linux-kernel - Re: [PATCH bpf-next v4 3/4] bpf: Free special fields when update local storage maps with BPF_F

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Message-ID: <22f12031-7a19-4824-a9cc-459fb63a5e0e@linux.dev>
Date: Mon, 3 Nov 2025 13:17:56 +0800
From: Leon Hwang <leon.hwang@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
 Andrii Nakryiko <andrii@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Kumar Kartikeya Dwivedi <memxor@...il.com>,
 Amery Hung <ameryhung@...il.com>, LKML <linux-kernel@...r.kernel.org>,
 kernel-patches-bot@...com
Subject: Re: [PATCH bpf-next v4 3/4] bpf: Free special fields when update
 local storage maps with BPF_F_LOCK



On 31/10/25 06:35, Alexei Starovoitov wrote:
> On Thu, Oct 30, 2025 at 8:25 AM Leon Hwang <leon.hwang@...ux.dev> wrote:
>>

[...]

>> @@ -641,6 +642,7 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
>>         if (old_sdata && (map_flags & BPF_F_LOCK)) {
>>                 copy_map_value_locked(&smap->map, old_sdata->data, value,
>>                                       false);
>> +               bpf_obj_free_fields(smap->map.record, old_sdata->data);
>>                 selem = SELEM(old_sdata);
>>                 goto unlock;
>>         }
>
> Even with rqspinlock I feel this is a can of worms and
> recursion issues.
>
> I think it's better to disallow special fields and BPF_F_LOCK combination.
> We already do that for uptr:
>         if ((map_flags & BPF_F_LOCK) &&
> btf_record_has_field(map->record, BPF_UPTR))
>                 return -EOPNOTSUPP;
>
> let's do it for all special types.
> So patches 2 and 3 will change to -EOPNOTSUPP.
>

Do you mean disallowing the combination of BPF_F_LOCK with other special
fields (except for BPF_SPIN_LOCK) on the UAPI side — for example, in
lookup_elem() and update_elem()?

If so, I'd like to send a separate patch set to implement that after the
series
“bpf: Introduce BPF_F_CPU and BPF_F_ALL_CPUS flags for percpu maps” is
applied.

After that, we can easily add the check in bpf_map_check_op_flags() for
the UAPI side, like this:

static inline int bpf_map_check_op_flags(...)
{
	if ((flags & BPF_F_LOCK) && !btf_record_has_field(map->record,
BPF_SPIN_LOCK))
		return -EINVAL;

	if ((flags & BPF_F_LOCK) && btf_record_has_field(map->record,
~BPF_SPIN_LOCK))
		return -EOPNOTSUPP;
}

Then we can clean up some code, including the bpf_obj_free_fields()
calls that follow copy_map_value_locked(), as well as the existing UPTR
check.

Thanks,
Leon