * device has not been completely initialized yet and it is still
> +        * missing from the list of children of its parent device.
> +        */
> +       if (dev == target || device_is_ancestor(dev, target))
>                 return 1;
>
>         ret = device_for_each_child(dev, target, device_is_dependent);
>
>

I think 4.14 device links API is so busted it's not worth patching
this. It's not a memory leak or any kind of security issue.

Stephan, are you seeing this issue in 4.14 or even care about it?

-Saravana