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

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

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()?

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