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: <CAADnVQJ9dFOou9jqmDaJKaYagF_KM0YXO+=r9uyM5r48+SFTuA@mail.gmail.com>
Date: Mon, 3 Nov 2025 09:24:31 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Leon Hwang <leon.hwang@...ux.dev>
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 Sun, Nov 2, 2025 at 9:18 PM Leon Hwang <leon.hwang@...ux.dev> wrote:
>
>
>
> 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()?

yes

> 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.

ok. fair enough.