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

Alexei Starovoitov
2025-11-03T17:24:48Z

On Sun, Nov 2, 2025 at 9:18 PM Leon Hwang <leon.hwang@linux.dev> wrote:
>
>
>
> On 31/10/25 06:35, Alexei Starovoitov wrote:
> > On Thu, Oct 30, 2025 at 8:25 AM Leon Hwang <leon.hwang@linux.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.