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

[Cheng-Yang Chou]

Hi Zhao,

On Thu, Apr 23, 2026 at 05:53:13PM +0800, Zhao Mengmeng wrote:
> On 4/23/26 17:48, Cheng-Yang Chou wrote:
> > 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.
> > 
> 
> I am working on a patch and is about to send it, I will add your Tested-by,
> WDYT? 

Thanks for catching this! Feel free to add,

Tested-by: Cheng-Yang Chou <yphbchou0911@xxxxxxxxx>

> >>>
> >>>> 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