linux-kernel - Re: [PATCH v1 3/7] powercap/dtpm: Fixup kfree for virtual node

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Date:   Thu, 17 Feb 2022 14:17:13 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     rjw@...ysocki.net, heiko@...ech.de, lukasz.luba@....com,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Daniel Lezcano <daniel.lezcano@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH v1 3/7] powercap/dtpm: Fixup kfree for virtual node

On Wed, 16 Feb 2022 at 19:10, Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
>
> On 16/02/2022 17:22, Ulf Hansson wrote:
> > On Sun, 30 Jan 2022 at 22:02, Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
> >>
> >> When the node is virtual there is no release function associated which
> >> can free the memory.
> >>
> >> Free the memory when no 'ops' exists.
> >>
> >> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> >> ---
> >>   drivers/powercap/dtpm.c | 4 ++--
> >>   1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/powercap/dtpm.c b/drivers/powercap/dtpm.c
> >> index 0b0121c37a1b..7bddd25a6767 100644
> >> --- a/drivers/powercap/dtpm.c
> >> +++ b/drivers/powercap/dtpm.c
> >> @@ -181,12 +181,12 @@ int dtpm_release_zone(struct powercap_zone *pcz)
> >>
> >>          if (dtpm->ops)
> >>                  dtpm->ops->release(dtpm);
> >> +       else
> >> +               kfree(dtpm);
> >>
> >
> > This doesn't look correct. Below you check dtpm against "root", which
> > may be after its memory has been freed.
> >
> > If the ->release() function should be responsible for freeing the
> > dtpm, it needs to be called after the check below.
>
> It is harmless, 'root' is not dereferenced but used as an ID
>
> Moreover, in the patch 5/7 it is moved out this function.

Right. It just looks a bit odd here.

>
>
> >>          if (root == dtpm)
> >>                  root = NULL;
> >>
> >> -       kfree(dtpm);

So then why doesn't this kfree do the job already?

kfree(NULL) works fine, if dtpm->ops->release(dtpm) already freed the data.

> >> -
> >>          return 0;
> >>   }
> >>

Kind regards
Uffe