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From:   Alessandro Carminati <alessandro.carminati@gmail.com>
Date:   Tue, 12 Sep 2023 19:05:19 +0200
Message-ID: <CAPp5cGSbdhLb2c5hQrP-O-+j63haXs0c6=JCoFxnAzF1mhspuA@mail.gmail.com>
Subject: Re: [PATCH] Sanitize possible_parent_show to Handle Return Value of of_clk_get_parent_name
To:     Stephen Boyd <sboyd@kernel.org>
Cc:     Philip Daly <pdaly@redhat.com>,
        Michael Turquette <mturquette@baylibre.com>,
        linux-clk@vger.kernel.org, linux-kernel@vger.kernel.org
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Hello Stephen,

Thank you for your time and your work.

Il giorno ven 8 set 2023 alle ore 23:25 Stephen Boyd
<sboyd@kernel.org> ha scritto:
>
> Quoting Alessandro Carminati (2023-09-08 01:36:14)
> > Hello Stephen,
> > Thank you for your review and the time you dedicated to it.
> >
> > Il giorno ven 8 set 2023 alle ore 00:33 Stephen Boyd
> > <sboyd@kernel.org> ha scritto:
> > >
> > > Quoting Alessandro Carminati (2023-09-07 07:15:36)
> > > > this is a gentle ping
> > > >
> > >
> > > I couldn't read your email because it was sent to nobody
> > > (unlisted-recipients). Can you resend with a proper To: line?
> > >
> > > >
> > > > Il giorno mar 22 ago 2023 alle ore 15:49 Alessandro Carminati
> > > > <alessandro.carminati@gmail.com> ha scritto:
> > > > >
> > > > > In the possible_parent_show function, ensure proper handling of the return
> > > > > value from of_clk_get_parent_name to prevent potential issues arising from
> > > > > a NULL return.
> > > > > The current implementation invokes seq_puts directly on the result of
> > > > > of_clk_get_parent_name without verifying the return value, which can lead
> > > > > to kernel panic if the function returns NULL.
> > > > >
> > > > > This patch addresses the concern by introducing a check on the return
> > > > > value of of_clk_get_parent_name. If the return value is not NULL, the
> > >
> > > Use of_clk_get_parent_name() to signify that it is a function.
> > >
> > > > > function proceeds to call seq_puts, providing the returned value as
> > > > > argument.
> > > > > However, if of_clk_get_parent_name returns NULL, the function provides a
> > > > > static string as argument, avoiding the panic.
> > > > >
> > > > > Reported-by: Philip Daly <pdaly@redhat.com>
> > > > > Signed-off-by: Alessandro Carminati (Red Hat) <alessandro.carminati@gmail.com>
> > > > > ---
> > >
> > > It needs a Fixes tag.
> > Sure!
> > I need to be more careful on this.
> >
> > >
> > > > >  drivers/clk/clk.c | 11 ++++++-----
> > > > >  1 file changed, 6 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> > > > > index c249f9791ae8..ab999644e185 100644
> > > > > --- a/drivers/clk/clk.c
> > > > > +++ b/drivers/clk/clk.c
> > > > > @@ -3416,6 +3416,7 @@ static void possible_parent_show(struct seq_file *s, struct clk_core *core,
> > > > >                                  unsigned int i, char terminator)
> > > > >  {
> > > > >         struct clk_core *parent;
> > > > > +       const char *tmp;
> > > > >
> > > > >         /*
> > > > >          * Go through the following options to fetch a parent's name.
> > > > > @@ -3436,12 +3437,12 @@ static void possible_parent_show(struct seq_file *s, struct clk_core *core,
> > > > >                 seq_puts(s, core->parents[i].name);
> > > > >         else if (core->parents[i].fw_name)
> > > > >                 seq_printf(s, "<%s>(fw)", core->parents[i].fw_name);
> > > > > -       else if (core->parents[i].index >= 0)
> > > > > -               seq_puts(s,
> > > > > -                        of_clk_get_parent_name(core->of_node,
> > > > > -                                               core->parents[i].index));
> > > > > -       else
> > > > > +       else if (core->parents[i].index >= 0) {
> > > > > +               tmp = of_clk_get_parent_name(core->of_node, core->parents[i].index);
> > > > > +               seq_puts(s, tmp ? tmp : "(none)");
> > >
> > > How about using seq_printf("%s", ...) instead? That should print out
> > > "(null)" in the case that it is NULL, instead of "(none)" and it is a
> > > one line change.
> >
> > I did consider using seq_printf("%s", ...) as an alternative approach to
> > address the issue initially.
> > However, after a review of the file's history, I arrived at a different
> > conclusion.
> >
> > The commit [1] that introduced this bug was primarily focused on replacing
> > seq_printf() with seq_putc().
> > I considered that to maintain code consistency and align with the intentions
> > of that commit, it may be more appropriate to continue using seq_putc() in
> > this particular instance.
> > I agree however with the idea of rolling back that particular change, but
> > in this case, we perhaps may want to consider also [2], a similar change made
> > in the same period.
> > I haven't proceeded with a patch submission for it[2], mainly due to the lack
> > of evidence of a kernel splash related to it and my uncertainty around the
> > fact that can exist use cases where the name field in the struct cgroup_subsys
> > can hit that code set to NULL.
>
> Is nothing actually wrong? And this is a speculative patch?

In the current state, Linux can crash, so I would say that there's something
wrong.
I submitted this patch in response to a specific bug report that caused a
kernel crash during testing.

>
> All other arms of this conditional statement check the validity of the
> pointer before printing the string. And when the parent isn't known we
> print "(missing)", so it looks like we should do that instead. How about
> this patch?

Indeed, your patch appears to provide a more verbose coding and accurate
message, which is valuable for troubleshooting.
So yeah, for what it is worth, I think it is good as well.

>
> ----8<---
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index c249f9791ae8..473563bc7496 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -3416,6 +3416,7 @@ static void possible_parent_show(struct seq_file *s, struct clk_core *core,
>                                  unsigned int i, char terminator)
>  {
>         struct clk_core *parent;
> +       const char *name = NULL;
>
>         /*
>          * Go through the following options to fetch a parent's name.
> @@ -3430,18 +3431,20 @@ static void possible_parent_show(struct seq_file *s, struct clk_core *core,
>          * registered (yet).
>          */
>         parent = clk_core_get_parent_by_index(core, i);
> -       if (parent)
> +       if (parent) {
>                 seq_puts(s, parent->name);
> -       else if (core->parents[i].name)
> +       } else if (core->parents[i].name) {
>                 seq_puts(s, core->parents[i].name);
> -       else if (core->parents[i].fw_name)
> +       } else if (core->parents[i].fw_name) {
>                 seq_printf(s, "<%s>(fw)", core->parents[i].fw_name);
> -       else if (core->parents[i].index >= 0)
> -               seq_puts(s,
> -                        of_clk_get_parent_name(core->of_node,
> -                                               core->parents[i].index));
> -       else
> -               seq_puts(s, "(missing)");
> +       } else {
> +               if (core->parents[i].index >= 0)
> +                       name = of_clk_get_parent_name(core->of_node, core->parents[i].index);
> +               if (!name)
> +                       name = "(missing)";
> +
> +               seq_puts(s, name);
> +       }
>
>         seq_putc(s, terminator);
>  }

Regards.
Alessandro