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Date:   Sun, 16 May 2021 17:37:28 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Khaled Romdhani <khaledromdhani216@gmail.com>
Cc:     mingo@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, linux-kernel@vger.kernel.org,
        kernel-janitors@vger.kernel.org
Subject: Re: [PATCH-next] sched: Fix Null pointer derefrence
Message-ID: <YKE8OHsq4IHN9q8W@hirez.programming.kicks-ass.net>
References: <20210515164645.22849-1-khaledromdhani216@gmail.com>
 <20210515210238.GG5618@worktop.programming.kicks-ass.net>
 <20210516130129.GA32445@ard0534>
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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.