Re: [PATCH 13/17] sched_ext: Forbid cpu-form kfuncs from cid-form schedulers

[Zhao Mengmeng]

On 4/24/26 09:32, Tejun Heo wrote:
> cid and cpu are both small s32s, trivially confused when a cid-form
> scheduler calls a cpu-keyed kfunc. Reject cid-form programs that
> reference any kfunc in the new scx_kfunc_ids_cpu_only at verifier load
> time.
> 
> The reverse direction is intentionally permissive: cpu-form schedulers
> can freely call cid-form kfuncs to ease a gradual cpumask -> cid
> migration.
> 
> The check sits in scx_kfunc_context_filter() right after the SCX
> struct_ops gate and before the any/idle allow and per-op allow-list
> checks, so it catches cpu-only kfuncs regardless of which set they
> belong to (any, idle, or select_cpu).
> 
> v2: Sync per-entry kfunc flags with their primary declarations (Zhao).
>     pahole intersects flags across BTF_ID_FLAGS() occurrences, so
>     omitting them drops the flags globally.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Reviewed-by: Cheng-Yang Chou <yphbchou0911@xxxxxxxxx>
> Cc: Zhao Mengmeng <zhaomzhao@xxxxxxx>
> ---
>  kernel/sched/ext.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 51 insertions(+)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index abc0c798150d..37f37f31b025 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -9979,6 +9979,47 @@ static const struct btf_kfunc_id_set scx_kfunc_set_any = {
>  	.filter			= scx_kfunc_context_filter,
>  };
>  
> +/*
> + * cpu-form kfuncs that are forbidden from cid-form schedulers
> + * (bpf_sched_ext_ops_cid). Programs targeting the cid struct_ops type must
> + * use the cid-form alternative (cid/cmask kfuncs).
> + *
> + * Membership overlaps with scx_kfunc_ids_{any,idle,select_cpu}; the filter
> + * tests this set independently and rejects matches before the per-op
> + * allow-list check runs.
> + *
> + * pahole/resolve_btfids scans every BTF_ID_FLAGS() at build time and
> + * intersects flags across duplicate entries, so each entry must carry the
> + * same flags as the kfunc's primary declaration; otherwise the flags get
> + * dropped globally.
> + */
> +BTF_KFUNCS_START(scx_kfunc_ids_cpu_only)
> +BTF_ID_FLAGS(func, scx_bpf_kick_cpu, KF_IMPLICIT_ARGS)
> +BTF_ID_FLAGS(func, scx_bpf_task_cpu, KF_RCU)
> +BTF_ID_FLAGS(func, scx_bpf_cpu_rq, KF_IMPLICIT_ARGS)
> +BTF_ID_FLAGS(func, scx_bpf_cpu_curr, KF_IMPLICIT_ARGS | KF_RET_NULL | KF_RCU_PROTECTED)
> +BTF_ID_FLAGS(func, scx_bpf_cpu_node, KF_IMPLICIT_ARGS)
> +BTF_ID_FLAGS(func, scx_bpf_cpuperf_cap, KF_IMPLICIT_ARGS)
> +BTF_ID_FLAGS(func, scx_bpf_cpuperf_cur, KF_IMPLICIT_ARGS)
> +BTF_ID_FLAGS(func, scx_bpf_cpuperf_set, KF_IMPLICIT_ARGS)
> +BTF_ID_FLAGS(func, scx_bpf_get_possible_cpumask, KF_ACQUIRE)
> +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_select_cpu_dfl, KF_IMPLICIT_ARGS | KF_RCU)
> +BTF_ID_FLAGS(func, __scx_bpf_select_cpu_and, KF_IMPLICIT_ARGS | KF_RCU)
> +BTF_ID_FLAGS(func, scx_bpf_select_cpu_and, KF_RCU)
> +BTF_ID_FLAGS(func, scx_bpf_get_idle_cpumask, KF_IMPLICIT_ARGS | KF_ACQUIRE)
> +BTF_ID_FLAGS(func, scx_bpf_get_idle_cpumask_node, KF_IMPLICIT_ARGS | KF_ACQUIRE)
> +BTF_ID_FLAGS(func, scx_bpf_get_idle_smtmask, KF_IMPLICIT_ARGS | KF_ACQUIRE)
> +BTF_ID_FLAGS(func, scx_bpf_get_idle_smtmask_node, KF_IMPLICIT_ARGS | KF_ACQUIRE)
> +BTF_ID_FLAGS(func, scx_bpf_put_idle_cpumask, KF_RELEASE)
> +BTF_ID_FLAGS(func, scx_bpf_test_and_clear_cpu_idle, KF_IMPLICIT_ARGS)
> +BTF_ID_FLAGS(func, scx_bpf_pick_idle_cpu, KF_IMPLICIT_ARGS | KF_RCU)
> +BTF_ID_FLAGS(func, scx_bpf_pick_idle_cpu_node, KF_IMPLICIT_ARGS | KF_RCU)
> +BTF_ID_FLAGS(func, scx_bpf_pick_any_cpu, KF_IMPLICIT_ARGS | KF_RCU)
> +BTF_ID_FLAGS(func, scx_bpf_pick_any_cpu_node, KF_IMPLICIT_ARGS | KF_RCU)
> +BTF_KFUNCS_END(scx_kfunc_ids_cpu_only)
> +

Hi Tejun,

Build it and check the vmlinux.h, no signature changes, selftests build well. So looks good to me.
 
Reviewed-by: Zhao Mengmeng <zhaomengmeng@xxxxxxxxxx>
>  /*
>   * Per-op kfunc allow flags. Each bit corresponds to a context-sensitive kfunc
>   * group; an op may permit zero or more groups, with the union expressed in
> @@ -10042,6 +10083,7 @@ int scx_kfunc_context_filter(const struct bpf_prog *prog, u32 kfunc_id)
>  	bool in_cpu_release = btf_id_set8_contains(&scx_kfunc_ids_cpu_release, kfunc_id);
>  	bool in_idle = btf_id_set8_contains(&scx_kfunc_ids_idle, kfunc_id);
>  	bool in_any = btf_id_set8_contains(&scx_kfunc_ids_any, kfunc_id);
> +	bool in_cpu_only = btf_id_set8_contains(&scx_kfunc_ids_cpu_only, kfunc_id);
>  	u32 moff, flags;
>  
>  	/* Not an SCX kfunc - allow. */
> @@ -10079,6 +10121,15 @@ int scx_kfunc_context_filter(const struct bpf_prog *prog, u32 kfunc_id)
>  	    prog->aux->st_ops != &bpf_sched_ext_ops_cid)
>  		return -EACCES;
>  
> +	/*
> +	 * cid-form schedulers must use cid/cmask kfuncs. cid and cpu are both
> +	 * small s32s and trivially confused, so cpu-only kfuncs are rejected at
> +	 * load time. The reverse (cpu-form calling cid-form kfuncs) is
> +	 * intentionally permissive to ease gradual cpumask -> cid migration.
> +	 */
> +	if (prog->aux->st_ops == &bpf_sched_ext_ops_cid && in_cpu_only)
> +		return -EACCES;
> +
>  	/* SCX struct_ops: check the per-op allow list. */
>  	if (in_any || in_idle)
>  		return 0;