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

[Zhao Mengmeng]

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.
> 
> > 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
>