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

Alexei Starovoitov
2025-10-30T22:35:20Z

On Thu, Oct 30, 2025 at 8:25 AM Leon Hwang <leon.hwang@linux.dev> wrote:
>
> When updating local storage maps with BPF_F_LOCK on the fast path, the
> special fields were not freed after being replaced. This could cause
> memory referenced by BPF_KPTR_{REF,PERCPU} fields to be held until the
> map gets freed.
>
> Similarly, on the other path, the old sdata's special fields were never
> freed when BPF_F_LOCK was specified, causing the same issue.
>
> Fix this by calling 'bpf_obj_free_fields()' after
> 'copy_map_value_locked()' to properly release the old fields.
>
> Fixes: 9db44fdd8105 ("bpf: Support kptrs in local storage maps")
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---
> kernel/bpf/bpf_local_storage.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
> index b931fbceb54da..9f447530f9564 100644
> --- a/kernel/bpf/bpf_local_storage.c
> +++ b/kernel/bpf/bpf_local_storage.c
> @@ -609,6 +609,7 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
> if (old_sdata && selem_linked_to_storage_lockless(SELEM(old_sdata))) {
> copy_map_value_locked(&smap->map, old_sdata->data,
> value, false);
> + bpf_obj_free_fields(smap->map.record, old_sdata->data);
> return old_sdata;
> }
> }
> @@ -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.

pw-bot: cr