Re: [PATCH bpf-next v5 03/11] bpf: Wire up JIT support for 16-byte kfunc returns

[Yonghong Song <yonghong.song@linux.dev>]

On 8/13/26 6:20 PM, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] The verifier fails to initialize BPF_REG_2 for 16-byte kfunc returns, and completely fails to initialize both BPF_REG_0 and BPF_REG_2 if the kfunc returns a 16-byte struct.
> --
>
> --- Patch [3]: [PATCH bpf-next v5 03/11] bpf: Wire up JIT support for 16-byte kfunc returns ---
> commit fc27c41d2405fa63fb5ef7aa5e34586ccf98e0bd
> Author: Yonghong Song <yonghong.song@linux.dev>
>
> bpf: Wire up JIT support for 16-byte kfunc returns
>
> This commit adds JIT support for 16-byte kfunc returns by placing the
> second half of the return value in the BPF R2 register. It introduces
> architecture-specific capabilities and enables support for x86-64, arm64,
> and riscv64. The verifier is also updated to reject unsupported 16-byte
> returns.
>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 57d14480ded29..161d77791bc62 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -2854,6 +2854,18 @@ int bpf_add_kfunc_call(struct bpf_verifier_env *env, u32 func_id, u16 offset)
>>   	err = btf_distill_func_proto(&env->log, kfunc.btf, kfunc.proto, kfunc.name, &func_model);
>>   	if (err)
>>   		return err;
>> +	if (func_model.ret_size > 8) {
>> +		if (kfunc.flags && (*kfunc.flags & KF_FASTCALL)) {
>> +			verbose(env, "kfunc %s with >8-byte return is not supported with KF_FASTCALL\n",
>> +				kfunc.name);
>> +			return -EOPNOTSUPP;
>> +		}
>> +		if (!bpf_jit_supports_kfunc_ret_reg_pair()) {
>> +			verbose(env, "kfunc %s with >8-byte return is not supported by JIT\n",
>> +				kfunc.name);
>> +			return -EOPNOTSUPP;
>> +		}
>> +	}
> [Severity: High]
> Does the verifier need updates in check_kfunc_call() to track the initialization
> of BPF_REG_2 for these larger returns?
>
> When evaluating the kfunc return type in check_kfunc_call(), the verifier
> currently appears to miss marking BPF_REG_2 as initialized for 16-byte scalar
> returns:
>
> kernel/bpf/verifier.c:check_kfunc_call() {
>      if (btf_type_is_scalar(t)) {
>          mark_reg_unknown(env, regs, BPF_REG_0);
>          ...
>      } else if (btf_type_is_ptr(t)) {
>          ...
>      } else if (btf_type_is_void(t)) {
>          ...
>      }
> }
>
> Furthermore, if the kfunc returns a 16-byte struct, the code falls through
> completely without initializing either BPF_REG_0 or BPF_REG_2.
>
> Since all CALLER_SAVED_REGS are cleared to NOT_INIT earlier during the call,
> won't this cause the verifier to incorrectly reject valid BPF programs that
> attempt to read these return values?
>
We should be okay here. The actual aggregate returning 16-byte struct is enabled
in later patch "bpf: Enable aggregate return types up to 16 bytes".