linux-kernel - Re: [PATCH 0/3] sched_ext: Harden scx_bpf_cpu

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Message-ID: <aIzRkEZLGSFybEfd@gpd4>
Date: Fri, 1 Aug 2025 16:39:12 +0200
From: Andrea Righi <arighi@...dia.com>
To: Christian Loehle <christian.loehle@....com>
Cc: linux-kernel@...r.kernel.org, sched-ext@...ts.linux.dev, tj@...nel.org,
	void@...ifault.com, mingo@...hat.com, peterz@...radead.org
Subject: Re: [PATCH 0/3] sched_ext: Harden scx_bpf_cpu_rq()

Hi Christian,

thanks for tackling this! Comments below.

On Fri, Aug 01, 2025 at 03:17:38PM +0100, Christian Loehle wrote:
> scx_bpf_cpu_rq() currently allows accessing struct rq fields without
> holding the associated rq.
> It is being used by scx_cosmos, scx_flash, scx_lavd, scx_layered, and
> scx_tickless. Fortunately it is only ever used to fetch rq->curr.
> So provide an alternative scx_bpf_remote_curr() that doesn't expose
> struct rq and harden scx_bpf_cpu_rq() by ensuring we hold the rq lock.
> 
> This also simplifies scx code from:
> 
> rq = scx_bpf_cpu_rq(cpu);
> if (!rq)
> 	return;
> p = rq->curr
> if (!p)
> 	return;
> /* ... Do something with p */
> 
> into:
> 
> p = scx_bpf_remote_curr(cpu);
> if (!p)
> 	return;
> /* ... Do something with p */

To be 100% correct I think we should do something similar to
bpf_task_from_pid(), acquire a reference to the task and release it via
bpf_task_release().

Basically:

 p = scx_bpf_remote_curr(cpu);
 if (!p)
 	return;
 /* ... Do something with p */
 bpf_task_release(p);

Thanks
-Andrea