Re: [PATCH 12/16] sched_ext: Forbid cpu-form kfuncs from cid-form schedulers

From: Cheng-Yang Chou

Date: Thu Apr 23 2026 - 06:07:03 EST

Hi Zhao,

On Thu, Apr 23, 2026 at 05:35:16PM +0800, Zhao Mengmeng wrote:
> On 2026-04-23 17:21 +0800, Cheng-Yang Chou wrote:
> > Hi Zhao,
> >
> > On Thu, Apr 23, 2026 at 05:03:23PM +0800, Zhao Mengmeng wrote:
> > > non_scx_kfunc_deny.bpf.c:17:6: error: conflicting types for 'scx_bpf_kick_cpu'
> > > 17 | void scx_bpf_kick_cpu(s32 cpu, u64 flags) __ksym;
> > > | ^
> > > /root/work/source-code/linux-next/tools/testing/selftests/sched_ext/build/include/vmlinux.h:136300:13: note: previous declaration is here
> > > 136300 | extern void scx_bpf_kick_cpu(s32 cpu, u64 flags, const struct bpf_prog_aux *aux) __weak __ksym;
> > > | ^
> > > non_scx_kfunc_deny.bpf.c:26:23: error: too few arguments to function call, expected 3, have 2
> > > 26 | scx_bpf_kick_cpu(0, 0);
> > > | ~~~~~~~~~~~~~~~~ ^
> > > /root/work/source-code/linux-next/tools/testing/selftests/sched_ext/build/include/vmlinux.h:136300:13: note: 'scx_bpf_kick_cpu' declared here
> > > 136300 | extern void scx_bpf_kick_cpu(s32 cpu, u64 flags, const struct bpf_prog_aux *aux) __weak __ksym;
> > >
> > > On the one hand, non_scx_kfunc_deny.bpf.c has it own problem, it should
> > > not declare scx_bpf_kick_cpu(), on the other hand, the root cause is after
> >
> > If your point is that a TCP program shouldn't declare scx_bpf_kick_cpu(),
> > you are correct. The main goal of non_scx_kfunc_deny is to ensure that,
> > witch commit 2d2b026c3ea7, no non-SCX programs can use SCX func.
>
> No, I mean this bpf program can just `#include <scx/common.bpf.h>` at the
> start of the file, remove the scx_bpf_kick_cpu() declaration.
>
> diff --git a/tools/testing/selftests/sched_ext/non_scx_kfunc_deny.bpf.c b/tools/testing/selftests/sched_ext/non_scx_kfunc_deny.bpf.c
> index 9f16d39255e7..0d6fcc8e5eb6 100644
> --- a/tools/testing/selftests/sched_ext/non_scx_kfunc_deny.bpf.c
> +++ b/tools/testing/selftests/sched_ext/non_scx_kfunc_deny.bpf.c
> @@ -9,12 +9,7 @@
> * Copyright (C) 2026 Cheng-Yang Chou <yphbchou0911@xxxxxxxxx>
> */
>
> -#include <vmlinux.h>
> -#include <bpf/bpf_helpers.h>
> -#include <bpf/bpf_tracing.h>
> -
> -/* SCX kfunc from scx_kfunc_ids_any set */
> -void scx_bpf_kick_cpu(s32 cpu, u64 flags) __ksym;
> +#include <scx/common.bpf.h>
>
> SEC("struct_ops/ssthresh")
> __u32 BPF_PROG(tcp_ca_ssthresh, struct sock *sk)
>
> But before this commit, your code builds well, after this commit, it
> failed. That's why I suggest Tejun to add the missing KF_IMPLICIT_ARGS, it
> changed the signature in generated vmlinux.h, which I think we don't need
> expose aux argument to bpf program.

Ahh, thanks for the explanation, now I get it.
Are you planning to send a patch to fix this, or would you like me to
spin one?

I tested the fix above, and everything works fine.

