linux-kernel - Re: [PATCH] clk: fix a panic error caused by accessing NULL pointer

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Date:   Tue, 5 Dec 2017 15:28:44 -0800
From:   Stephen Boyd <sboyd@...eaurora.org>
To:     Chunyan Zhang <zhang.lyra@...il.com>
Cc:     Chunyan Zhang <chunyan.zhang@...eadtrum.com>,
        Michael Turquette <mturquette@...libre.com>,
        linux-clk <linux-clk@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Cai Li <cai.li@...eadtrum.com>,
        Orson Zhai <orson.zhai@...eadtrum.com>
Subject: Re: [PATCH] clk: fix a panic error caused by accessing NULL pointer

On 11/21, Chunyan Zhang wrote:
> On 21 November 2017 at 16:57, Chunyan Zhang <zhang.lyra@...il.com> wrote:
> > On 21 November 2017 at 03:12, Stephen Boyd <sboyd@...eaurora.org> wrote:
> >> On 11/20, Chunyan Zhang wrote:
> >>> From: Cai Li <cai.li@...eadtrum.com>
> >>>
> >>> In some cases the clock parent would be set NULL when doing re-parent,
> >>> it will cause a NULL pointer accessing if clk_set trace event is enabled,
> >>> since the trace event function would not check the input parameter.
> >>>
> >>> Signed-off-by: Cai Li <cai.li@...eadtrum.com>
> >>> Signed-off-by: Chunyan Zhang <chunyan.zhang@...eadtrum.com>
> >>
> >> Fixes: tag?
> >>
> >>> ---
> >>>  drivers/clk/clk.c | 9 ++++-----
> >>>  1 file changed, 4 insertions(+), 5 deletions(-)
> >>>
> >>> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> >>> index c8d83ac..64efaf0 100644
> >>> --- a/drivers/clk/clk.c
> >>> +++ b/drivers/clk/clk.c
> >>> @@ -1242,13 +1242,12 @@ static int __clk_set_parent(struct clk_core *core, struct clk_core *parent,
> >>>
> >>>       old_parent = __clk_set_parent_before(core, parent);
> >>>
> >>> -     trace_clk_set_parent(core, parent);
> >>> -
> >>>       /* change clock input source */
> >>> -     if (parent && core->ops->set_parent)
> >>> +     if (parent && core->ops->set_parent) {
> >>> +             trace_clk_set_parent(core, parent);
> >>>               ret = core->ops->set_parent(core->hw, p_index);
> >>> -
> >>> -     trace_clk_set_parent_complete(core, parent);
> >>> +             trace_clk_set_parent_complete(core, parent);
> >>> +     }
> >>
> >> Is the problem that parent may be NULL and the tracepoint
> >> dereferences it?
> >
> > Yes, I think that probably is uncommon usage though, it revealed that
> > the tracepoint could be stronger :)
> 
> The reason we need to set the parent as NULL is to disable the clk for
> the purpose of saving power.

Do you have drivers calling set_parent with NULL to save power?
Seems sort of odd. Why not do something when disabling the clk in
clk_disable() path instead? Either way, I'll apply the patch to
clk-fixes.

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