linux-kernel - Re: [PATCH-next] sched: Fix Null pointer derefrence

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Date:   Sun, 16 May 2021 17:37:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Khaled Romdhani <khaledromdhani216@...il.com>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH-next] sched: Fix Null pointer derefrence

On Sun, May 16, 2021 at 02:01:29PM +0100, Khaled Romdhani wrote:
> On Sat, May 15, 2021 at 11:02:38PM +0200, Peter Zijlstra wrote:
> > On Sat, May 15, 2021 at 05:46:45PM +0100, Khaled ROMDHANI wrote:
> > > The 'curr' variable could be NULL and derefrenced by
> > > pick_next_entity. Fix this by adding a check that prevent
> > > the invocation of pick_next_entity with a NULL passed argument.
> > 
> > And why exactly is that a problem?
> >
> Within pick_next_entity, we could have 'left = curr'. 
> Thus the function wakeup_preempt_entity which is invoked 
> from pick_next_entity, derefrence the the sched entity 'left' 
> that may be a NULL passed argument.

We call pick_next_entity() from pick_task_fair() (or
pick_next_task_fair(), same argument for both), we only call it when
cfs_rq->nr_running != 0. IOW we *know* there is at least one task.

Therefore, if curr == NULL, we *must* have left.

If you can't read code, stay away from Coverity, it's crap.