Re: [PATCH] bpf: add KF_SPIN_LOCK flag for kfuncs under bpf_spin_lock

[Leon Hwang <leon.hwang@linux.dev>]

[PATCH bpf-next] bpf: ...

On 4/8/26 16:32, Kaitao Cheng wrote:
> From: Kaitao Cheng <chengkaitao@kylinos.cn>
> 
> Introduce the KF_SPIN_LOCK kfunc metadata flag in BTF so kfuncs may be
> explicitly marked as safe to call while holding bpf_spin_lock.
> 
> Allow kfuncs defined in kernel modules to be marked with KF_SPIN_LOCK.


Sounds reasonable to me. I did see the requirement when implementing a 
custom struct_ops.

A selftest is required for this new flag.

Thanks,
Leon

> 
> Example: BTF_ID_FLAGS(func, $kfunc_name, KF_SPIN_LOCK)
> 
> Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
> ---
>   include/linux/btf.h   |  1 +
>   kernel/bpf/verifier.c | 20 +++++++++++++++-----
>   2 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index c09b7994de4e..380dc10b6750 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -79,6 +79,7 @@
>   #define KF_ARENA_ARG1   (1 << 14) /* kfunc takes an arena pointer as its first argument */
>   #define KF_ARENA_ARG2   (1 << 15) /* kfunc takes an arena pointer as its second argument */
>   #define KF_IMPLICIT_ARGS (1 << 16) /* kfunc has implicit arguments supplied by the verifier */
> +#define KF_SPIN_LOCK    (1 << 17) /* kfunc is allowed inside bpf_spin_lock-ed region */
>   
>   /*
>    * Tag marking a kernel function as a kfunc. This is meant to minimize the
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 7aa47342dc65..9f3feba3c2fe 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -11657,11 +11657,21 @@ static bool is_bpf_stream_kfunc(u32 btf_id)
>   	       btf_id == special_kfunc_list[KF_bpf_stream_print_stack];
>   }
>   
> -static bool kfunc_spin_allowed(u32 btf_id)
> +static bool kfunc_spin_allowed(struct bpf_verifier_env *env, s32 func_id, s16 offset)
>   {
> -	return is_bpf_graph_api_kfunc(btf_id) || is_bpf_iter_num_api_kfunc(btf_id) ||
> -	       is_bpf_res_spin_lock_kfunc(btf_id) || is_bpf_arena_kfunc(btf_id) ||
> -	       is_bpf_stream_kfunc(btf_id);
> +	struct bpf_kfunc_meta kfunc;
> +	int err;
> +
> +	if (is_bpf_graph_api_kfunc(func_id) || is_bpf_iter_num_api_kfunc(func_id) ||
> +	    is_bpf_res_spin_lock_kfunc(func_id) || is_bpf_arena_kfunc(func_id) ||
> +	    is_bpf_stream_kfunc(func_id))
> +		return true;
> +
> +	err = fetch_kfunc_meta(env, func_id, offset, &kfunc);
> +	if (err || !kfunc.flags)
> +		return false;
> +
> +	return *kfunc.flags & KF_SPIN_LOCK;
>   }
>   
>   static bool is_sync_callback_calling_kfunc(u32 btf_id)
> @@ -17289,7 +17299,7 @@ static int do_check_insn(struct bpf_verifier_env *env, bool *do_print_state)
>   				     insn->imm != BPF_FUNC_spin_unlock &&
>   				     insn->imm != BPF_FUNC_kptr_xchg) ||
>   				    (insn->src_reg == BPF_PSEUDO_KFUNC_CALL &&
> -				     (insn->off != 0 || !kfunc_spin_allowed(insn->imm)))) {
> +				     !kfunc_spin_allowed(env, insn->imm, insn->off))) {
>   					verbose(env,
>   						"function calls are not allowed while holding a lock\n");
>   					return -EINVAL;