> >
> > > this commit, the signature of scx_bpf_kick_cpu() changes from
> > >
> > > `extern void scx_bpf_kick_cpu(s32 cpu, u64 flags) __weak __ksym;` to
> > > `extern void scx_bpf_kick_cpu(s32 cpu, u64 flags, const struct bpf_prog_aux *aux) __weak __ksym`
> >
> > Regarding the non_scx_kfunc_deny.bpf.c build failure, I'll fix it. If
> > needed, we can test with another SCX function that doesn't require the
> > *aux parameter.
> >
> > Thanks.
>
> Like I said above, it builds well on for-next branch, just this commit
> trigger the failure. I believe your test case is very reasonable.
>
> Best regards,
> --
> Zhao Mengmeng <zhaomengmeng@xxxxxxxxxx>
> > >
> > > After code analysis and test, I believe scx_bpf_kick_cpu miss the
> > > KF_IMPILCIT_ARGS, just like the defination in scx_kfunc_ids_any.
> > >
> > > So here misses KF_IMPLICIT_ARGS
> > > > +BTF_ID_FLAGS(func, scx_bpf_task_cpu)
> > > Missing KF_RCU.
> > > > +BTF_ID_FLAGS(func, scx_bpf_cpu_rq)
> > > Missing KF_IMPLICIT_ARGS
> > > > +BTF_ID_FLAGS(func, scx_bpf_cpu_curr)
> > > Missing KF_IMPLICIT_ARGS | KF_RET_NULL | KF_RCU_PROTECTED
> > > > +BTF_ID_FLAGS(func, scx_bpf_cpu_node)
> > > > +BTF_ID_FLAGS(func, scx_bpf_cpuperf_cap)
> > > Missing KF_IMPLICIT_ARGS
> > > > +BTF_ID_FLAGS(func, scx_bpf_cpuperf_cur)
> > > Missing KF_IMPLICIT_ARGS
> > > > +BTF_ID_FLAGS(func, scx_bpf_cpuperf_set)
> > > Missing KF_IMPLICIT_ARGS
> > > > +BTF_ID_FLAGS(func, scx_bpf_get_possible_cpumask)
> > > Missing KF_ACQUIRE
> > > > +BTF_ID_FLAGS(func, scx_bpf_get_online_cpumask)
> > > Missing KF_ACQUIRE
> > > > +BTF_ID_FLAGS(func, scx_bpf_put_cpumask)
> > > Missing KF_RELEASE
> > > > +BTF_ID_FLAGS(func, scx_bpf_select_cpu_dfl)
> > > Missing KF_IMPLICIT_ARGS | KF_RCU
> > > > +BTF_ID_FLAGS(func, __scx_bpf_select_cpu_and)
> > > Missing KF_IMPLICIT_ARGS | KF_RCU
> > > > +BTF_ID_FLAGS(func, scx_bpf_select_cpu_and)
> > > Missing KF_RCU
> > >
> > > Please correct me if I miss something.
> > > > +BTF_ID_FLAGS(func, scx_bpf_get_idle_cpumask)
> > > > +BTF_ID_FLAGS(func, scx_bpf_get_idle_cpumask_node)
> > > > +BTF_ID_FLAGS(func, scx_bpf_get_idle_smtmask)
> > > > +BTF_ID_FLAGS(func, scx_bpf_get_idle_smtmask_node)
> > > > +BTF_ID_FLAGS(func, scx_bpf_put_idle_cpumask)
> > > > +BTF_ID_FLAGS(func, scx_bpf_test_and_clear_cpu_idle)
> > > > +BTF_ID_FLAGS(func, scx_bpf_pick_idle_cpu)
> > > > +BTF_ID_FLAGS(func, scx_bpf_pick_idle_cpu_node)
> > > > +BTF_ID_FLAGS(func, scx_bpf_pick_any_cpu)
> > > > +BTF_ID_FLAGS(func, scx_bpf_pick_any_cpu_node)
> > > > +BTF_KFUNCS_END(scx_kfunc_ids_cpu_only)
> > > > +
> > > > /*
> > > > * 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
> > > > @@ -10031,6 +10067,7 @@ 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. */
> > > > @@ -10068,6 +10105,15 @@ 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;
> > > > --
> > > > 2.53.0
> > > >
> > > >
> > >
> > >
> > >
> >
> > --
> > Cheers,
> > Cheng-Yang
> >
>
>

--
Cheers,
Cheng-Yang