Re: [PATCH] sched_ext: Mark scx_bpf_cpu_rq as NULL returnable

[Andrea Righi <arighi@nvidia.com>]

Hi Christian,

On Tue, Jul 29, 2025 at 05:03:36PM +0100, Christian Loehle wrote:
> scx_bpf_cpu_rq() obviously returns NULL on invalid cpu.
> Mark it as such.

Makes sense. Maybe add something like the following to the commit message
to better justify why we want the verifier to enforce the NULL check:

While scx_ops_error() is triggered on invalid CPUs, the BPF scheduler
doesn't exit immediately, so there's a window where scx_bpf_cpu_rq() may
return NULL and still be dereferenced.

With something like that:

Acked-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> 
> Cc: stable@vger.kernel.org
> Fixes: 6203ef73fa5c ("sched/ext: Add BPF function to fetch rq")
> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
> ---
>  kernel/sched/ext.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 7dd5cbcb7a06..b734f55f3318 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -7599,7 +7599,7 @@ BTF_ID_FLAGS(func, scx_bpf_get_online_cpumask, KF_ACQUIRE)
>  BTF_ID_FLAGS(func, scx_bpf_put_cpumask, KF_RELEASE)
>  BTF_ID_FLAGS(func, scx_bpf_task_running, KF_RCU)
>  BTF_ID_FLAGS(func, scx_bpf_task_cpu, KF_RCU)
> -BTF_ID_FLAGS(func, scx_bpf_cpu_rq)
> +BTF_ID_FLAGS(func, scx_bpf_cpu_rq, KF_RET_NULL)
>  #ifdef CONFIG_CGROUP_SCHED
>  BTF_ID_FLAGS(func, scx_bpf_task_cgroup, KF_RCU | KF_ACQUIRE)
>  #endif
> -- 
> 2.34.1