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

[Kaitao Cheng]

在 2026/8/4 16:43, Kumar Kartikeya Dwivedi 写道:
> On Tue Aug 4, 2026 at 10:32 AM CEST, Kaitao Cheng wrote:
>> From: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
>>
>> 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.
>>
>> Example: BTF_ID_FLAGS(func, $kfunc_name, KF_SPIN_LOCK)
>>
>> Signed-off-by: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
>> ---
>>  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;
> 
> Why not apply it to all of the existing kfuncs? Also, use [PATCH bpf-next] subject.

I do not think all kfuncs are suitable for the KF_SPIN_LOCK context.
Kfuncs marked with KF_SLEEPABLE are an obvious example. Developers
should add KF_SPIN_LOCK with great care and ensure that the kfunc
does not sleep and is safe to call in atomic context; otherwise, it
could easily introduce deadlocks.

However, the kfuncs explicitly allowed by kfunc_spin_allowed(), such
as those covered by is_bpf_graph_api_kfunc, is_bpf_iter_num_api_kfunc,
is_bpf_res_spin_lock_kfunc, is_bpf_arena_kfunc and is_bpf_stream_kfunc,
can be marked with KF_SPIN_LOCK. We can then simplify kfunc_spin_allowed()
to rely solely on the flag.

-- 
Thanks
Kaitao Cheng