Re: [PATCH bpf-next v7 1/2] bpf: Introduce jit_required flag and refactor kfunc path

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

On 2026/7/3 14:59, Tiezhu Yang wrote:
[...]
>>> ```
>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>> index f496b45b9da4..1f5824c1c691 100644
>>> --- a/kernel/bpf/verifier.c
>>> +++ b/kernel/bpf/verifier.c
>>> @@ -2772,7 +2772,7 @@ int bpf_add_kfunc_call(struct bpf_verifier_env
>>> *env, u32 func_id, u16 offset)
>>>
>>>   bool bpf_prog_has_kfunc_call(const struct bpf_prog *prog)
>>>   {
>>> -       return !!prog->aux->kfunc_tab;
>>> +       return prog->jit_required && !!prog->aux->kfunc_tab;
>>
>>
>> When 'prog->jit_required' is used for JIT-inlineable helper call, this
>> change could also cause false positive for the above pruned kfunc case.
>>
>> If you don't want bpf_fixup_call_args() rejects the program with -EINVAL
>> for the pruned kfunc case, suggest moving 'if (!func_id && !offset)'
>> before the tab allocation in bpf_add_kfunc_call().
> 
> How about this:
> 
> ```
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 25aea4271cd0..c34cc524651a 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2770,7 +2770,7 @@ int bpf_add_kfunc_call(struct bpf_verifier_env
> *env, u32 func_id, u16 offset)
> 
>  bool bpf_prog_has_kfunc_call(const struct bpf_prog *prog)
>  {
> -       return !!prog->aux->kfunc_tab;
> +       return prog->aux->kfunc_tab && prog->aux->kfunc_tab->nr_descs > 0;


NIT: drop '> 0'

Looks better.

>  }
> 
>  static int add_subprog_and_kfunc(struct bpf_verifier_env *env)
> ```
> IMO, there are no side effects for the following four cases:
> 
> 1. Pure JIT-inlined Helper
> 2. Pure Pruned kfunc
> 3. Pruned kfunc + Inlined Helper
> 4. Active (Unpruned) kfunc
> 

This change allows pruned kfunc + interpreter fallback and pruned kfunc
+ bpf_fixup_call_args(), when CONFIG_BPF_JIT_ALWAYS_ON is off.

Does it look like a pre-existing issue?

Pls read "Support kernel module function calls from eBPF" [1] to
understand the background of introducing func_id=0 kfunc.

[1] https://lore.kernel.org/bpf/20211002011757.311265-1-memxor@gmail.com/

Thanks,
Leon