Re: [RFC PATCH 1/4] bpf, cgroup: fix cgroup struct_ops query for a second attach type

[Yafang Shao <laoar.shao@gmail.com>]

On Tue, Sep 22, 2026 at 3:31 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> Two things in __cgroup_bpf_query() work only because CGROUP_TCP_SOCK_OPS is
> the only one struct_ops attach type.
>
> It calls cgroup_bpf_enabled(atype) with an atype that
> find_atype_by_struct_ops_id() works out at runtime.  That macro is an asm
> goto and needs a constant.  Today the compiler can see there is only one
> value; add a second type and the build breaks with "impossible constraint in
> 'asm'".  Add cgroup_bpf_enabled_runtime(), which reads the key instead, and
> use it here.  This is a syscall path, so the cost does not matter.
>
> And find_atype_by_struct_ops_id() matches on type_id alone.  An attach type
> whose subsystem is not built keeps type_id 0, so a query for type 0 finds it
> and returns success with nothing instead of -ENOENT.  Skip such slots.
>
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>

Acked-by: Yafang Shao <laoar.shao@gmail.com>

Could you please send this fix separately from the series so it can
land in bpf-next on its own?

> ---
>  include/linux/bpf-cgroup.h | 9 +++++++++
>  kernel/bpf/cgroup.c        | 3 ++-
>  2 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
> index 8a75a6cd7309..3b2c127d401d 100644
> --- a/include/linux/bpf-cgroup.h
> +++ b/include/linux/bpf-cgroup.h
> @@ -76,6 +76,14 @@ to_cgroup_bpf_attach_type(enum bpf_attach_type attach_type)
>
>  extern struct static_key_false cgroup_bpf_enabled_key[MAX_CGROUP_BPF_ATTACH_TYPE];
>  #define cgroup_bpf_enabled(atype) static_branch_unlikely(&cgroup_bpf_enabled_key[atype])
> +/*
> + * Same test when @atype is not a constant.  cgroup_bpf_enabled() uses
> + * static_branch_unlikely which creates jump-label site and requires statically
> + * selected key. Since key is selected dynamically, use static_key_enabled here
> + * and keep it off fast paths.
> + */
> +#define cgroup_bpf_enabled_runtime(atype) \
> +       static_key_enabled(&cgroup_bpf_enabled_key[atype])
>
>  struct bpf_cgroup_storage_map;
>
> @@ -508,6 +516,7 @@ static inline int cgroup_bpf_struct_ops_attach(struct bpf_map *map,
>  }
>
>  #define cgroup_bpf_enabled(atype) (0)
> +#define cgroup_bpf_enabled_runtime(atype) (0)
>  #define BPF_CGROUP_RUN_SA_PROG_LOCK(sk, uaddr, uaddrlen, atype, t_ctx) ({ 0; })
>  #define BPF_CGROUP_RUN_SA_PROG(sk, uaddr, uaddrlen, atype) ({ 0; })
>  #define BPF_CGROUP_PRE_CONNECT_ENABLED(sk) (0)
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 696b27383974..99b4e96f5db7 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -45,6 +45,7 @@ static enum cgroup_bpf_attach_type find_atype_by_struct_ops_id(u32 type_id)
>
>         for (atype = 0; atype < MAX_CGROUP_BPF_ATTACH_TYPE; atype++) {
>                 if (cgroup_bpf_is_struct_ops_atype(atype) &&
> +                   cgroup_struct_ops[atype].type_id &&
>                     cgroup_struct_ops[atype].type_id == type_id)
>                         return atype;
>         }
> @@ -1448,7 +1449,7 @@ static int __cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
>                         return -ENOENT;
>                 from_atype = to_atype = atype;
>                 flags = 0;
> -               if (!cgroup_bpf_enabled(atype))
> +               if (!cgroup_bpf_enabled_runtime(atype))
>                         goto skip_count;
>         } else if (type == BPF_LSM_CGROUP) {
>                 if (!effective_query && attr->query.prog_cnt &&
> --
> 2.53.0-Meta
>
>


-- 
Regards
Yafang