linux-kernel - Re: [PATCH v2 sched_ext/for-6.18] sched_ext: Fix NULL dereference in scx_bpf_cpu

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Message-ID: <aLn2T4LUePv9gAwc@slm.duckdns.org>
Date: Thu, 4 Sep 2025 10:27:59 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	Christian Loehle <christian.loehle@....com>,
	Jake Hillion <jakehillion@...a.com>, sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 sched_ext/for-6.18] sched_ext: Fix NULL dereference in
 scx_bpf_cpu_rq() warning

On Thu, Sep 04, 2025 at 08:23:48PM +0200, Andrea Righi wrote:
> When printing the deprecation warning for scx_bpf_cpu_rq(), we may hit a
> NULL pointer dereference if the kfunc is called before a BPF scheduler
> is fully attached, for example, when invoked from a BPF timer or during
> ops.init():
> 
>  [   50.752775] BUG: kernel NULL pointer dereference, address: 0000000000000331
>  ...
>  [   50.764205] RIP: 0010:scx_bpf_cpu_rq+0x30/0xa0
>  ...
>  [   50.787661] Call Trace:
>  [   50.788398]  <TASK>
>  [   50.789061]  bpf_prog_08f7fd2dcb187aaf_wakeup_timerfn+0x75/0x1a8
>  [   50.792477]  bpf_timer_cb+0x7e/0x140
>  [   50.796003]  hrtimer_run_softirq+0x91/0xe0
>  [   50.796952]  handle_softirqs+0xce/0x3c0
>  [   50.799087]  run_ksoftirqd+0x3e/0x70
>  [   50.800197]  smpboot_thread_fn+0x133/0x290
>  [   50.802320]  kthread+0x115/0x220
>  [   50.804984]  ret_from_fork+0x17a/0x1d0
>  [   50.806920]  ret_from_fork_asm+0x1a/0x30
>  [   50.807799]  </TASK>
> 
> Fix this by only printing the warning once the scheduler is fully
> registered.
> 
> Fixes: 5c48d88fe0049 ("sched_ext: deprecation warn for scx_bpf_cpu_rq()")
> Cc: Christian Loehle <christian.loehle@....com>
> Signed-off-by: Andrea Righi <arighi@...dia.com>

Applied to sched_ext/for-6.18.

Thanks.

-- 
tejun