Re: [PATCH 0/3] sched_ext: Harden scx_bpf_cpu_rq()

[Andrea Righi <arighi@nvidia.com>]

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